FPGA-Neural/hardware/v2/docs/OPEN_ITEMS.md

FPGA-Neural V2 — OPEN ITEMS

Consolidated from HARDWARE_FREEZE.md, PINOUT.md, CLOCK_ARCHITECTURE.md, POWER_ARCHITECTURE.md, SCHEMATIC_READINESS.md. Classified per the governing spec's own rule: BLOCKER / CRITICAL / WARNING / OPEN / FUTURE.

BLOCKER (impede la realizzazione o il funzionamento del chip)

0. STEP20 update: a real SPI host interface (spi_host_bridge.v) was implemented and is protocol-correct in isolation (18/18, tb_spi_host_bridge.v), but a real, disclosed, UNRESOLVED defect (errors.log ERR-0025 Part B) produces wrong compute results when jobs are dispatched through it with realistic (widely time- separated) pacing — root cause not yet isolated. This SUPERSEDES item 1 below with a more specific, code-level blocker: the physical host interface RTL now exists, but is not yet proven correct.
1. No physical host interface exists. The RTL's own "host" ports are a 110-pin raw parallel job-registration bus (reg_valid/reg_node_id/reg_required/reg_producer_ids/ reg_x_base/reg_w_base/reg_n_tiles/reg_result_addr) — a simulation/testbench convenience, not a real board protocol. No RTL exists to serialize it (e.g. SPI, matching V1's own spi_neuron_top.v precedent).
2. The 110-pin host/registration bus has no real ball assignment (deliberately — it is not yet a real physical protocol, see item 1). The SDRAM bus (37 signals) and clk/rst (2 signals) now DO have a real, sourced, P&R-verified assignment (hardware/v2/constraints/ v2_unified.lpf, from the real Lattice pinout CSV found at ~/Downloads/FPGA-SC-02034-3-0-ECP5U-45-Pinout.csv during this step's own pre-commit review) — this item is narrower than originally scoped.
3. No schematic exists; no PCB has been started.

CRITICAL (rischio elevato, deve essere risolto prima del freeze)

1. Timing closure is MARGINAL, with an unfavorable pass rate. 8 real P&R seeds for the frozen N=4 single-SDRAM design: only 1/8 reach ≥80MHz (66.97–81.84MHz range). This is WORSE than STEP18's own dual-memory design (5/8 pass). The critical path itself is unchanged (still dependency_manager.v's own pre-existing first_ready_idx/reg_ready chain) — the regression is attributed to added overall die/routing pressure from consolidation, not a new RTL defect, but it is real and unresolved.
2. Clock source/oscillator gap. The RTL requires a direct ≥80MHz clock (no PLL exists anywhere in the hierarchy — confirmed via EHXPLLL: 0/4 in every real synthesis run). Prior project memory records a 16MHz board oscillator. Neither "source an 80MHz+ oscillator" nor "add a real PLL to the RTL" has been decided.
3. Two physical memories were required through STEP18 — RESOLVED this round (DEC-0034): the V2 physical path no longer instantiates hardware/v1/rtl/psram_controller.v at all. Kept here only as a closed CRITICAL item for the historical record.

WARNING (non blocca il prototipo ma deve essere documentato)

1. N=2's real Fmax (86.04MHz in STEP18's own dual-memory design) and the STEP19 single-SDRAM N=2 config were not both measured with the same best-of-N-seed rigor as N=4 — a real, disclosed gap in measurement thoroughness, not a functional issue.
2. W_ENTRIES/cache sizing in the weight-fetch path was set to match N_SLOTS (4) by construction reasoning, not swept for optimality.
3. I/O standard (LVCMOS33 assumed for all 149 signals) has not been verified per real VCCIO bank once ball assignment becomes possible.

OPEN (decisione ancora da prendere)

1. Configuration-flash part number / SPI-flash-boot vs JTAG-only bring-up.
2. Power regulator topology and part numbers (the previously-recorded ../basic-ecp5-pcb reference design is not accessible this session to confirm as a concrete plan).
3. Real current budget (requires running a real power-estimation tool against the actual synthesized netlist — not done this round).
4. Decoupling/bulk capacitance values (depend on regulator selection).
5. Reset synchronization to a real external POR/supervisor source.
6. Real per-bank VCCIO/I-O-standard verification once ball data is available.

FUTURE EVOLUTION (miglioramento post-freeze — explicitly deferred)

1. N=8 evaluation.
2. A smarter W/AR priority scheme in sdram_unified_backend.v to recover some of the +10.8% (N=4) / +5.1% (N=2) cycle-count cost of single-SDRAM unification (STEP18 EXP-0046's own packing win is still present — this is about the NEW W-vs-AR contention specifically). generic
3. Page-mode / keep-row-open SDRAM controller redesign (STEP18's own identified next bottleneck for raw memory bandwidth, independent of the single-vs-dual-memory question).
4. True multi-outstanding SDRAM request pipelining (STEP18 Part E's own documented, deliberately out-of-scope boundary).
5. A real physical host-interface RTL bridge (SPI or similar), resolving BLOCKER #1 above.
6. Floorplanning / seed-pinning work to convert the current MARGINAL timing result into a reliable PASS.