micheleandClaude Sonnet 5
8e014d8d49
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
92 lines
4.5 KiB
Markdown
92 lines
4.5 KiB
Markdown
# NMS Weight Datapath Scaling (STEP14 Part A)
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Status: architectural requirement established and proven (simulation),
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**not realizable on real hardware today** (fixed 16-bit physical
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PSRAM). Full data: `hardware/v2/reports/step14_weight_scaling.csv`.
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Full narrative: `hardware/v2/logs/experiments.log` (EXP-0032, EXP-0033),
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`decisions.log` (DEC-0028).
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## Question answered
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*At what weight-path width does the processor stop being fundamentally
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starved by weight delivery?* **64 bits** — exactly `P_IN × DATA_WIDTH`
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(8 × 8). Proven by direct cycle-exact simulation, not assumed.
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## What was built
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`weight_prefetch_engine_wide.v` — a parameterized (`MEM_DATA_WIDTH`)
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generalization of the real `weight_prefetch_engine.v`'s continuous
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cross-tile-boundary streaming design, simulation-only/exploratory
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(same status as `ideal_memory_model.v`). `WORDS_PER_TILE =
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ceil(P_IN*DATA_WIDTH / MEM_DATA_WIDTH)`, clamped to a minimum of 1.
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`nms_memory_manager_stream_wide.v` pairs it with STEP13's own streaming
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memory manager unchanged (A2's requirement), on a *separate* logical
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wide port from the real 16-bit result-write-back port.
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A real bug was found and fixed during development: address stepping
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initially used `WORDS_PER_TILE × BYTES_PER_WORD` as the inter-tile
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byte stride, which over-counts whenever the bus is wider than one full
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tile (the 128-bit case, `WORDS_PER_TILE=1` but `BYTES_PER_WORD=16`
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while the tile itself is only 8 bytes) — this skips over the next
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tile's actual data in the packed backing store. Fixed by defining
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`TILE_BYTES = TILE_BITS/8` as the canonical, width-independent stride.
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## Results (bit-exact + ideal-memory cycle count)
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| Width | Words/tile | Steady-state cycles/tile | 16-tile job total |
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|---|---|---|---|
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| 16-bit | 4 | 4 | 80 |
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| 32-bit | 2 | 2 | 48 |
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| **64-bit** | **1** | **1** | **32** |
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| 128-bit | 1 | 1 | 32 |
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All four widths pass bit-exact correctness (9/9 tests each, including
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under injected extra memory latency). 64-bit achieves a clean,
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cycle-exact **1 cycle/tile** — 100% of `neural_processor.v`'s own
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theoretical per-tile acceptance rate, exactly matching the streaming
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memory manager's own ceiling (EXP-0027, STEP13). 128-bit gives **zero**
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further benefit: a bus wider than one full tile still delivers exactly
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one tile per transaction in this single-tile-per-request design (no
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multi-tile bursting was attempted).
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## The critical distinction: logical vs. physical bandwidth (A5)
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STEP14 explicitly warned against assuming a wider logical interface
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means the real memory can deliver it. It cannot, here: **the real V1
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PSRAM chain is fixed at 16 bits** — a real chip
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(ISSI IS66WVE4M16EBLL-70BLI, x16), not an RTL parameter. The
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already-existing, already-verified `weight_prefetch_engine.v` (real,
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used throughout STEP11-13) *is* exactly what a "64-bit logical / 16-bit
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physical" packing adapter would produce: it assembles one 64-bit
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logical tile from 4 real sequential 16-bit word transactions. Its real,
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repeatedly-measured result is 4 cycles/tile — identical to the ideal
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16-bit row above, because the real transaction count is unchanged
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regardless of what the logical interface upstream claims. **A logical
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wide interface backed by a physically-narrow bus delivers exactly the
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narrow bus's own throughput.** No new "packing adapter" module was
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built for this reason — the real engine already demonstrates the
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answer, conclusively, without further RTL.
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## Answer to the primary research questions
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- **Is 16-bit weight delivery fundamentally insufficient for P_IN=8?**
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Yes — it costs 4 cycles/tile, 4× the achievable minimum.
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- **Is 32-bit enough?** No — still 2× the achievable minimum (2
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cycles/tile).
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- **Is 64-bit the natural architectural point?** Yes, exactly — proven
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cycle-exact, not approximate.
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- **Does wider logical delivery actually improve real throughput?**
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**Not on this hardware.** Realizing the 64-bit ideal requires a
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matching *physical* bandwidth increase (a real 64-bit-wide external
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bus, or multiple parallel 16-bit PSRAM chips banked together) — a
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board/silicon-level change, outside this project's own RTL scope.
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## Recommendation
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The 64-bit requirement is now precisely quantified and should inform
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any future hardware revision (wider PSRAM, multiple banks). No RTL
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change is warranted on the current board: `weight_prefetch_engine.v`
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(real, 16-bit) remains the correct, already-optimal implementation
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given the fixed physical bus width — STEP13's streaming-manager fix
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already extracts everything available from the real interface.
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