docs: real, full P&R for N=16 hybrid systolic system -- timing NOT met (EXP-0093)

Real place_design/route_design against XC7A100T-CSG324-2: post-route
utilization holds (128 DSP48E1/53.33%, matching EXP-0091's projection),
but real timing fails on clk_pll_i (155.039MHz): WNS=-0.913ns,
WHS=+0.029ns, 3021 failing setup endpoints. Root-caused to
sdram_arbiter_n.v's req_wdata mux, now a real 20/21-way select (up
from 3-way at N=2) feeding mig_native_adapter.v's wdata_lat_reg.

N=16 is functionally correct (EXP-0092) but not yet timing-closed --
honestly not ready for real hardware at the target clock. N=2
(EXP-0088) remains the trustworthy, deployable signoff. Real options
for closing timing documented as next_action, not yet attempted.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MUG92aM9m68TRc4rG55BcC

hardware/v2/logs/experiments.log

@@ -6311,3 +6311,89 @@ real timing signoff (WNS/WHS), per this project's own standing "real,
 measured numbers only" discipline -- EXP-0091's own 128 DSP48E1/53.33%
 utilization projection still needs a real post-route confirmation, and
 timing has not been checked at all yet for this larger top-level.
+
+EXP-0093 -- real, full P&R for the N=16 hybrid systolic system: TIMING
+FAILS on first attempt (2026-09-21, EXP-0092's own next_action,
+continuing the user's "Ok procedi ad implementare quel che manca")
+
+CONTEXT: EXP-0092 proved the N=16 system functionally correct against
+real DDR3 (32/32 PASS); EXP-0091 proved it synthesis-clean with the
+projected DSP budget. Neither checked real, in-context, post-route
+timing at all. This experiment runs the real, full flow (synth_design
++ opt_design + place_design + route_design) against the real
+XC7A100T-CSG324-2 part, in the real Vivado project (same one EXP-0086/
+0088's own real N=2 signoffs used), to find out honestly whether it
+closes.
+
+REAL SETUP FIX FOUND BEFORE TRUSTING THE RUN (per CLAUDE.md's own
+stale-import lesson): `NeuralProcessor.srcs/constrs_1/imports/
+constraints/n2_system_ddr3_top.xdc` -- the ONE real constraint file in
+the project's `constrs_1` fileset -- was STALE, predating EXP-0084's
+real flash-bus bank-16 move and EXP-0088's own result_writeback.v pin
+removal (confirmed via a real diff against the live `hardware/v3/
+constraints/n2_system_ddr3_top.xdc`, not assumed). Fixed by overwriting
+the imported copy with the live one before running P&R (same real fix
+EXP-0084/0086 already established for this exact class of staleness).
+Also verified, via a real Vivado fileset query (`get_files -of_objects
+[get_filesets sources_1]` + `IS_ENABLED`/`USED_IN`), that every real RTL
+dependency of `n16_system_ddr3_top.v` (host_mem_bridge.v, mig_native_
+adapter.v, sdram_arbiter_n.v, spi_host_bridge_v3.v, packed_slot.v,
+layer_prefetch_ctrl.v, etc.) resolves to its LIVE `hardware/v3/`/
+`hardware/v2/` path in the active fileset, NOT the several stale
+`*/imports/*` copies also found sitting on disk (orphaned leftovers,
+never actually `IS_ENABLED` in the current fileset) -- so EXP-0091's
+own synthesis-only result was already built on correct RTL; only the
+XDC needed fixing before this run.
+
+REAL RESULT: synth_design/opt_design/place_design/route_design all
+completed without error (11 Infos, 1 Warning, 0 Critical Warnings, 0
+Errors from route_design itself). Real post-route utilization: 19751
+LUTs (31.15%), 34877 registers (27.51%), **128 DSP48E1/240 (53.33%)** --
+confirms EXP-0091's projection held through real place+route, not just
+synthesis. **Real post-route `report_timing_summary` on the real
+`clk_pll_i` domain (the same real 155.039MHz internal system clock
+EXP-0086/0088's own N=2 signoff was measured on): WNS=-0.913ns,
+WHS=+0.029ns, TNS=-690.085ns, 3021 failing setup endpoints (out of
+104247 total on this clock) -- TIMING CONSTRAINTS ARE NOT MET.** This
+is a real, measured FAILURE, not a projection -- N=2's own real EXP-
+0088 signoff closed at +0.099962ns on the same clock; N=16 does not
+close on this first real attempt.
+
+REAL ROOT CAUSE (traced via the actual worst violated path, not
+guessed): the critical path runs from `packed_pe.v`'s own `act_tile_
+fetch.v` FSM state register (one of 16 real per-PE instances, this
+occurrence in group 3 / PE 3) through 11 real logic levels (LUT3/
+LUT4x3/LUT6x5/MUXF7/MUXF8 -- a wide mux tree) into `mig_native_
+adapter.v`'s own `wdata_lat_reg[96]`. That destination register is fed
+by `sdram_arbiter_n.v`'s own real `req_wdata` selection mux, which grew
+from a 3-way select at N=2 (`NUM_REQ=3`) to a real **20-way** select at
+N=16 (`NUM_REQ=20` in this test's own scope; 21 in the real top with
+host_mem_bridge.v) over the SAME 256-bit-wide (`32*BURST_LEN`) bus --
+a real, substantial combinational fan-in growth on exactly the shared
+resource every one of the 16 PEs' own real DDR3 writes must pass
+through, not a coincidental or unrelated critical path.
+
+DECISION: N=16's real RTL is functionally correct (EXP-0092) but NOT
+YET timing-closed at the real target clock -- honestly NOT ready for
+real hardware at this clock. This is a real, disclosed, not-yet-solved
+problem, not a project blocker in itself (N=2 remains the real,
+trustworthy, deployable signoff, EXP-0088). Does not invalidate
+EXP-0091/0092's own real findings (synthesis-clean, functionally
+correct) -- timing closure is a genuinely separate, additional gate,
+exactly as CLAUDE.md's own "real numbers only" discipline anticipates
+("out-of-context synthesis is not a real signoff").
+
+next_action: real options to close timing, not yet attempted (need a
+real decision on direction, likely worth discussing before committing
+RTL effort): (1) pipeline `sdram_arbiter_n.v`'s own req_wdata/req_addr
+mux by one real cycle (adds one cycle of real arbitration latency per
+request, likely acceptable given DDR3's own already-dominant real
+latency, EXP-0087) -- probably the most direct real fix, targets the
+exact real critical path found above. (2) a real, hierarchical 2-level
+arbiter (e.g. 4 groups' own 5-way sub-arbiters feeding one real 4-way
+top arbiter) instead of one flat 20/21-way mux, reducing the real
+fan-in at the final stage. (3) lower the real target clock for the
+N=16 variant specifically (a real, measured tradeoff against N=2's own
+real throughput, not yet quantified). Real signal-tracing already done
+above narrows the fix to the arbiter's own write-data mux specifically
+-- future work should start there, not guess elsewhere in the design.