V2.0.0 hardware freeze - single SDRAM

FASE #1 hardware freeze for FPGA-Neural V2, N4/P8, single external
SDRAM (Alliance Memory AS4C4M16SA-6TIN) serving weights, activations,
and results through one physical sdram_controller.v instance. Removes
the PSRAM dependency (hardware/v1/rtl/psram_controller.v +
memory_interface.v) from the V2 physical path entirely -- V1 itself
remains fully unmodified, the golden reference.

New RTL: sdram_unified_backend.v (2-way W/AR arbitration over one
SDRAM controller, real per-byte DQM write masking added to
sdram_controller.v for correct single-byte result writes with no
read-modify-write), nms_neural_multiprocessor_sdram_unified.v (the
frozen top-level). Two real bugs found and fixed via full-system
testing before being accepted (ERR-0023): a deadlock and an off-by-one
data-shift bug in the new arbitration logic.

Real results: N=4 and N=2 D-Stress bit-exact (256/256 neurons), 40
real AUTO REFRESH events interleaved with zero corruption, real
Yosys+nextpnr-ecp5 synthesis/P&R for LFE5U-45F-8CABGA381 (149/245
TRELLIS_IO, a real 45-pin reduction from the prior dual-memory
design). Timing is MARGINAL (1/8 P&R seeds >=80MHz), reported honestly
rather than masked by the best seed.

Real, sourced ball-level pinout for the SDRAM bus + clk/rst (39/149
signals, P&R-verified) using the official Lattice ECP5U-45 pinout CSV
found on disk during this step's own pre-commit review -- corrects an
earlier draft that wrongly assumed no real pinout data was available.

Chip readiness: NO. Real, disclosed blockers remain (no physical host
interface exists yet -- the RTL's own reg_* ports are a 110-pin raw
test-harness bus; clock source/PLL decision; power/configuration
component selection) -- see hardware/v2/docs/{HARDWARE_FREEZE,
CHIP_READINESS,OPEN_ITEMS}.md for the complete, itemized status.

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

docs/architecture/weight_datapath_scaling.md

@@ -0,0 +1,91 @@
+# NMS Weight Datapath Scaling (STEP14 Part A)
+
+Status: architectural requirement established and proven (simulation),
+**not realizable on real hardware today** (fixed 16-bit physical
+PSRAM). Full data: `hardware/v2/reports/step14_weight_scaling.csv`.
+Full narrative: `hardware/v2/logs/experiments.log` (EXP-0032, EXP-0033),
+`decisions.log` (DEC-0028).
+
+## Question answered
+
+*At what weight-path width does the processor stop being fundamentally
+starved by weight delivery?* **64 bits** — exactly `P_IN × DATA_WIDTH`
+(8 × 8). Proven by direct cycle-exact simulation, not assumed.
+
+## What was built
+
+`weight_prefetch_engine_wide.v` — a parameterized (`MEM_DATA_WIDTH`)
+generalization of the real `weight_prefetch_engine.v`'s continuous
+cross-tile-boundary streaming design, simulation-only/exploratory
+(same status as `ideal_memory_model.v`). `WORDS_PER_TILE =
+ceil(P_IN*DATA_WIDTH / MEM_DATA_WIDTH)`, clamped to a minimum of 1.
+`nms_memory_manager_stream_wide.v` pairs it with STEP13's own streaming
+memory manager unchanged (A2's requirement), on a *separate* logical
+wide port from the real 16-bit result-write-back port.
+
+A real bug was found and fixed during development: address stepping
+initially used `WORDS_PER_TILE × BYTES_PER_WORD` as the inter-tile
+byte stride, which over-counts whenever the bus is wider than one full
+tile (the 128-bit case, `WORDS_PER_TILE=1` but `BYTES_PER_WORD=16`
+while the tile itself is only 8 bytes) — this skips over the next
+tile's actual data in the packed backing store. Fixed by defining
+`TILE_BYTES = TILE_BITS/8` as the canonical, width-independent stride.
+
+## Results (bit-exact + ideal-memory cycle count)
+
+| Width | Words/tile | Steady-state cycles/tile | 16-tile job total |
+|---|---|---|---|
+| 16-bit | 4 | 4 | 80 |
+| 32-bit | 2 | 2 | 48 |
+| **64-bit** | **1** | **1** | **32** |
+| 128-bit | 1 | 1 | 32 |
+
+All four widths pass bit-exact correctness (9/9 tests each, including
+under injected extra memory latency). 64-bit achieves a clean,
+cycle-exact **1 cycle/tile** — 100% of `neural_processor.v`'s own
+theoretical per-tile acceptance rate, exactly matching the streaming
+memory manager's own ceiling (EXP-0027, STEP13). 128-bit gives **zero**
+further benefit: a bus wider than one full tile still delivers exactly
+one tile per transaction in this single-tile-per-request design (no
+multi-tile bursting was attempted).
+
+## The critical distinction: logical vs. physical bandwidth (A5)
+
+STEP14 explicitly warned against assuming a wider logical interface
+means the real memory can deliver it. It cannot, here: **the real V1
+PSRAM chain is fixed at 16 bits** — a real chip
+(ISSI IS66WVE4M16EBLL-70BLI, x16), not an RTL parameter. The
+already-existing, already-verified `weight_prefetch_engine.v` (real,
+used throughout STEP11-13) *is* exactly what a "64-bit logical / 16-bit
+physical" packing adapter would produce: it assembles one 64-bit
+logical tile from 4 real sequential 16-bit word transactions. Its real,
+repeatedly-measured result is 4 cycles/tile — identical to the ideal
+16-bit row above, because the real transaction count is unchanged
+regardless of what the logical interface upstream claims. **A logical
+wide interface backed by a physically-narrow bus delivers exactly the
+narrow bus's own throughput.** No new "packing adapter" module was
+built for this reason — the real engine already demonstrates the
+answer, conclusively, without further RTL.
+
+## Answer to the primary research questions
+
+- **Is 16-bit weight delivery fundamentally insufficient for P_IN=8?**
+  Yes — it costs 4 cycles/tile, 4× the achievable minimum.
+- **Is 32-bit enough?** No — still 2× the achievable minimum (2
+  cycles/tile).
+- **Is 64-bit the natural architectural point?** Yes, exactly — proven
+  cycle-exact, not approximate.
+- **Does wider logical delivery actually improve real throughput?**
+  **Not on this hardware.** Realizing the 64-bit ideal requires a
+  matching *physical* bandwidth increase (a real 64-bit-wide external
+  bus, or multiple parallel 16-bit PSRAM chips banked together) — a
+  board/silicon-level change, outside this project's own RTL scope.
+
+## Recommendation
+
+The 64-bit requirement is now precisely quantified and should inform
+any future hardware revision (wider PSRAM, multiple banks). No RTL
+change is warranted on the current board: `weight_prefetch_engine.v`
+(real, 16-bit) remains the correct, already-optimal implementation
+given the fixed physical bus width — STEP13's streaming-manager fix
+already extracts everything available from the real interface.