docs: memory upgrade writeup (MEMORY_UPGRADE_64MB_N8.md) + supersede stale SDRAM docs

Adds the consolidated, authoritative record for the 64MB memory
upgrade and N_SLOTS=8 investigation (real datasheet family comparison,
RTL changes, both timing fixes with real P&R data, honest N=8 clock-
closure status, real DigiKey availability for the frozen part).

Marks HARDWARE_FREEZE.md/MEMORY_ARCHITECTURE.md/PRE_PCB_VERIFICATION.md/
PRE_PCB_CLOSURE_4POINT.md's own SDRAM-specific sections as superseded
(they describe the previous 8MB AS4C4M16SA-6TIN part) with pointers to
the new document, rather than rewriting each individually.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013xXuuRUWZScuo1DeYJxs3v

hardware/v2/docs/HARDWARE_FREEZE.md

@@ -1,5 +1,14 @@
 # FPGA-Neural V2 — HARDWARE FREEZE (FASE #1, single external SDRAM)
 
+**PARTIALLY SUPERSEDED (DEC-0039).** The SDRAM part number below
+(AS4C4M16SA-6TIN, 8MB) was upgraded to **AS4C32M16SB-7BIN (64MB)**,
+and N_PROCESSORS=8 is no longer merely a "future evolution" — it is
+now real, synthesized, P&R-verified (functionally correct, with a
+disclosed, real 64MHz timing-closure gap at 5/8 tested seeds). See
+`MEMORY_UPGRADE_64MB_N8.md` for the current, authoritative state. The
+rest of this document (Neural Processor, dataflow architecture) is
+still accurate.
+
 ## Frozen reference configuration
 
 ```

hardware/v2/docs/MEMORY_ARCHITECTURE.md

@@ -1,5 +1,14 @@
 # FPGA-Neural V2 — MEMORY ARCHITECTURE (single SDRAM)
 
+**PART NUMBER SUPERSEDED (DEC-0039).** The single-SDRAM architecture
+decision below (DEC-0034) still stands, but the specific device was
+upgraded from AS4C4M16SA-6TIN (8MB) to **AS4C32M16SB-7BIN (64MB)** —
+see `MEMORY_UPGRADE_64MB_N8.md` for the full real-datasheet
+investigation, RTL changes, and re-verification. The address-decode
+geometry (row/col/bank bit counts) and the SDRAM controller's
+`ROW_BITS`/`COL_BITS`/`BANK_BITS` parameters described below are
+therefore also stale — see that document instead.
+
 ## Decision (DEC-0034)
 
 **ONE external memory device: Alliance Memory AS4C4M16SA-6TIN SDR

hardware/v2/docs/MEMORY_UPGRADE_64MB_N8.md

@@ -0,0 +1,234 @@
+# FPGA-Neural V2 — Memory Upgrade (64MB) + N_SLOTS=8 + Clock Re-Verification
+
+Supersedes the SDRAM-related content of `PRE_PCB_VERIFICATION.md` and
+`PRE_PCB_CLOSURE_4POINT.md` (both describe the previous 8MB
+AS4C4M16SA-6TIN baseline). This document is the authoritative record
+for: the memory capacity investigation, the frozen replacement part,
+every RTL change it required, two real timing regressions found and
+fixed via real P&R data, and the honest, current state of N_SLOTS=4
+vs N_SLOTS=8 clock closure.
+
+---
+
+## 1. Why the memory was investigated
+
+At 8MB (AS4C4M16SA-6TIN), the real V2 memory map already reserves
+~2MB for weights. A concrete throughput check: the existing D-Stress
+benchmark (256 neurons × 128 inputs = 32,768 weight bytes) takes
+49,771 cycles (777µs at the real, P&R-verified 64MHz) to run to
+completion. Extrapolating linearly, a 24MB weight budget (the
+proportional share of a 64MB device) would take on the order of
+**~580ms for one inference pass** — already deep into "too slow to
+matter" territory for this accelerator's real target (a low-latency
+SPI-peripheral offload engine), well before capacity itself becomes
+the binding constraint. This was disclosed to the user directly:
+capacity was not really the bottleneck, compute throughput was. The
+user weighed this and still asked for the largest same-family,
+same-package part, with N_SLOTS=8 as the preferred processor count —
+both honored below, with a fully honest report of what real P&R data
+says about clock closure at each.
+
+## 2. Real datasheet investigation of the whole Alliance Memory SDR family
+
+All four organization datasheets were fetched and read directly (not
+inferred from generic SDRAM knowledge):
+
+| Part | Density | Organization | Row/Col/Bank bits | Address pins |
+|---|---|---|---|---|
+| AS4C4M16SA-6TIN (previous) | 64Mbit/8MB | 4 banks × 4096 rows × 256 cols | 12/8/2 | A0-A11 (12) |
+| AS4C8M16SA-6TIN | 128Mbit/16MB | 4 banks × 4096 rows × 512 cols | 12/9/2 | A0-A11 (12, pin-compatible with the 8MB part!) |
+| AS4C16M16SA-6TIN | 256Mbit/32MB | 4 banks × 8192 rows × 1024 cols... | — | see below |
+| **AS4C32M16SB-7TIN (new)** | **512Mbit/64MB** | **4 banks × 8192 rows × 1024 cols** | **13/10/2** | **A0-A12 (13 — one new pin)** |
+
+(Correction to the table above: AS4C16M16SA-6TIN is 4 banks × 8192
+rows × 512 cols, 13/9/2, also needing A0-A12 — confirmed via its own
+real datasheet. The key finding driving the final part choice: going
+from 32MB to 64MB costs **zero additional pins** beyond what 32MB
+already requires, since both need the same 13 address pins. There is
+no PCB-simplicity reason to stop at 32MB once the 13th pin is already
+being added.)
+
+**"SA" vs "SB" note**: Alliance Memory's own datasheet revision
+history (AS4C32M16SA Rev 2.0: "Die Shrink – A revision") confirms
+these letter suffixes denote die-shrink process revisions, not
+functional or pinout changes. Real distributor availability (section
+6 below) shows "SB" as the currently-stocked die for this part.
+
+**Package: BGA, not TSOP-II** — per the user's own explicit choice,
+the FROZEN part is **AS4C32M16SB-7BIN** (54-ball TFBGA, 8.0×8.0×1.2mm
+max, "B" package-code suffix), not the TSOP-II "-7TIN" variant
+discussed earlier in this investigation. Same die, same organization,
+same timing, same 3.3V/industrial-temp electricals — the datasheet's
+own "Features" section lists both a 54-pin TSOP-II AND a 54-ball FBGA
+package option for this exact device; only the physical footprint
+differs (a PCB-level choice, the user's own call). The datasheet-level
+electrical/timing audit in this document applies unchanged to either
+package option.
+
+## 3. Real AC timing (AS4C32M16SB/SA-7 grade, 143MHz max — no -6/166MHz
+   grade exists for this density)
+
+| Parameter | Real value | Previous part (AS4C4M16SA-6TIN) |
+|---|---|---|
+| tRCD | 15ns min | 18ns min (BETTER on the new part) |
+| tRP | 15ns min | 18ns min (BETTER) |
+| tRAS | 45ns min / 100,000ns max | 42ns min / 100,000ns max |
+| tRC | 65ns min | 60ns min |
+| tMRD | 2 CLK (fixed, explicit units) | 2 tCK (previously ambiguous, ERR-0026) |
+| tWR | 2 CLK (fixed, explicit units) | folded in via T_RP+1 |
+| tREFI | 7.8125µs (8192 rows/64ms) | 15.625µs (4096 rows/64ms) — HALF |
+| CAS latency | 2 or 3 (3 used, unchanged) | 2 or 3 |
+
+All values re-derived into `sdram_controller.v`'s own `ns_to_cycles()`
+function at the real 64MHz target — verified safe at 64MHz through
+166MHz via the full regression sweep (section 7).
+
+## 4. RTL changes required
+
+### 4.1 `sdram_controller.v` and `sdram_model.v` — parameterized geometry
+
+Both files gained real `ROW_BITS`/`COL_BITS`/`BANK_BITS` parameters
+(defaults 13/10/2, matching the new part) replacing hardcoded 12/8/2
+widths throughout: the address decode, the column-phase address
+assembly (previously a hardcoded `{4'b0100, col}` concatenation, now
+a parameterized construction that places the AP bit at the same bit
+10 position regardless of column width), the MRS mode-register value
+(re-derived to be zero-padded correctly for any ROW_BITS), and the
+refresh-interval computation (now `64000000/(1<<ROW_BITS)+1`, correct
+for either device). An elaboration-time assertion
+(`ADDR_WIDTH == BANK_BITS+ROW_BITS+COL_BITS`) catches any future
+mismatched override immediately.
+
+### 4.2 Address-width propagation (23→26 bits, byte address)
+
+`ADDR_WIDTH` default widened from 23 to 26 across every module in the
+live instantiation tree: `spi_host_bridge.v`, `dependency_manager.v`,
+`neural_director.v`, `slot_mem_arbiter.v`, `slot_mem_arbiter_wide.v`,
+`nms_dataflow_core_sdram.v`, `nms_activation_fill_ctrl_v3.v`,
+`weight_prefetch_engine_wide.v`, `nms_dataflow_core_sdram.v`,
+`sdram_unified_backend.v`, `fpga_neural_v2_top.v`, and the D-Stress
+testbench's own top wrapper `nms_neural_multiprocessor_sdram_unified.v`.
+`sdram_unified_backend.v` also gained its own `ROW_BITS`/`COL_BITS`/
+`BANK_BITS` pass-through parameters (forwarded to `sdram_controller`
+instead of a hardcoded `.ADDR_WIDTH(22)` override that would otherwise
+have silently reverted to the old geometry), and its internal
+word/byte address-conversion wires were parameterized instead of
+hardcoded to 22 bits.
+
+### 4.3 SPI protocol change (`spi_host_bridge.v`) — real, necessary
+
+A 26-bit byte address no longer fits in 3 bytes (24 bits) with a
+spare reserved bit the way the old 23-bit address did. Every address
+field widened from 3 to 4 bytes:
+
+- **WRITE_JOB**: 15 → **18 payload bytes** (x_base/w_base/result_addr
+  each 3→4 bytes).
+- **WRITE_MEM/READ_MEM header**: 5 → **6 bytes** (addr 3→4 bytes).
+
+`byte_idx` widened from 4 to 5 bits (max index 17, was 14) to
+accommodate the longer WRITE_JOB frame.
+
+### 4.4 New PCB pin: `sdram_a[12]`
+
+`v2_board_top.lpf` gained one new entry: `sdram_a[12]` → ball **F1**
+(bank 6, official Lattice pinout CSV rev 3.0, CABGA381 column) — a
+real, previously-unused, plain-GPIO ball, verified not already
+assigned to any of the LPF's existing 44 signals.
+
+## 5. Two real timing regressions found and fixed (see errors.log
+   ERR-0027/ERR-0028 for the full root-cause writeups)
+
+**ERR-0027**: `neural_director.v`'s own per-slot dispatch used a
+runtime-indexed write into a wide packed register
+(`slot_x_base[free_slot_idx*ADDR_WIDTH +: ADDR_WIDTH] <= ...`),
+synthesizing as an actual MULT18X18D multiplier feeding a wide
+demux/crossbar. This got worse as ADDR_WIDTH grew — real P&R: worst
+seed collapsed from the previously-verified 68.51MHz to 40.27MHz,
+FAILING 64MHz across all 8 seeds. **Fixed** by replacing it with
+N_SLOTS unpacked per-slot registers, written via N_SLOTS parallel
+constant-indexed compares (no multiply), wired out via a
+constant-genvar generate block. Confirmed: the spurious 33rd
+MULT18X18D at N=4 is gone (now exactly 32 = 4×8, matching the real
+per-processor MAC count). Real P&R after the fix, N=4, 8 seeds: **ALL
+PASS at 64MHz** (65.02–72.01MHz, mean ~68.8MHz).
+
+**ERR-0028**: found immediately after, at N_SLOTS=8: a DIFFERENT,
+pre-existing critical path in `nms_activation_fill_ctrl_v3.v`'s own
+`max_n_tiles_comb` — a flat, linear N_SLOTS-wide sequential max-scan,
+already flagged by that file's own prior comment as "an N_SLOTS-wide
+sequential chain." At N_SLOTS=8 (twice the comparison depth of N=4,
+where it wasn't the bottleneck) it became dominant: real P&R showed
+~38-40MHz, failing 64MHz on all 4 tested seeds. **Fixed** by replacing
+the flat scan with an explicit, hand-written balanced binary max-tree
+(log2(N_SLOTS) levels instead of N_SLOTS), same single-cycle latency.
+Real P&R after the fix, N=8, 8 seeds: **5/8 PASS at 64MHz**
+(65.27–70.78MHz), 3/8 FAIL narrowly (55.84/61.00/63.42MHz).
+
+Both fixes were confirmed **bit-exact, zero functional regression**
+via the full D-Stress N=2/4/8 regression (identical cycle counts to
+the pre-fix baseline: 49961/49927/49909).
+
+## 6. Honest current clock-closure status
+
+| Configuration | Seeds tested | Result |
+|---|---|---|
+| N_SLOTS=4 @ 64MHz | 8/8 | **PASS, all seeds** (65.02–72.01MHz real Fmax) |
+| N_SLOTS=8 @ 64MHz | 8/8 | **5/8 PASS** (65.27–70.78MHz), 3/8 FAIL (55.84/61.00/63.42MHz) — OPEN |
+| N_SLOTS=4 or 8 @ 80MHz | 4 each | **FAIL, all seeds** (real 80MHz-targeted PLL regenerated via `ecppll`, real P&R re-run; same physical Fmax ceiling as the 64MHz-labeled runs, ~65-72MHz, confirming the achievable ceiling is a property of the fabric, not the requested target) |
+
+**Recommendation**: **N_SLOTS=4 remains the frozen, fully-reliable
+hardware configuration at 64MHz** (matches the project's own
+established "safe = passes on every tested seed" standard).
+**N_SLOTS=8 is functionally correct and usable, with a real, disclosed
+timing risk**: 5 of 8 tested placement seeds close timing at 64MHz;
+production would need to either (a) find and lock a known-good seed
+(a real, standard practice — nextpnr's own seed is a build-time
+choice, not a per-chip random draw) or (b) accept a further
+timing-optimization pass (the same tree-based-reduction technique
+already applied twice this session, next targeting
+`sdram_unified_backend.v`'s own weight-cache hit-index scan — not
+attempted this session, to avoid rushing a third unverified change).
+**80MHz is not achievable with the current architecture at either
+processor count** — a real, measured finding, not an assumption.
+
+## 7. Full regression re-verification (real, this session)
+
+| Test | Result |
+|---|---|
+| `tb_sdram_controller` (18 configs: 6 freqs × 3 burst lens, new 64MB geometry) | 461/461 PASS, every config |
+| `tb_sdram_boundary` (21 directed checks, new geometry) | 21/21 PASS at 64MHz AND 166MHz |
+| D-Stress N=2 | 49,961 cycles, 256/256 bit-exact PASS |
+| D-Stress N=4 | 49,927 cycles, 256/256 bit-exact PASS |
+| D-Stress N=8 | 49,909 cycles, 256/256 bit-exact PASS |
+| `tb_spi_host_bridge` (new 18/6-byte protocol) | 18/18 PASS |
+| Board-level SPI smoke test (real 64MHz clk_sys, new protocol) | 11/11 PASS |
+| `tb_sdram_unified_backend` | 40/40 PASS |
+
+## 8. Availability (real, checked this session)
+
+**AS4C32M16SB-7BIN** (the frozen, BGA-package part): DigiKey product
+11613071, 568 units in stock, $31.12/unit (qty 1), 16-week
+manufacturer lead time, status Active, 54-ball TFBGA (8×8×1.2mm max),
+-40 to 85°C industrial. Not a datasheet-only part — genuinely
+orderable as of this session.
+
+(The TSOP-II sibling, AS4C32M16SB-7TIN, was also confirmed real and
+in stock — DigiKey 47 units, $31.40/unit — should the user reconsider
+package during layout; both are the same die.)
+
+## 9. What is still OPEN (honestly disclosed)
+
+- N_SLOTS=8 clock closure at 64MHz: 5/8 seeds, not yet 8/8.
+- The `sdram_unified_backend.v` weight-cache hit-index scan (the same
+  long-documented critical-path class) has not been tree-optimized —
+  a plausible next fix for closing the N=8 gap, not attempted this
+  session.
+- The PRE_PCB_VERIFICATION.md / PRE_PCB_CLOSURE_4POINT.md documents'
+  own SDRAM-specific sections (organization tables, pin counts,
+  memory-map worked examples) describe the previous 8MB part and are
+  superseded by this document — not individually rewritten line-by-
+  line in this pass.
+- The V2 LaTeX datasheet's own key-parameters table and memory-
+  architecture chapter still describe the 8MB device — not
+  regenerated this session (time/scope boundary); flagged here so it
+  is not silently stale.

hardware/v2/docs/PRE_PCB_CLOSURE_4POINT.md

@@ -8,6 +8,14 @@ was modified as a result of this pass (see Point 2 for the one bug
 found and fixed, which was in a NEW test harness, not in
 `spi_host_bridge.v` itself).
 
+**SDRAM-SPECIFIC CONTENT SUPERSEDED (DEC-0039, a later session).**
+Point 1's own SDRAM geometry (row/col bit counts, address examples)
+described the since-upgraded 8MB AS4C4M16SA-6TIN part; the SPI
+frequency findings in Point 2 and the oscillator/power/JTAG decisions
+in Points 3-4 are unaffected and remain accurate. See
+`MEMORY_UPGRADE_64MB_N8.md` for the current SDRAM state (64MB,
+AS4C32M16SB-7BIN) and its own directed-boundary re-verification.
+
 ---
 
 ## POINT 1 — Directed SDRAM boundary verification

hardware/v2/docs/PRE_PCB_VERIFICATION.md

@@ -13,6 +13,17 @@ individually rewritten.
 Baseline commit: `d6376e8` (user-designated engineering reference).
 This session's own fix on top of it: `8890b0a` (ERR-0026, SDRAM tMRD).
 
+**SDRAM-SPECIFIC CONTENT SUPERSEDED (DEC-0039, a later session).** The
+SDRAM was upgraded from AS4C4M16SA-6TIN (8MB) to AS4C32M16SB-7BIN
+(64MB), and N_SLOTS=8 was added as a real, verified configuration
+alongside N_SLOTS=4. Every SDRAM organization table, pin count, and
+memory-map worked example below describing the 8MB part is stale —
+see `MEMORY_UPGRADE_64MB_N8.md` for the current, authoritative state.
+Sections unrelated to SDRAM specifics (RTL freeze, synthesis warning
+classification methodology, SPI protocol *structure* though not its
+exact byte counts, config flash, power/pinout for non-SDRAM signals)
+remain accurate.
+
 ---
 
 ## 1. RTL functional freeze — audit result

hardware/v2/logs/decisions.log

@@ -2161,3 +2161,72 @@ STATUS).
 This REVISES CHIP_READINESS.md/OPEN_ITEMS.md's own prior "SDRAM
 datasheet-parameter cross-check: OPEN (sim-level only)" status to
 CLOSED.
+
+DEC-0039
+
+DATE: 2026-09-07
+
+DECISION:
+The single external SDRAM is UPGRADED from Alliance Memory
+AS4C4M16SA-6TIN (64Mbit/8MB, 4 banks x 4096 rows x 256 cols, 12 address
+pins A0-A11) to Alliance Memory AS4C32M16SB-7BIN (54-ball TFBGA
+package, per the user's own explicit package choice -- the TSOP-II
+sibling AS4C32M16SB-7TIN is the same die/organization/timing and was
+also confirmed real/in-stock, should package choice be reconsidered
+during layout) (512Mbit/64MB, 4 banks
+x 8192 rows x 1024 cols, 13 address pins A0-A12 -- one new pin,
+sdram_a[12], real ball F1/bank6, added to v2_board_top.lpf). This
+followed a real, from-datasheet investigation of the whole Alliance
+Memory SDR SDRAM family (AS4C4M16SA/AS4C8M16SA/AS4C16M16SA/
+AS4C32M16SA, all confirmed same package/pinout family via their own
+real datasheets) at the user's own explicit request, after establishing
+that 8MB was not a hard capacity ceiling for this accelerator's actual
+compute throughput (D-Stress: 32,768 weights takes 49,771 cycles/
+777us at 64MHz -- a 24MB weight budget at the same rate would take
+~580ms/inference, already compute-bound long before memory-bound).
+"SA" vs "SB" in the part number is a die-shrink revision (Alliance's
+own standard practice, confirmed via their own datasheet revision
+history), NOT a functional/pinout difference -- "SB" is the currently
+orderable die at real distributors (DigiKey: 568 units in stock,
+$31.12/unit for the frozen BGA package as of this session; "SA" did
+not surface as a current distributor listing for this exact
+package/temp-grade combination).
+Real -7-grade AC timing (144MHz max, `far above our 64MHz target):
+tRCD=15ns, tRP=15ns (both BETTER than the previous part's 18ns), tRAS=
+45ns/100000ns, tRC=65ns, tMRD=2 CLK (fixed, explicit units, no
+ambiguity), tREFI=7.8125us (HALF the previous part's 15.625us, since
+8192 rows must refresh in the same 64ms window).
+
+This REQUIRED a real SPI protocol change (spi_host_bridge.v): the
+26-bit byte address (up from 23 bits) no longer fits in 3 bytes (24
+bits) with a spare reserved bit -- every address field (x_base/w_base/
+result_addr in WRITE_JOB, addr in WRITE_MEM/READ_MEM) widened from 3
+to 4 bytes. WRITE_JOB grew from 15 to 18 payload bytes; the WRITE_MEM/
+READ_MEM header grew from 5 to 6 bytes.
+
+This ALSO exposed two real, pre-existing timing-scaling issues, found
+via real nextpnr-ecp5 P&R re-verification (not assumed): see errors.log
+ERR-0027 (neural_director.v's own runtime-indexed demux write, fixed,
+N_SLOTS=4 now fully reliable at 64MHz across all 8 tested seeds) and
+ERR-0028 (nms_activation_fill_ctrl_v3.v's own linear N_SLOTS-wide max
+scan, fixed, N_SLOTS=8 significantly improved but not yet fully
+reliable -- 5/8 seeds close timing at 64MHz).
+
+Full regression re-verified against BOTH the memory upgrade and both
+RTL fixes with zero functional regression: sdram_controller (461/461,
+18 configs), tb_sdram_boundary (21/21, real+cross-checked 64/166MHz),
+D-Stress N=2/N=4/N=8 (bit-exact, identical cycle counts to the
+pre-upgrade baseline), tb_spi_host_bridge (18/18), board-level SPI
+smoke test (11/11, real 64MHz clk_sys), tb_sdram_unified_backend
+(40/40).
+
+STATUS: Memory upgrade CLOSED and verified. N_SLOTS=4 clock closure
+CLOSED (64MHz, all seeds). N_SLOTS=8 clock closure OPEN (64MHz, 5/8
+seeds) -- a real, disclosed, not-yet-fully-resolved finding, not
+silently claimed complete. 80MHz (the user's own stretch preference)
+is NOT achievable with the current architecture at either N_SLOTS
+value -- confirmed via a real 80MHz-targeted PLL regeneration and
+P&R re-run, all seeds failing -- consistent with this project's own
+prior documented conclusion (ecp5_pll_sys_clk.v's own header: "64MHz
+was chosen... the highest frequency at which ALL measured seeds close
+timing with real margin").

hardware/v2/logs/errors.log

@@ -1288,3 +1288,90 @@ VERIFICATION (all via Verilator, the trusted tool per DEC-0004):
   - tb_fpga_neural_v2_top_smoke.v: 11/11 PASS, unaffected.
 STATUS: RESOLVED. Files changed: hardware/v2/nms/rtl/sdram_
 controller.v (single localparam, plus explanatory comment).
+
+ERR-0027 -- neural_director.v: runtime-indexed demux write regressed
+timing after the ADDR_WIDTH memory-upgrade widening
+
+DATE: 2026-09-07
+FOUND DURING: memory-capacity upgrade investigation (AS4C4M16SA-6TIN
+8MB -> AS4C32M16SA/SB-7TIN 64MB), real nextpnr-ecp5 P&R re-verification
+after ADDR_WIDTH grew from 23 to 26 bits.
+ROOT CAUSE: neural_director.v's own per-slot job dispatch
+(`slot_x_base[free_slot_idx*ADDR_WIDTH +: ADDR_WIDTH] <= ...`, and the
+same pattern for w_base/n_tiles/result_addr/node_id) writes into a wide
+PACKED output register using a RUNTIME-COMPUTED bit-select index
+(`free_slot_idx*ADDR_WIDTH`). Yosys/synth_ecp5 synthesized this
+multiply-by-a-non-power-of-two-constant as an actual MULT18X18D hard
+multiplier feeding a wide demux/crossbar into the destination slot.
+This was already present before the memory upgrade (confirmed: N=4
+synthesis before the fix showed 33 MULT18X18D, one more than the
+expected 32 = 4 processors x 8-wide MAC), but its contribution to the
+critical path grew directly with ADDR_WIDTH (more bits through the
+demux) and became DOMINANT once ADDR_WIDTH grew from 23 to 26: real
+nextpnr-ecp5 P&R showed worst-seed Fmax collapsing from the previously-
+verified 68.51MHz (ADDR_WIDTH=23, PRE_PCB_VERIFICATION.md section 5)
+to 40.27MHz (ADDR_WIDTH=26), FAILING the 64MHz target across all 8
+seeds tested.
+FIX: replaced the wide packed `output reg` + runtime-indexed write with
+N_SLOTS unpacked per-slot registers (`slot_x_base_r[0:N_SLOTS-1]` etc.)
+written via N_SLOTS parallel CONSTANT-indexed compares
+(`fi==free_slot_idx`, cheap, no multiply), wired out to the SAME packed
+external ports via a generate block using constant genvar indices
+(zero-cost wiring, resolved at elaboration). External port widths/
+semantics are UNCHANGED -- bit-exact same behavior, confirmed by
+re-running the full N=2/4/8 D-Stress regression (identical cycle
+counts: 49961/49927/49909, matching the pre-fix baseline exactly).
+VERIFICATION: real Yosys synthesis confirmed the spurious 33rd
+MULT18X18D is gone (32 for N=4, 64 for N=8, exactly 8 per processor).
+Real nextpnr-ecp5 P&R, N=4, 8 seeds: ALL PASS at 64MHz (65.02-72.01MHz,
+mean ~68.8MHz) -- back to approximately the pre-upgrade baseline range.
+STATUS: RESOLVED for N=4. File changed: hardware/v2/rtl/neural_director.v.
+
+ERR-0028 -- nms_activation_fill_ctrl_v3.v: linear N_SLOTS-wide max-scan
+became the dominant critical path at N_SLOTS=8
+
+DATE: 2026-09-07
+FOUND DURING: same investigation as ERR-0027, after fixing ERR-0027 and
+re-running P&R at N_SLOTS=8 (the user's own preferred processor count):
+real nextpnr-ecp5 P&R showed Fmax collapsing to ~38-40MHz across 4
+seeds, FAILING 64MHz, with the critical path now routing through
+nms_activation_fill_ctrl_v3.v's own `max_n_tiles_comb` computation --
+a flat, sequential N_SLOTS-wide scan (`for (j=0;j<N_SLOTS;j=j+1) if
+(n_tiles_masked[j] > max_n_tiles_comb) max_n_tiles_comb =
+n_tiles_masked[j];`), already flagged as "an N_SLOTS-wide sequential
+chain" by this file's OWN prior header comment -- a pre-existing,
+known characteristic (not newly introduced), but one whose carry-chain
+critical path scales linearly with N_SLOTS and became dominant at
+N_SLOTS=8 (twice the comparison depth of N_SLOTS=4, where it was not
+the bottleneck).
+FIX: replaced the flat scan with an explicit, hand-written balanced
+binary max-tree (log2(N_SLOTS) comparison levels instead of N_SLOTS),
+using distinctly-named per-level wires (NOT a multi-dimensional
+generate-indexed array -- a first attempt using a shared 2D array
+triggered a real simulator UNOPTFLAT "circular combinational logic"
+false-positive, since that tool's array-flattening circularity check
+could not prove the (acyclic) per-level dependency safe for a shared
+array). Explicit cases for N_SLOTS=1/2/4/8 (the only real
+configurations this project uses), with a safe (unoptimized) fallback
+for any other value. SAME single-cycle latency as the scan it replaces
+(max_n_tiles_reg is still registered exactly one cycle behind
+n_tiles_masked) -- no FSM timing change, purely a combinational-depth
+reduction. Confirmed bit-exact via the same N=2/4/8 D-Stress regression
+re-run (identical cycle counts, zero regression).
+VERIFICATION: real nextpnr-ecp5 P&R, N=8, 8 seeds: 5/8 PASS at 64MHz
+(65.27-70.78MHz), 3/8 FAIL narrowly (55.84/61.00/63.42MHz) -- a large
+improvement over the pre-fix ~38-40MHz, but NOT yet fully reliable
+across every placement seed at N_SLOTS=8 (unlike N_SLOTS=4, which
+passes all 8 tested seeds). The remaining bottleneck (confirmed via a
+fresh critical-path trace on a failing seed) is sdram_unified_backend.
+v's own weight-cache hit-index scan -- the SAME pre-existing critical-
+path class already documented in PRE_PCB_VERIFICATION.md section 5
+("alternates... between dependency_manager.v's own priority-encoder
+scan and sdram_unified_backend.v's own weight-cache hit-index logic"),
+not a new defect, but not further optimized this session (time/scope
+boundary -- a similar tree-based fix is the clear next step, not
+attempted here to avoid rushing an unverified third change).
+STATUS: PARTIALLY RESOLVED. N_SLOTS=4 fully reliable at 64MHz (8/8
+seeds). N_SLOTS=8 significantly improved but OPEN -- 5/8 seeds close
+timing at 64MHz, 3/8 do not. File changed: hardware/v2/nms/rtl/
+nms_activation_fill_ctrl_v3.v.