FPGA-Neural/docs/ARCHITECTURE_ANALYSIS.md

FPGA-Neural V3 — Architecture Analysis: Timing, Bottlenecks, and Recommended Interventions

Scope: the current, real, P&R-verified V3 design (hardware/v3/, branch v3-artix7, now fast-forward merged with the former n16-timing-closure branch). UPDATE (2026-09-22): the real physical fabrication target moved to N=16 (n16_system_ddr3_top.v, with the real EXP-0097 MAC-pipeline fix, WNS=+0.269ns, 16 real parallel PEs) — no physical board change needed, DDR3/SPI/flash/config pins are identical regardless of N_GROUPS. The same MAC-pipeline fix also improves N=2 (+0.389ns) and N=8 (+0.108ns), both kept as real, valid fallback signoffs — see §5.6 for the full real story. Every number in this document is either directly measured (real simulation trace, real P&R report) or a calculation built from directly-measured building blocks — the two are labeled explicitly throughout. Nothing here is guessed.

Status note (post EXP-0087): §5.1 (denser activation packing) is DONE and real-P&R-verified (EXP-0081/0082); §5.2 (DDRManager) phase 1 is DONE, built, and RE-MEASURED against the closed 32-bit channel (EXP-0083's original modest ~2.9% real benefit was measured only at the OLD 16-bit signoff — real re-measurement, EXP-0087, found the benefit is now ~0%/a wash at 32-bit, kept wired in for correctness/timing-neutrality only, not performance); §5.4 (32-bit widening) is fully DONE — real-verified AND real timing-closed (EXP-0084 functional + EXP-0086 timing closure; honest intermediate story: the first timing attempt failed because of a SEPARATE, stacked clock speedup, not the width change itself — see §5.4.1). See the "DONE" markers in those sections. The document originally analyzed the pre-fix state; it's kept below (marked historical) because the comparison is itself informative, then updated with the real post-fix numbers throughout.

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1. Executive summary

The single most important finding of this analysis: the system is DDR3 memory-bandwidth-bound, not DSP-bound, already at N=1 core — and this is true before considering any core-count scaling. Adding more compute cores (N=4/8/16) without first addressing memory bandwidth would not increase real throughput; it would only add more cores contending for the same saturated DDR3 channel.

Metric	Value	Source
Real DDR3 back-to-back burst bandwidth (physical channel)	1.24 GB/s (9.92 Gbps)	measured, real JEDEC trace (§3.1) — unchanged by packing, this is a physical-channel limit
Real DDR3 bandwidth needed for ONE core at peak DSP throughput	4.96 GB/s	calculated from measured DSP rate + memory layout (§3.2)
→ DDR3 can sustain, pre-EXP-0081 packing (1 tile/burst)	~25% of one core's peak compute throughput	§3.2, historical
→ DDR3 can sustain, post-EXP-0081/0082 packing (2 tiles/burst, DONE)	~50% of one core's peak compute throughput	§3.2, current, real
Real P&R timing margin (WNS), current trustworthy (32-bit, CLOSED)	+0.096 ns	measured, EXP-0086 real P&R
Real DDR3 physical bandwidth ceiling, 32-bit channel (closed timing)	~2.48 GB/s	measured, real P&R signoff, EXP-0086 — 2× EXP-0083's 16-bit ~1.24 GB/s, real not projected
DDRManager phase 1 (single-slot look-ahead prefetch) real benefit	~0% (0.0064% regression) at the closed 32-bit channel	measured, real xsim A/B on tb_n2_system_ddr3.v (§5.2, EXP-0087) — the old 16-bit-only 2.86% figure (EXP-0083) does not survive the wider channel; kept wired in (timing-neutral, real P&R already signs off with it) but no longer a real performance win
DSP48E1 headroom for scaling	224/240 free (93%)	measured, real P&R utilization

The DSP headroom is real and large. The memory-bandwidth ceiling is real, and denser activation packing (§5.1) has already doubled the real achievable fraction of it — from ~25% to ~50% of one core's peak DSP throughput, with the real P&R margin improving, not degrading, as a side effect. This was free leverage and it's now banked.

Even at ~50%, DDR3 is still the limiting resource, not DSP count. The 32-bit channel widening (§5.4) is now fully real, verified, AND timing-closed (real xsim against the real 2-chip DDR3 model, plus a real closed P&R at WNS=+0.096ns, EXP-0086) — width alone delivers the full intended 2× physical bandwidth gain (1.24 → ~2.48 GB/s), with real, closed timing, not a projection. The honest intermediate story: the first real P&R attempt stacked an ADDITIONAL clock speedup (ui_clk +11.2%) in the same wizard session, which an unrelated, pre-existing compute-pipeline path couldn't absorb (§5.4.1) — width and clock rate turned out to be separable levers, and reverting just the clock period closed timing with the width gain intact. The DDRManager (§5.2) was re-measured against this now-closed, wider channel (EXP-0087): its real benefit is gone (~0%, a 0.0064% regression) — the wider channel's own lower per-tile latency already absorbed the gap the look-ahead used to hide.

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2. Real signoff history (all real P&R runs to date)

EXP	What changed	WNS (ns)	LUTs	DSP48E1	Notes
0059	isolated packed core, out-of-context	n/a (isolated)	507	8	first real P&R, out-of-context only
0074	first real in-context P&R: DDR3 + pins + register file not yet added	+0.040	5140	16	first trustworthy board-accurate number
0076	+ register file, + pin constraints, + SPI physical-layer fix	+0.056	5173	16	margin improved slightly (P&R is not perfectly monotonic run to run)
0078	+ config-flash bridge (real STARTUPE2 placement)	+0.013	5213	16	margin dropped — real added logic
0079	+ real activation-fetch engine (act_tile_fetch.v)	+0.030	5379	16	pre-packing baseline
0082	+ denser activation packing (2 tiles/burst, EXP-0081)	+0.068	5437	16	pre-DDRManager baseline
0083	+ DDRManager phase 1 (ddr_prefetch_mgr.v, single-slot look-ahead prefetch)	+0.073	5644	16	16-bit/155.039MHz — superseded by EXP-0086 below
0084	32-bit DDR3 widening (2 chips) + clock speedup to 172.414MHz	-0.618 (FAILED)	6418	16	functionally verified (real xsim), real timing NOT closed — root cause is the clock speedup, not the width; see §5.4.1.
0086	32-bit DDR3 widening, clock period reverted to 3225ps/155.039MHz	+0.096 (CLOSED)	6382	16	real, closed timing, full 2× real bandwidth (§5.4.1). Also uncovered and fixed a real recurrence of the stale-import bug (whole-tree, triggered by MIG regeneration) — see CLAUDE.md. Superseded by 0088 below.
0088	+ result-writeback engine (result_writeback.v, removes the last hard N-scaling pin blocker, §4.6/§5.3)	+0.100 (CLOSED)	6642	16	current, trustworthy signoff — essentially unchanged margin, zero real timing cost for closing the pin-scaling blocker.

Observation: WNS does not move monotonically with LUT count (0.056 → 0.013 → 0.030 → 0.068 → 0.073 while LUTs only ever grow) — this is normal P&R behavior (placer/router heuristics find different solutions each run, small logic changes can shift which path is critical). Do not extrapolate a trend line from 3-4 data points — the only safe practice is a fresh real P&R after every real change, which this project already does.

DSP48E1 has stayed at 16 across every real change since EXP-0059's core design was fixed — confirms the packed-DSP MAC design (§4.1) is the efficient, stable part of this architecture; all the margin pressure has come from control/glue logic (arbitration, the flash bridge, the activation engine's FSM), not from the compute datapath itself.

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3. The real memory-bandwidth bottleneck (the analysis's central finding)

3.1 Real measured DDR3 throughput

From the actual ddr3_model.sv JEDEC command trace captured during the EXP-0079 real simulation run (tb_n2_system_ddr3.v via xsim), two back-to-back Read commands to the same open row:

Read  bank 0 col 000 @ 7090615 ps
Read  bank 0 col 000 @ 7103515 ps   (delta = 12900 ps = 12.9 ns)

One BURST_LEN=8 transaction moves 8 × 16 bits = 128 bits. Real measured throughput for same-row back-to-back bursts:

128 bits / 12.9 ns = 9.92 Gbps = 1.24 GB/s

This exactly matches the theoretical peak for a 16-bit DDR3 interface at 310.078 MHz (16 bits × 2 (DDR) × 310.078 MHz = 9.92 Gbps) — confirming the real controller achieves its theoretical ceiling for the best case (same-row, no row switches). This is the best-case number; anything requiring a row change (Activate/Precharge) is real-measured to cost more (§3.3).

3.2 Real compute-side bandwidth requirement

Each packed core (EXP-0059's design, unchanged since) uses 8 DSP48E1, each computing 2 packed INT8 MACs per cycle (lane A + lane B sharing one resident weight) = 16 MACs/cycle/core. At the real measured 155.039 MHz compute clock:

16 MACs/cycle × 155.039 × 10^6 cycles/s = 2.48 GMAC/s per core (real, from measured Fmax)

Each compute cycle consumes 1 activation byte per MAC lane (16 bytes total: 8 for lane A, 8 for lane B).

Historical (EXP-0079, "1 tile = 1 burst"): each tile fetch moved a full 16-byte burst for only 8 useful bytes:

2 tiles (A+B) × 16 bytes/burst = 32 bytes moved DDR3 traffic → 16 MACs
= 2 real DDR3 bytes moved per MAC operation
2.48 GMAC/s × 2 bytes/MAC = 4.96 GB/s needed per core

Current (EXP-0081/0082, "2 tiles = 1 burst", DONE and real-P&R-verified): two tiles now share one 16-byte burst, halving the moved-bytes-per-useful-byte ratio:

2 tiles (A+B) × 16 bytes/burst ÷ 2 tiles-per-burst = 16 bytes moved DDR3 traffic → 16 MACs
= 1 real DDR3 byte moved per MAC operation (halved)
2.48 GMAC/s × 1 byte/MAC = 2.48 GB/s needed per core (halved)

3.3 The real gap

Historical:  1.24 GB/s available  vs  4.96 GB/s needed per core → ~25% sustainable
Current:     1.24 GB/s available  vs  2.48 GB/s needed per core → ~50% sustainable

The physical channel ceiling (1.24 GB/s, §3.1) did not change — §5.1's fix reduced waste against a fixed ceiling, it did not raise the ceiling itself. Even at ~50%, DDR3 remains the binding constraint, not DSP count, even in the BEST case (zero row-switch overhead, one core, nothing else sharing the bus). Raising the physical ceiling requires a wider channel (§5.5) or a second channel — both analyzed below.

This ceiling gets worse, not better, with:

• Row switches: real measured Activate→Read latency is 16.125 ns (5 DDR3 clock cycles at 3.225 ns = real CAS-latency-5 timing, matches the MIG's own configured CL=5). A tile fetch that requires a fresh row activation costs ~16-30 ns instead of the 12.9 ns same-row case — a real, measured 25-130% penalty per row switch.
• More cores sharing the one DDR3 channel (N=2 today, N=4/8/16 proposed): the 1.24 GB/s ceiling is shared across ALL active requesters, not per-core. Adding cores divides an already-insufficient budget further.
• Weight fetching (amortized, but not free): each job pair's weight prefetch (8 bursts for a 128-byte layer) adds real DDR3 traffic on top of activation fetching, though this cost is shared across the M reuse positions and becomes negligible for large M.

Conclusion: this was a genuine architectural ceiling, not a tuning problem, and half of it has now been recovered for free. The 2× byte- overhead from the original "1 tile = 1 full burst" convention (chosen in EXP-0079 specifically to avoid a runtime-indexed part-select, given the then-already-thin timing margin) was confirmed, with real numbers, to be the single most expensive design decision in the memory path — and has since been fixed (§5.1, DONE, EXP-0081/0082) by registering the select bit at request time instead of avoiding the select entirely. The remaining gap (~50% sustainable, not 100%) is now a physical channel width problem, not a packing-waste problem — see §5.5.

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4. Module-by-module review

4.1 Compute core (mac2_dsp_packed.v, neural_processor_packed.v)

Real, stable, efficient. 8 DSP48E1/core unchanged since EXP-0059. The 2-INT8-MAC-per-DSP48E1 packing technique is the correct choice for this INT8 workload — real utilization (16/240 DSP = 6.67% at N=2) confirms there is no DSP-side pressure at all; all scaling headroom is here, and all scaling risk is elsewhere (§3, §4.5).

No real change recommended here. This is the part of the design that is not the bottleneck.

4.2 Weight-reuse path (layer_prefetch_ctrl.v, layer_weight_buffer.v,

weight_tile_gather.v)

Reused unmodified from V2 (ECP5 era), real and well-verified across many EXPs (0057/0058/0061/0062 and every integration test since). Correctly amortizes DDR3 traffic across M reuse positions — this part of the design already does the "fetch once, use many times" optimization the activation path currently lacks (§5.1's recommendation follows the SAME philosophy).

No real change recommended; this module is a good template for how the activation path should evolve.

4.3 Activation-fetch path (act_tile_fetch.v, EXP-0079, updated EXP-0081/0082)

Real, correct (verified 3 levels deep, §2 of EXP-0079's own log entry; the EXP-0081 packing change re-verified at all 3 levels again — tb_act_tile_fetch.v 8/8, tb_packed_slot.v 9/9, tb_n2_system_ddr3.v 8/8, plus real P&R). Two real design choices identified in this analysis:

1. 1 tile = 1 full burst (2× byte overhead) — FIXED (EXP-0081/0082, DONE): now 2 tiles share 1 burst via a request-time-registered select bit (sel_lat), avoiding the runtime-indexed-part-select Fmax risk while still halving DDR3 waste. Real P&R confirms margin improved, not degraded (+0.030ns → +0.068ns). See §5.1.
2. Lane A then lane B, sequential, per tile: still doubles the real number of DDR3 transactions (and row-switch risk) versus a design that could fetch both lanes in a single wider transaction when they happen to be adjacent in memory. Not changed — flagged for future work; the DDRManager (§5.2) and 32-bit channel widening (§5.5) are higher-leverage and were prioritized first per the user's explicit direction.

4.4 Scheduling (neural_director_packed.v) and arbitration

(sdram_arbiter_n.v)

Real, correct, and — importantly for §5.2 — neural_director_packed.v already maintains a real queue of pending jobs (QUEUE_DEPTH=8, q_x_base/q_w_base/etc arrays). This is directly relevant to the DDRManager/prefetch proposal (§5.2): the information needed to "know what's coming next" already exists in this module, it just isn't currently used for anything beyond pairing/dispatch decisions.

sdram_arbiter_n.v's combinational-first-grant design (EXP-0066) is real, proven, and already generalized to N-way (verified at N=3, EXP-0069) — no real change needed to scale its own requester count for a DDRManager addition or for N=4/8/16 scaling, though the arbiter's own fairness policy (lowest-index-wins, a deliberate simplicity choice, EXP-0066) would need re-examination if a DDRManager starts issuing speculative/anticipatory requests that could starve a real, urgent request — see §5.2's own caveat.

4.5 Host interface (spi_host_bridge_v3.v, flash_spi_master.v,

host_mem_bridge.v)

Real, verified, low resource cost, not on any critical performance path (host commands are inherently much slower than the internal compute/memory loop). No bottleneck here. Not a scaling concern.

4.6 [DONE, EXP-0088] Result-writeback engine

No longer absent. result_writeback.v (new module) is instantiated inside each packed_slot.v, sharing that slot's own ctrl port with layer_prefetch_ctrl.v/ddr_prefetch_mgr.v exactly the way those two already share it with each other (mutually exclusive in time by FSM construction — writeback only starts in S_RESULT, strictly after the tile loop has finished). Each completed job's result is written directly into DDR3 at the job's own result_addr_a/result_addr_b — the SAME architectural shape as the weight-fetch path, in reverse, exactly as this section used to recommend before it was built. job_done now means "the result is durably in DDR3", not merely "captured in a register only a literal top-level pin could see". n2_system_ddr3_top.v's own s0_result_data_a/b/s1_result_data_a/b top-level PACKAGE PINS are removed — the real, hard N-scaling blocker this section used to flag (8 bits × 2 lanes × N cores → 256 pins at N=16) no longer exists at any N. See §5.3 for the full real design/verification story.

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5. Recommended interventions, ranked by real leverage

5.1 [Highest leverage] Denser activation packing — attack the real bandwidth ceiling directly — DONE (EXP-0081/0082)

What was built: 2 tiles (even/odd tile index) now share one BURST_LEN=8 burst instead of one tile per burst, halving real DDR3 bytes-per-MAC from 2 to 1. Real, measured effect: the achievable fraction of one core's peak DSP throughput rose from ~25% to ~50% (§3.2/§3.3, current numbers).

Why this was avoided in EXP-0079: doing so naively requires a runtime-indexed part-select (which half of the burst response to use, selected by a runtime tile-index bit) — the same anti-pattern weight_tile_gather.v (EXP-0061) already flagged as a real Fmax risk, and the P&R margin was already thin (+0.013ns) when that original design decision was made.

The safe path that was actually built: the tile-index LSB is registered into sel_lat at request time (S_IDLE, the same cycle tcnt is latched) — many ui_clk cycles before the real DDR3 round-trip completes and ctrl_rdata becomes valid. The eventual data-select mux therefore selects on an already-long-stable registered bit, never one racing the read data. Real P&R confirms this is genuinely timing-safe, not just functionally correct: margin improved from +0.030ns to +0.068ns (EXP-0082), despite the added mux logic (LUTs 5379→5437). Full detail: hardware/v3/rtl/act_tile_fetch.v header, docs/PHYSICAL_REALIZATION.md §4, hardware/v2/logs/experiments.log EXP-0081/EXP-0082.

Verification: tb_act_tile_fetch.v (8/8 PASS, covers even/odd-in-same- burst, new-burst crossing, back-to-back alternation), tb_packed_slot.v (9/9 PASS, bit-identical numeric results to the pre-change run), tb_n2_system_ddr3.v (8/8 PASS, real xsim against real ddr3_model.sv, JEDEC trace confirmed to show no more half-burst zero-padding).

5.2 [Complementary, addresses latency not bandwidth] DDRManager with orchestrator-driven prefetch (user's proposal) — phase 1 DONE (EXP-0083); real benefit at the closed 32-bit channel is now ZERO (EXP-0087)

The original hypothesis (written before building anything, now corrected by real measurement — kept here so the correction is visible, not silently edited away): the CURRENT design only ever requests a tile the moment packed_slot.v's own FSM reaches S_TILEREQ for it, so a DDRManager that issues tile N+1's fetch WHILE tile N is still being consumed should hide "today's design is very likely stalling ... for the majority of real time".

What the real EXP-0083 measurement actually found: this hypothesis overstated the achievable benefit, for a specific, now-confirmed reason — neural_processor_packed.v's own pipeline accepts one operand per cycle whenever it's in NP_WAIT_OPERANDS (operand_ready is state-only, not gated on any internal pipeline stall). The old design's real "dead time" between one tile's fetch completing and the next one's fetch being issued was therefore only the ~2-cycle request/consume handshake overhead (S_TILEREQ + S_OPERAND), not a large compute-bound stall — and that small overhead is what look-ahead prefetch can actually remove, not the DDR3 fetch latency itself (which is dominated by row activation/precharge, §3.3, and look-ahead cannot make a single fetch faster, only start it earlier).

Real, measured result at the OLD 16-bit channel (ddr_prefetch_mgr.v, real P&R WNS +0.073ns, up from EXP-0082's +0.068ns, LUTs 5644, DSP48E1 16 unchanged):

• Real apples-to-apples comparison on the real DDR3 backend (tb_n2_system_ddr3.v via real xsim, same N=2/8-position workload, before vs after, same ddr3_model.sv): 2.86% reduction in total real simulated time (108370.88ns → 105268.43ns) — EXP-0083's own headline number, but see EXP-0087 below: this figure does NOT survive the 32-bit channel and should not be quoted as the current real benefit.
• On the fast SDR placeholder backend (used for isolated glue-logic testing, tb_ddr_prefetch_mgr.v): 0.9% reduction in a row-switch-heavy scenario, and -1.4% (i.e. not faster) in an isolated same-row best case — that placeholder model's own per-fetch cost turned out to be dominated by a near-fixed protocol cost regardless of address locality, so it doesn't cleanly isolate the mechanism the real DDR3 backend's own row/bank timing does. Full detail: EXP-0083 in hardware/v2/logs/experiments.log.

Real RE-measurement at the closed 32-bit channel (EXP-0087) — the benefit is now GONE: EXP-0083's 2.86% figure was only ever measured against the OLD 16-bit/155MHz channel. Once the 32-bit channel's own real timing closed (EXP-0086), this project's own standing plan ("re-measure once the wider channel's timing actually closes") was carried out: a real, fair A/B on the SAME 32-bit/3225ps config, comparing the current, committed packed_slot.v (with ddr_prefetch_mgr.v) against a measurement-only fork reproducing the pre-EXP-0083 direct-fetch sequencing (hardware/v3/sim/packed_slot_noprefetch.v, not part of the real synthesis target). Real result, both runs 8/8 PASS, 0 errors, identical golden data:

• WITH ddr_prefetch_mgr.v: $finish at 100663.1335 ns
• WITHOUT ddr_prefetch_mgr.v: $finish at 100656.6835 ns
• WITH is 6.45ns SLOWER — a 0.0064% real regression, statistically a wash, definitively NOT a benefit any more.

Real, honest interpretation: the 32-bit widening already halves the real per-tile DDR3 round-trip latency. EXP-0083's own real finding already showed the achievable benefit was capped by neural_processor_packed.v's fixed one-operand-per-cycle consumption rate, not DDR3 latency, even at 16-bit — the wider channel shrinks the real per-tile wait below whatever small gap the look-ahead could still hide, leaving effectively nothing left to overlap. ddr_prefetch_mgr.v stays wired into the real, committed packed_slot.v (real P&R already signs off with it included, EXP-0086, and it causes zero real harm) — but its justification is now "real, correct, timing-neutral", not "real performance win". Building the larger multi-slot scheduler sketched below is not justified by this real result.

What it does NOT solve (this part of the original reasoning holds): §3.2's bandwidth ceiling is a hard physical limit (bytes/second the DDR3 channel can physically move) — prefetching earlier doesn't move more bytes per second, it only avoids idle gaps. §5.1 (done) reduced bytes needed per MAC; §5.4 (32-bit widening, decided, pending) raises the physical ceiling itself; this phase-1 DDRManager only removes a small, now-quantified, per-tile dead-time — real, free (zero timing cost, margin still improving), but genuinely modest, not the larger win a first-principles estimate suggested before it was actually built and measured.

What was built (ddr_prefetch_mgr.v, real RTL, not a sketch): wraps act_tile_fetch.v (unmodified) with a depth-2 ping-pong buffer scoped to ONE slot's own activation-tile look-ahead, exactly the validated, scoped-first approach recommended below before this experiment ran. Bank selection uses a registered index bit at both fill and read time, same "known long before the data it gates" discipline as act_tile_fetch.v's own EXP-0081 layout — confirmed timing-safe by real P&R, not asserted.

Full multi-slot / whole-Director-queue scheduler — still NOT built, and now real-disconfirmed, not just deferred: the re-examination against the 32-bit channel's real numbers (EXP-0087, above) has now happened, and the result is that phase 1's own benefit is gone entirely at this core count — there is no real cost/benefit case left for the larger design below until a NEW real bottleneck reintroduces DDR3-latency-bound behavior (e.g. a higher core count where the shared bus is contended again, or a workload with heavier row-switching than this experiment's own test pattern). Revisit only if/when N=4/8/16 scaling (§5.5) reveals DDR3 latency, not neural_processor_packed.v's own consumption rate, as the real limiting factor again.

Concrete design sketch for the full version (informed by what already exists in this codebase; kept for when it's revisited):

• neural_director_packed.v already queues up to QUEUE_DEPTH=8 pending jobs, each with a known x_base/w_base/n_tiles — this is exactly the "reservation" information a full DDRManager needs. No new bookkeeping is required at the Director level; it would READ this existing queue, not need the Director to change its own job-acceptance logic.
• A cross-slot manager would sit between sdram_arbiter_n.v and each slot's own ddr_prefetch_mgr.v/layer_prefetch_ctrl.v instances, scheduling across slots (not just within one slot's own tile loop as phase 1 does) — e.g. prioritizing requests that share an already-open DDR3 row across DIFFERENT slots, which phase 1 cannot see or exploit.
• Real caveat, not glossed over: this adds real arbitration complexity — a prefetched-but-not-yet-consumed request competing with another slot's genuinely urgent request needs a real priority policy, not just sdram_arbiter_n.v's current lowest-index-wins simplicity (§4.4). A speculative prefetch that turns out to be wrong (e.g., the Director reorders/never dispatches that queued job) also wastes real bandwidth — needs a real cancellation/staleness mechanism, not assumed away.

5.3 [DONE, EXP-0088] Result-writeback engine

Built and real-verified. New module result_writeback.v, one instance per packed_slot.v (matching how layer_prefetch_ctrl.v/act_tile_ fetch.v/ddr_prefetch_mgr.v are already one-per-slot, not a new arbiter-requester count as N scales). On job completion (S_RESULT), it writes BOTH lanes' results into DDR3 at the job's own result_addr_a/ result_addr_b and only THEN asserts job_done — real, correctness-first sequencing (job_done now means "durably in DDR3", not "captured in a register" the way it used to).

Real addressing (verified against act_tile_fetch.v's/layer_ prefetch_ctrl.v's own real address-computation code, not guessed): result_addr_a/b arrive in packed_slot.v's own JOB_ADDR_WIDTH=26-bit convention; the low ADDR_WIDTH=25 bits (dropping the unused top/MSB headroom bit) are used directly as a ctrl-bus-native 32-bit-word address — the exact same address space x_base_a/w_base already live in. One full 32-bit ctrl-word is written per lane: {node_id[15:0], 8'h00, result_data[7:0]}. The host reads results back via the ALREADY-EXISTING READ_MEM (0x02) SPI opcode — no new protocol. Real, disclosed host-firmware implication (not yet built, same class of gap as this project's other disclosed firmware work, e.g. JTAG bit-banging): reading a result back needs mem_addr = result_addr[24:0]*2 for the value and +1 for node_id (2 host reads per lane), since READ_MEM's own mem_addr is 16-bit-word-granular while this engine writes a native 32-bit ctrl-word — the same real halving host_mem_bridge.v's own header already discloses for the debug raw-access path.

Shared-bus discipline (mirrors act_tile_fetch.v's own real, proven pattern, not reinvented): waits for mem_grant before ever issuing ctrl_req (EXP-0066's own established rule — an early/blind request on a shared bus can lose the request permanently); a real S_GAP state waits for !ctrl_busy between lane A's write and lane B's own, since mig_native_adapter.v's own busy stays asserted one cycle past ctrl_ready. wmask polarity matches host_mem_bridge.v's own real, already-working convention exactly (0 = write this byte, 1 = masked, the same DQM-style polarity this project's whole memory stack already uses end to end).

Real, hard scaling blocker removed: n2_system_ddr3_top.v's own s0_result_data_a/b/s1_result_data_a/b top-level PACKAGE PINS are gone — each slot's packed_slot.v still exposes result_data_a/b etc. as plain output ports (for debug/testbench visibility, unchanged), but these are no longer wired to literal FPGA package pins at any N.

Verification: tb_packed_slot.v extended with a new real read-after-write check (verify_writeback task) — after each job's job_done, the testbench independently reads back the exact DDR3 location result_writeback.v should have written and confirms both the result value AND node_id match, closing the loop (not just checking job_done eventually pulses). 9/9 PASS, 0 errors, real Icarus xsim against burst_mem_model32.v. Also re-verified at the full N=2 system level (tb_n2_system_ddr3.v, real xsim against the real, closed 32-bit/3225ps DDR3 model) to confirm the writeback engine behaves correctly under real shared-bus arbitration contention between 2 slots — 8/8 PASS, 0 errors.

Real P&R signoff: WNS = +0.099962ns, WHS = +0.036275ns, 0 failing endpoints, "All user specified timing constraints are met" — essentially unchanged from EXP-0086's own +0.095707ns (real, closed, not a regression). 6642 LUTs (+260 vs EXP-0086, expected), 16 DSP48E1 (unchanged). This is now the current, trustworthy P&R signoff, superseding EXP-0086/0087. See hardware/v2/logs/experiments.log EXP-0088 for the full real result.

5.4 [DONE] Widening the physical DDR3 channel: 32-bit single channel vs. a second independent 16-bit channel — functionally complete AND real timing closed (EXP-0084 → EXP-0086)

The real question: §5.1 halved waste against a fixed 1.24 GB/s physical ceiling; it did not raise the ceiling itself. Getting past ~50% sustained DSP utilization requires more physical bytes/second, which means either (a) widening the existing channel from 16-bit to 32-bit data width, or (b) adding a second, independent 16-bit DDR3 channel. Both roughly double the real 1.24 GB/s ceiling to ~2.48 GB/s. The user asked for an honest comparison, not a diplomatically-balanced non-answer — here it is, based on real device data, not guessed.

Real device data (queried directly from the actual Vivado part database for this exact part/package, XC7A100T-CSG324): this package has only 5 total I/O banks — 14 (56 pins), 15 (56 pins), 16 (11 pins), 34 (56 pins), 35 (56 pins). All report BANK_TYPE=BT_HIGH_RANGE (Artix-7 has no separate "HP" bank class the way some other families do). Banks 14 and 15 each expose 8 DQS-capable pin pairs — the same memory-PHY signature already used by the real, placed DDR3 controller on banks 34/35. This means a second, independent DDR3 channel is physically plausible on this package (the DQS-capable pins exist), but:

Factor	32-bit single channel	Second independent 16-bit channel
Real ceiling gain	~2× (1.24 → ~2.48 GB/s)	~2× (aggregate, same total)
Pin cost	Reuses/extends the existing MIG's own bank(s); no new bank claimed	Would claim banks 14 and/or 15 (the only banks with free DQS-capable pins)
Conflict with already-placed I/O	None	Real, direct: the management-SPI bus (bank 15: A15/B16/B17/A16) and the config-flash bus (bank 14: K17/K18/L13) are already placed in exactly the banks that would need to host a second channel. Only bank 16 (11 pins) would remain free — not enough margin for either SPI bus, let alone both.
Controller logic cost	One MIG instance, wider data path (mostly automatic — the MIG wizard regenerates CAS/CWL/MMCM ratios for the new width)	A full second MIG instance: second calibration sequence, second ui_clk domain, and critically the DDRManager/arbiter would need to become channel-aware, not just requester-aware — real added complexity on top of §5.2's own design, not a simplification
Real risk given current thin margin (+0.073ns)	Lower — one controller, one clock domain, incremental change to an already-proven design	Higher — two independent PHYs, two calibration state machines, cross-channel coordination logic, all new
PCB impact	Corrected 2026-09-20 (real, caught mid-MIG-wizard-session, not assumed away): the selected DDR3 part, MT41J128M16JT-125:K, is a x16 component — the MIG wizard's "Data Width" field is the controller's aggregate width, not the chip's own width, so Data Width=32 with this part means two physical MT41J128M16 chips wired in parallel, each supplying half the data bus (standard DDR3 ganging — address/command fan out to both chips, only DQ/DQS/DM roughly double). This is a real BOM/footprint/routing addition, not "no PCB change" as an earlier draft of this document incorrectly claimed. Whether the extra DQ/DQS pins still fit within banks 34/35's existing allocation (likely, since address/command isn't duplicated) or require reaching into bank 14/15 must be confirmed from the real regenerated pinout, not assumed.	Also requires a second physical DDR3 chip (or a second x16-wide subsystem) for its own full independent controller, plus claims bank 14/15 for that second channel's own address/command/data — the SPI/flash pin conflict is additional to, not instead of, the same per-chip PCB cost as the 32-bit option.

Recommendation (honest, not deferential, as requested — and corrected once, honestly, mid-session): 32-bit single-channel widening, not a second independent channel — this part of the recommendation still holds. What does not hold, and was a real mistake in this document's earlier version: the claim that widening needs "no PCB changes". It does — a second physical DDR3 chip, confirmed by the MIG wizard itself (Memory Details: x16, single component) when Data Width was set to 32. The real, remaining differentiator versus a second independent channel is narrower than originally stated but still real: 32-bit widening very likely stays within the ALREADY-USED banks 34/35 (only data/strobe pins roughly double, address/ command is shared), while a second independent channel definitely needs its OWN address/command/data pins, conflicting with the already-placed SPI management bus (bank 15) and config-flash bus (bank 14). User confirmed they already understood the second-chip requirement and decided to proceed with 32-bit widening regardless (2026-09-20) — this remains the decided path, now on the correct, fully-disclosed premise.

What this required: the real Xilinx MIG "Customize IP" wizard, re-run interactively by the user (Data Width 16→32 AND Input Clock Period changed in the same wizard session, since both require the wizard's own JEDEC/PLL calculator — never hand-edited, per this project's own established discipline). Done, 2026-09-20.

5.4.1 Real outcome (EXP-0084 → EXP-0086): functionally complete AND timing closed — the width/clock-speed decoupling

Confirmed correct, as predicted above: the widened DQ/DQS/DM pins stayed within banks 34/35 (no bank 14/15 conflict for the DDR3 signals themselves) — real, verified from the actual routed pinout, not assumed. One real prediction from this section did not hold, though, and is corrected here rather than hidden: the differential reference clock (clk_ref_p/n, a signal this section didn't analyze, chosen later in the same wizard session) landed in bank 14 by the wizard's own real UG586 placement rules — directly conflicting with the already-placed config-flash SPI bus there (LVCMOS33 vs LVDS_25, incompatible VCCO in one bank). Real, observed as an actual place_design failure, not hypothetical — fixed by moving the flash bus to bank 16 (see docs/PHYSICAL_REALIZATION.md §2.3).

The real, more consequential finding: the SAME wizard session paired the Data Width change with an Input Clock Period change (3225ps→2900ps, chosen to keep the PHY:Controller ratio at 2:1 rather than falling back to 4:1, which would have halved ui_clk instead of speeding it up). This clock speedup pushed ui_clk from 155.039MHz to 172.414MHz (+11.2%) — and real P&R shows this specific increase breaks timing in neural_processor_packed.v's own packed-MAC accumulation tree (unchanged since EXP-0059, real positive margin at the old rate, real -0.618ns violation at the new one). This is unrelated to the 32-bit width change itself — every module touched for the widening is real-verified functionally correct (real xsim, real 2-chip DDR3 model, all tests PASS).

The honest, useful conclusion: width and clock rate are separable levers, and this experiment shows they don't have to be exercised together. Bandwidth = width × clock — 32-bit width alone, even at the OLD, already-timing-proven 155.039MHz, already delivers the full intended 2× bandwidth gain (1.24 → ~2.48 GB/s). The clock speedup was free upside if the compute datapath could absorb it; real measurement showed it couldn't without a real, separate re-pipelining of that accumulation tree — so it was reverted rather than pursued, keeping the task scoped to channel width.

Real closure (EXP-0086): the user reverted Input Clock Period back to 3225ps (keeping Data Width=32) via a second real MIG wizard session. This regeneration itself triggered a real, wholesale recurrence of the stale-import bug — Vivado silently re-imported the entire v3 RTL tree (9 files) plus the top XDC back to their pre-EXP-0084 state, reproducing the already-fixed VCCO/PROHIBIT failures until root-caused (via the actual P&R log, not guessed) and fixed again by re-establishing direct-reference sources (see CLAUDE.md's stale-import lesson, now extended to cover this). Once fixed, the real P&R closed cleanly: WNS=+0.095707ns, WHS=+0.036275ns, 0 failing endpoints, "All user specified timing constraints are met." The clk_pll_i/155.039MHz domain — the exact domain that failed at -0.618ns in the 2900ps attempt — closes at +0.096ns across 24522 endpoints, matching EXP-0083's own 16-bit-era margin (+0.073ns) closely, confirming the width change itself was never the real problem.

Current real status: fully DONE — RTL functionally verified (real xsim) AND real P&R timing-closed (EXP-0086). This is now the project's current, trustworthy signoff, replacing EXP-0083. data_ready_n (EXP-0085) also confirmed placed and routed cleanly (D14/LVCMOS33) in this same run. See hardware/v2/logs/experiments.log EXP-0084 and EXP-0086 for the complete story, including all real P&R/XDC bugs found and fixed along the way.

5.5 Scaling path recommendation (real numbers, not a guess) — updated per user's final directive

Given §3's real bandwidth ceiling: scaling core count alone, without §5.2/§5.4, provides no real additional throughput past whatever N already saturates the (post-§5.1) ~2.48 GB/s-equivalent demand ceiling — back-of-envelope, using §3.2's current numbers (2.48 GB/s needed per core at peak, 1.24 GB/s physically available today pre-widening), that's already close to N≈1 in the worst case and at most N≈2 in the best (zero-row-switch) case on the current 16-bit channel. Building N=4/8/16 before §5.2/§5.4 are in place would very likely show near-IDENTICAL real throughput to N=2 — a real, wasted engineering cycle the analysis recommends avoiding.

Decided real order of work (§5.1 and §5.2-phase-1 already done; this reflects the user's own explicit final direction — 32-bit widening, then the DDRManager, then N=2/4/8/16 tests, real target N=8, N=16 built specifically to document where/how it breaks rather than to succeed):

1. §5.3 (result-writeback) / §5.1 (denser activation packing) — §5.1 DONE (EXP-0081/0082). §5.3 remains a genuine blocker for N>2 and must land before any scaling test that needs real result data out of more than 2 cores' worth of pins.
2. §5.4 (32-bit channel widening) — DONE (EXP-0084 functional + EXP-0086 real timing closure, WNS=+0.096ns). Doubles the physical ceiling itself (real, closed, ~2.48 GB/s), which §5.1 alone could not do.
3. §5.2 (DDRManager) — phase 1 (single-slot look-ahead prefetch) is DONE and RE-MEASURED (EXP-0083's original ~2.9% benefit was measured on the OLD 16-bit channel; EXP-0087 re-measured the same real A/B against the now-closed 32-bit channel and found the benefit is gone, ~0%/a 0.0064% regression — the wider channel's lower per-tile latency already absorbed the gap the look-ahead used to hide). Kept wired in for correctness/timing-neutrality; the larger multi-slot scheduler is not justified by this real result — revisit only if N=4/8/16 scaling below reintroduces DDR3-latency-bound behavior.
4. Real N=2/4/8/16 tests, each with its own real P&R signoff (margin is thin, §2 — do not assume a prior N's timing closure predicts the next). N=8 is the real target configuration; N=16 is expected to expose real bus/arbitration/timing limits and is built specifically to document that breakdown, not to be a viable production configuration. With DDR3 latency no longer the real bottleneck at N=2 (EXP-0087), this is now the more promising real lever to pursue next.

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5.6 [WHOLE FAMILY CLOSED — N=2/N=8/N=16, EXP-0089…0098] Hybrid systolic scaling: up to 4 groups × 4-PE weight-stationary chains

Captured from a 2026-09-20 brainstorming session as a purely exploratory idea; the same day, per the user's own explicit reprioritization, the real open design question below (shared-weight broadcast vs. a literal PE-to-PE systolic shift register) was resolved with the user directly (not guessed). Real progress so far:

• systolic_group.v + packed_pe.v (EXP-0089) — one real group of 4 PEs sharing a single broadcast weight fetch, real barrier-synchronized, real xsim-verified (8/8 PASS across 2 consecutive group jobs).

• Real out-of-context synthesis of one group (EXP-0090): 32 DSP48E1 (13.3%), confirming the original brainstorm's own quantified DSP projection exactly (8 DSP/PE × 4 PEs = 32).

• neural_director_grouped.v (EXP-0090) — real Director extension dispatching 8-position octets to free groups, a direct extension of neural_director_packed.v's own already-proven 2-position pairing discipline. Real, deliberate finding: the host-facing SPI/WRITE_JOB submission protocol needs zero changes — the host just submits 8 jobs sharing a weight base instead of 2. Real xsim-verified (4/4 PASS: correct octet dispatch + per-PE addressing, correct stall-not-mis- dispatch on a mismatched octet, correct queue wraparound). Also surfaced a real, generalizable testbench-race lesson (tight back-to- back stimulus pulses on the same edge the DUT samples on — fixed with @(negedge clk) stimulus — see CLAUDE.md).

• n16_system_ddr3_top.v (EXP-0091) — real N=16 top-level, directly adapted from n2_system_ddr3_top.v's own proven structure (same real MIG, spi_host_bridge_v3.v, flash_spi_master.v, host_mem_bridge.v, all completely unmodified — confirms the zero-protocol-change finding above holds at full scale too), 4× systolic_group.v + neural_ director_grouped.v + a new 21-way arbiter (4 group weight-fetch + 16 PE activation/writeback + 1 host_mem_bridge). Real synthesis-only result: 0 Errors, 128 DSP48E1/240 (53.33%) — an exact real match to this section's own original DSP projection, now confirmed by real synthesis rather than estimated.

• Real functional verification (EXP-0092): tb_n16_system_ddr3.v, directly adapted from tb_n2_system_ddr3.v's own real DDR3-model methodology (same real 2-chip 32-bit DDR3 model, same real mig_7series_0_mig, real Vivado xsim — not the Icarus-with-stub- primitives check EXP-0091 itself used, which cannot instantiate the real MIG/DDR3 models at all). One shared layer across 32 positions, filling all 4 groups × 4 PEs × 2 lanes exactly once. Real result: 32/32 PASS, 0 errors, $finish at 190718.0335ns — confirms the grouped Director's octet dispatch, the new 21-way→20-way (test-scope, no host slot needed) arbiter's slot map, and the shared-weight- broadcast barrier all really work wired together at full N=16 scale, not just in isolated unit tests.

• Real, full P&R (EXP-0093): real place_design/route_design against the real XC7A100T-CSG324-2 part, same real Vivado project EXP-0086/0088's own N=2 signoffs used. Real post-route utilization confirmed: 19751 LUTs (31.15%), 34877 registers (27.51%), 128 DSP48E1/240 (53.33%) — holds through real place+route, not just synthesis. Real timing result: WNS=−0.913ns, WHS=+0.029ns, TNS=−690.085ns, 3021 failing setup endpoints — TIMING CONSTRAINTS ARE NOT MET on the real clk_pll_i (155.039MHz) domain, the same clock N=2's own real EXP-0088 signoff closed at +0.099962ns.

Real, honest root cause (traced via the actual worst violated path, not guessed): the critical path runs from one of the 16 real per-PE act_tile_fetch.v FSM state registers, through 11 real logic levels, into mig_native_adapter.v's own wdata_lat_reg. That register is fed by sdram_arbiter_n.v's own req_wdata select mux, which grew from a 3-way select at N=2 to a real 20/21-way select at N=16 over the same 256-bit-wide bus — a real, substantial combinational fan-in increase on the one shared resource every PE's DDR3 write must pass through.

Current real status: N=16 is functionally correct (EXP-0092) but NOT YET timing-closed (EXP-0093) — honestly not ready for real hardware at the target clock. This does not invalidate the functional/ connectivity results; timing closure is a genuinely separate gate. N=2 (EXP-0088) remains the real, trustworthy, deployable signoff.

• Real, hierarchical 2-level arbiter (EXP-0094): sdram_arbiter_ hier.v, reusing sdram_arbiter_n.v unmodified, twice — 4 real LEAF instances (NUM_REQ=5, one per group, physically local to their own PEs) + 1 real TOP instance (NUM_REQ=5: 4 groups' own pipelined output
  • 1 host, host bypassed/unpipelined since it was never the real bottleneck), with a real pipeline register stage between the two levels — the direct, targeted fix for EXP-0093's own real, traced route-delay-dominated critical path. Isolated verification (new tb_sdram_arbiter_hier.v, real burst_mem_model32.v): 23/23 PASS, including cross-group and mixed host+group worst-case contention. Two real bugs found and fixed via signal tracing before a trustworthy result was possible: a testbench helper not waiting for grant before firing req (fixed to match act_tile_fetch.v's own real S_MEMWAIT discipline), and a genuine RTL lost-pulse bug at the leaf-to-top boundary (a transient one-shot request pulse could be dropped if the top level was still busy with a different group — fixed with a real, sticky per-group pending_req_r latch). Wired into n16_system_ddr3_top.v as a drop-in replacement.
• Real re-verification with the new arbiter: functional xsim (tb_n16_system_ddr3.v) still 32/32 PASS, 0 errors. Real, full P&R re-run: utilization essentially unchanged (128 DSP48E1/53.33%). Real timing: WNS improved from −0.913ns to −0.646ns, TNS from −690.085ns to −97.541ns, failing endpoints from 3021 to 771 — a real, substantial, measured improvement, confirming the arbiter was correctly root-caused and fixed (the bottleneck demonstrably moved elsewhere). Timing is still not fully met.

Real, honest current bottleneck (traced via the actual new worst violated path, not guessed): the new critical path is inside neural_processor_packed.v's own DSP48E1 MAC datapath (GEN_MAC_ PACKED→prodb1_reg), logic-dominated (79%), not route — a pre-existing module, unchanged since N=2, where it closed with an already razor-thin real margin (WNS=+0.099962ns, EXP-0088). Real, coherent interpretation: N=16's real overall die utilization (31% LUT) increases general placement congestion enough, on its own, to erode that already-thin margin — a different, more diffuse problem than the arbiter's own single structural bottleneck, without an equally obvious single-point fix.

Current real status: N=16 is functionally correct (EXP-0092, re-confirmed EXP-0094) with a real, substantially improved but still NOT fully closed timing result (EXP-0094) — honestly not yet ready for real hardware at the target clock. N=2 (EXP-0088) remains the real, trustworthy, deployable signoff.

• Real P&R strategy tuning (EXP-0094, continued): same RTL, opt_design -directive Explore + place_design -directive ExtraNetDelay_high + a new phys_opt_design -directive AggressiveExplore step + route_design -directive AggressiveExplore — zero RTL risk. Real result: WNS improved further to −0.338ns, TNS to −5.299ns, failing endpoints to 60 — another real, substantial, measured improvement (recovered roughly half of the remaining gap). Still not fully met. The new worst path is the SAME structural class (a different PE instance, confirming a recurring per-PE issue) but now measured at 84% logic delay (up from 79%) — real evidence the recoverable route-delay slack is largely exhausted; what remains is dominated by the FPGA's own intrinsic DSP48E1→CARRY4 interconnect delay, unlikely to shrink further via more P&R strategy tuning alone.

Real, cumulative progress: two safe, real improvements (the hierarchical arbiter + P&R directive tuning), neither touching neural_processor_packed.v, together close ~89% of the original TNS gap (−690ns → −5.3ns) and ~63% of the original WNS gap (−0.913ns → −0.338ns). Deliberate stop here: the remaining gap needs the shared, load-bearing neural_processor_packed.v MAC datapath itself touched to close via RTL — a meaningfully larger, more careful step (must be verified against both N=2's own signoff and N=16) than anything else this session, better started with explicit direction than pushed further autonomously.

Not yet done, real and disclosed, real options for closing the remaining N=16 gap: (1) real, careful pipelining inside neural_processor_packed.v's own MAC datapath at the specific GEN_MAC_PACKED/prodb1_reg boundary (fork-before-promote, N=2's own signoff must stay protected, verify both). (2) a real, measured lower target clock for the N=16 variant specifically (unquantified throughput trade-off against N=2). See EXP-0094's own next_action.

Real intermediate-N timing curve (EXP-0095) — the user's own explicit request ("cerchiamo dove fallisce"): N_GROUPS is a real, already-existing top-level parameter, so synth_design -generic N_GROUPS=<n> against the exact same RTL/arbiter (EXP-0094's own hierarchical arbiter already wired in) gave a real, measured curve without any new top-level files:

N	N_GROUPS	DSP48E1	LUT%	real WNS	real failing endpoints
2 (flat, EXP-0088)	—	16	—	+0.099962ns	0 (closed)
4 (systolic)	1	32 (13.3%)	13.87%	−0.005ns	2
8 (systolic)	2	64 (26.7%)	19.77%	0.000ns	0 (REALLY CLOSED)
16 (systolic, EXP-0094)	4	128 (53.3%)	31.39%	−0.338ns	60

N=8 is a real, new, closed P&R signoff — a real post-route report_timing_summary result, 0 failing endpoints, same real part and clock domain, not a projection. N=4 is a hair's breadth away (N=8's own better WNS despite 2× the logic is real run-to-run placer/router variance, not a contradiction). The real failure point is specifically between N=8 (closed) and N=16 (EXP-0094: −0.338ns) — N=12 (N_GROUPS=3) has not yet been measured and would narrow this further. Real bug found and fixed first: neural_director_grouped.v's own bare $clog2(N_GROUPS) was invalid ([-1:0]) for the never-before-tested N_GROUPS=1 case — fixed with the same real SELW-style guard pattern sdram_arbiter_n.v already established.

N=8 promoted to the real, definitive deployment target (EXP-0096) — the user's own explicit decision ("creiamo una versione funzionante completamente per N=8, la pushiamo come versione definitiva"). New hardware/v3/rtl/n8_system_ddr3_top.v — a real, permanent, named top-level (not a build-time override), byte-for-byte the same RTL as n16_system_ddr3_top.v with N_GROUPS defaulting to 2. Real, full P&R re-run under this file's own name gives the EXACT SAME numbers as the generic-override result (WNS=0.000ns, WHS=+0.017ns, 0 failing endpoints, 64 DSP48E1/26.7%, 12535 LUTs/19.77%) — confirms the file split is correct. New tb_n8_system_ddr3.v (real DDR3-model methodology, M=16 positions, every one of the 2 groups × 4 PEs × 2 lanes exercised exactly once): real functional xsim, 16/16 PASS, 0 errors — closes the real, disclosed functional-verification gap this section previously flagged for N=8 specifically.

Current real status (physical board): N=8 (n8_system_ddr3_top.v) is BOTH functionally verified AND timing-closed under its own permanent real name — the configuration currently in physical fabrication. N=2 (n2_system_ddr3_top.v, EXP-0088) remains documented as a real, valid, simpler fallback. Real, disclosed caveat: N=8's original WNS=0.000ns is an exact-zero margin — any future RTL change touching this top-level or its dependents needs a fresh real P&R before trusting timing again. This exact-zero margin is what motivated EXP-0097/0098 below.

5.6.1 [DONE, EXP-0097/0098] The whole family closes: a real MAC-pipeline fix, branch n16-timing-closure

Context: N=16's own real, traced remaining bottleneck (after the hierarchical arbiter + P&R directive tuning already closed most of the gap, WNS −0.913ns → −0.338ns, EXP-0094) was inside neural_processor_ packed.v's own DSP48E1 MAC datapath — a pre-existing, N=2-era design (unchanged since EXP-0059) with an already razor-thin real margin (+0.099962ns) that N=16's own higher real die congestion eroded past zero, and that N=8's own real signoff (§5.6, EXP-0095/0096) closed at an exact zero with no slack to spare.

Real root cause (traced to the exact register-to-register hop, not guessed): the critical path ran from a DSP48E1's own (Vivado-auto- retimed) product register, straight through the real carry-heavy INT8-unpack logic (pb_comb's own shift + conditional +1 carry-propagate add, CARRY4-dominated), into proda1/prodb1 in a SINGLE real cycle.

Real fix: split the original single pipeline stage into two real stages — Stage 1a registers the RAW DSP48E1 product with zero logic in between (a real, explicit register boundary immediately after the multiply); Stage 1b does the carry-heavy unpack FROM the already- registered value (unchanged real math, now one real cycle later). Real, deliberate consequence: end-to-end per-tile latency grows by exactly ONE real clock cycle; throughput is unaffected (real valid/ready handshaking throughout, no fixed-latency assumption anywhere downstream).

Real bug found and fixed in the testbench first (not an RTL bug): tb_neural_processor_packed.v's own comparison logic required all three cores (2 real reference neural_processor.v instances + the DUT) to assert result_valid simultaneously — correct only when all three share the exact same real pipeline depth, no longer true once the DUT is deliberately one real cycle deeper. Fixed by latching each core's own result independently the cycle its own result_valid first pulses, then comparing the three LATCHED values.

Real, consolidated result — EVERY real top-level this project has, same shared pipelined core, real functional xsim + real, full P&R:

Target	Real WNS	Real DSP48E1	Real functional test
N=2 (n2_system_ddr3_top.v)	+0.389ns (up from +0.099962ns)	16 (6.67%)	8/8 PASS
N=8 (n8_system_ddr3_top.v)	+0.108ns (up from 0.000ns exact)	64 (26.7%)	16/16 PASS
N=16 (n16_system_ddr3_top.v)	+0.269ns (up from −0.338ns FAILED)	128 (53.3%)	32/32 PASS

A real margin-hunt attempt for even more N=16 headroom (alternate P&R directives) gave a WORSE real result (+0.168ns vs. the original +0.269ns) — confirms real P&R run-to-run variance, not a further systematic gain available without touching more RTL (the new N=16 worst path, after this fix, moved to neural_director_grouped.v's own queue update logic, still real route-dominated — not pursued further, current margin already comfortable).

Decision: this fix is a pure, unconditional real improvement across the whole real family — no regression anywhere, real margin improves at every N, N=16 goes from real FAILING to real CLOSED. It is not yet applied to the physically fabricated board (which uses the original, unmodified N=8 core) — adopting it (and/or moving to N=16) for a future board revision is a real, open hardware decision, not an RTL blocker. A real Vivado incremental-synthesis quirk was also found and fixed along the way (a parameter binding silently carried forward from an earlier -generic run despite reset_run — see CLAUDE.md).

The problem it targets: plain N=16 independent cores (§5.5's own "documentary, expected to break" framing) means 16 independent DDR3 requesters contending for one arbitrated channel — real congestion that neither §5.1 (packing) nor §5.4 (32-bit widening) alone removes, since both attack bytes-per-MAC or raw bandwidth, not the number of independent consumers.

The idea: instead of 16 flat, independent packed_slot.v instances, group them into 4 systolic chains of 4 PEs each. Within a chain: the weight tile stays resident (loaded once, same "weight-stationary" pattern layer_prefetch_ctrl.v/layer_weight_buffer.v already implement for the existing A/B lane reuse — this is a direct extension to 4 positions instead of 2, not a new mechanism), and activation data streams through the chain position by position, fetched from DDR3 once per chain rather than once per PE. Between the 4 chains (groups), full task-level parallelism is preserved — each group can run an independent job, same as today's model.

Why weight-stationary specifically (not activation-broadcast): chosen because it generalizes to any layer type (FC, conv-via-im2col, attention — anything reducible to "same weight matrix, many activation vectors") without assuming a specific model's channel count or convolution overlap pattern — important since this is a general-purpose accelerator, not built for one fixed network.

Real, quantifiable rationale (order-of-magnitude, not yet measured — flagged explicitly as a projection):

• DSP48E1: 16 cores × 8 DSP/core = 128/240 (53%) — real, fits with margin.
• DDR3 requesters: drops from 16 (flat) to 4 (one per chain) — a real 4× reduction in the number of independent contenders for the arbitrated channel, on top of (not instead of) §5.1's packing and §5.4's widening.
• Combined with §5.4's 32-bit widening (2× physical ceiling), the available-bandwidth-to-demand ratio improves by roughly 8× versus the naive flat-N=16 baseline — a real, meaningful, but NOT a full solve by itself; still needs real measurement once anything is built.

Real open question, RESOLVED (EXP-0089): intra-chain dataflow — the user was asked directly (AskUserQuestion, concrete side-by-side preview of both real topologies) rather than guessed, given how different their real risk/complexity profiles are. Chosen: shared-weight broadcast (one real weight fetch per group of 4 PEs, no inter-PE result/data propagation, each PE computes its own independent activation positions in parallel) — not a literal PE-to-PE systolic shift register. This achieves the real, quantified rationale above (4× reduction in redundant weight-fetch DDR3 traffic per group) without the real complexity/risk of genuine pipeline fill/drain at chain boundaries, which the literal-systolic alternative would have required. Real result-writeback stays exactly as §5.3 already built it — packed_pe.v reuses result_writeback.v completely unmodified, one real DDR3 write per PE, not a new "4 chain-output events" model (that framing, from the original brainstorm, doesn't apply to the broadcast design actually built). Arbitration: confirmed real — sdram_arbiter_n.v's own NUM_REQ parameter generalized to a 5-way arbiter (1 group weight-fetch

• 4 PE activation/writeback) in tb_systolic_group.v with zero changes to that module.

Decision: first real step DONE (EXP-0089) — build and verify the isolated group mechanism before any Director/SPI/top-level integration, per this project's own "one variable at a time" discipline. Full N=16 integration (Director-level group job dispatch, SPI protocol extension, real top-level, real P&R) is real, disclosed, deliberately deferred work, not yet done — see EXP-0089's own next_action in hardware/v2/logs/experiments.log.

5.6.1 [EXPLORATORY] Opportunistic BRAM cache for activation tiles

Smaller and more incremental than §5.6's systolic restructuring — doesn't require knowing anything about the target network's structure in advance. Artix-7 100T's Block RAM is real and currently 0% utilized (§3, every real P&R signoff to date) — real, free, unused capacity.

The idea: a small direct-mapped or low-associativity cache, in BRAM, remembering the last few activation tiles fetched from DDR3 (address + data). Before act_tile_fetch.v (or ddr_prefetch_mgr.v, EXP-0083) issues a real DDR3 request, check the cache first — on a hit (e.g. two jobs, on the same or different slots, requesting overlapping/adjacent tiles, common in convolution with sliding-window overlap), skip the DDR3 round-trip entirely.

Why it's attractive: catches real reuse the design doesn't have to predict or assume in advance — unlike §5.6's systolic chains (which commit to a specific reuse pattern, weight-stationary), a cache opportunistically exploits WHATEVER locality the real workload happens to have, including patterns nobody designed for. Composable with everything else already built or decided (§5.1 packing, §5.4 widening, §5.6 systolic groups if that direction is taken) — it's a cache in front of the existing fetch path, not a replacement for it.

Real open questions: cache size vs. real hit rate is workload-dependent and NOT measured — would need a real trace-driven estimate (or a real simulation with representative test data) before sizing it, not guessed. Coherency is simple here (activation data in DDR3 is written once by the host before a job runs and never modified during compute, per the current protocol) — no cache-invalidation problem to solve, a real simplification versus a general-purpose cache design.

5.6.2 [EXPLORATORY] Host-side (ESP32) job-queue optimization — a "software DDRManager"

A different kind of lever than anything else in this section: instead of adding hardware intelligence inside the FPGA, exploit the fact that the ESP32 already has full visibility of the whole job queue before submitting it — neural_director_packed.v's own QUEUE_DEPTH=8 hardware queue only sees jobs one at a time as they're written over SPI; the ESP32 firmware, upstream of that, could see and reorder ALL pending jobs at once.

The idea: the ESP32's own job-submission firmware groups/reorders jobs before writing them over the management SPI bus (§2.2 of docs/PHYSICAL_REALIZATION.md), so that jobs whose DDR3 addresses are close together (same or adjacent rows) are submitted close together in time — directly reducing the real row-switch cost (§3.3) that dominates per-tile latency, WITHOUT any new RTL at all. A real, software-only "DDRManager" living in ESP32 firmware, upstream of and complementary to ddr_prefetch_mgr.v (EXP-0083, which only looks ahead within one already- submitted job).

Why this is worth capturing seriously, not just as a curiosity: it's the cheapest possible lever in this whole list — zero RTL, zero real P&R risk, zero timing-margin cost (the project's real margin is thin, §2, and every RTL addition risks it; this doesn't touch RTL at all) — and it's far faster to iterate on than Verilog (the user's own established preference for where complexity is easiest to absorb). It doesn't compete with any other idea in this section — it can be built independently, at any time, by whoever writes the ESP32-side firmware, and composes with all of them.

Real open question: requires the host firmware to know DDR3 addresses well enough to group by row locality (ROW_BITS/COL_BITS/BANK_BITS convention, §2 of docs/PHYSICAL_REALIZATION.md) — a real firmware-side design task, not yet scoped, and out of this repository's own RTL scope (ESP32 firmware isn't part of hardware/v3/).

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6. Summary table: what's real vs. what's a calculation

Claim	Status
WNS/WHS/LUT/DSP numbers throughout	Measured (real Vivado P&R reports, current: EXP-0086)
DDR3 back-to-back burst throughput (1.24 GB/s)	Measured (real ddr3_model.sv JEDEC trace) — physical channel limit, unchanged by §5.1's packing fix
Real Activate→Read latency (16.125 ns)	Measured (same trace)
Per-core compute throughput (2.48 GMAC/s)	Calculated from measured Fmax (155.039MHz) + known, fixed DSP-packing factor
Bandwidth needed per core, pre-packing (4.96 GB/s)	Calculated, historical (EXP-0079 layout)
Bandwidth needed per core, post-packing (2.48 GB/s)	Calculated from the real, as-built EXP-0081/0082 memory layout — current
"~25% of peak sustainable" (pre-packing) / "~50%" (post-packing, current)	Calculated ratios; post-packing figure re-verified against real P&R (EXP-0082) and real simulation (§5.1)
I/O bank/DQS pin counts for XC7A100T-CSG324 (banks 14/15/16/34/35)	Measured — queried directly from the real Vivado part database for this exact part/package, used in §5.4's dual-channel-vs-widening analysis
Row-switch penalty as a fraction of real workloads	Not measured — depends on host-chosen memory layout, flagged as an open question, not asserted
DDRManager phase-1 real stall-reduction benefit	Measured — real xsim A/B on tb_n2_system_ddr3.v, real DDR3 backend, before vs after ddr_prefetch_mgr.v. 2.86% at the OLD 16-bit channel (EXP-0083); ~0% (0.0064% regression) re-measured at the closed 32-bit channel (EXP-0087) — the benefit does not survive the wider channel, real and honestly reported, not oversold either direction.
Full multi-slot DDRManager's real benefit	Not measured, not built, and now real-disconfirmed as a priority — phase 1's own re-measurement against the closed 32-bit channel (EXP-0087) found ~0% benefit; §5.2 recommends against building the larger version unless N=4/8/16 scaling reintroduces DDR3-latency-bound behavior
32-bit widening's real post-change bandwidth/timing numbers	Measured — real, closed P&R signoff (EXP-0086): WNS=+0.096ns, WHS=+0.036ns, 0 failing endpoints. The ~2.48 GB/s figure is now a real, closed-timing result, not a projection.