FPGA-Neural/docs/FPGA-NeuralNetwork-Engine.md

FPGA Neural Network Engine

Hardware Neural Network Engine based on FPGA + dedicated RAM.

The project implements a parametric hardware accelerator for neural networks, designed to be reusable across different embedded systems and applications.

The fundamental design principle is that the neural-network computation is performed entirely inside the FPGA, while the host system communicates with the engine through a simple hardware-independent interface such as SPI.

---

1. Project Goal

The goal of this project is to develop a reusable Neural Network Engine implemented in FPGA hardware.

The engine is composed of:

• FPGA;
• dedicated RAM connected to the FPGA;
• host interface, initially SPI and potentially Dual SPI.

The host system is not part of the neural-network computational datapath.

Possible host systems include:

• Linux SoCs;
• Raspberry-Pi-like systems;
• ESP32;
• microcontrollers;
• other embedded processors;
• development PCs.

The same Neural Network Engine architecture should therefore be usable in completely different systems.

                    HOST SYSTEM
             ┌─────────────────────────┐
             │                         │
             │ Linux / ESP32 / MCU     │
             │                         │
             │ Configuration           │
             │ Training                │
             │ Control                 │
             └────────────┬────────────┘
                          │
                       SPI / Dual SPI
                          │
                          ▼
             ┌─────────────────────────┐
             │          FPGA           │
             │                         │
             │ Neural Network Engine   │
             │                         │
             │ Compute / Control       │
             │                         │
             └────────────┬────────────┘
                          │
                          │
                    Dedicated RAM

---

2. Architectural Principle

The FPGA is the actual neural-network accelerator.

The RAM required by the neural network is physically associated with the FPGA and is not part of the host system memory.

The host only provides:

• configuration;
• network parameters;
• input data;
• control;
• result retrieval.

The neural-network calculations themselves are executed by the FPGA.

This separation is a fundamental architectural requirement.

---

3. Hardware Configuration vs Network Configuration

An important distinction is made between the hardware architecture of the accelerator and the parameters of the neural network.

3.1 FPGA hardware configuration

The physical architecture of the Neural Network Engine is defined when the FPGA design is synthesized and implemented.

Typical hardware parameters include:

N_INPUTS
N_NEURONS
N_LAYERS
PARALLEL
DATA_WIDTH
ACCUMULATOR_WIDTH

These parameters can therefore be Verilog/SystemVerilog parameters or equivalent synthesis-time configuration values.

For example:

N_INPUTS  = 32
N_NEURONS = 4
PARALLEL  = 8

defines a specific hardware implementation optimized for that architecture.

The resulting FPGA bitstream contains the corresponding datapath.

---

3.2 Neural-network configuration

Once the FPGA has been configured and initialized, the actual neural-network parameters can be loaded through the host interface.

These parameters may include:

• weights;
• biases;
• activation parameters;
• quantization parameters;
• network-specific constants.

These values are stored in the RAM associated with the FPGA.

Therefore:

FPGA BITSTREAM
        │
        │ defines hardware architecture
        ▼
┌─────────────────────┐
│ Neural Network      │
│ Hardware Engine     │
└──────────┬──────────┘
           │
           │ loads
           ▼
┌─────────────────────┐
│ Dedicated RAM       │
│                     │
│ weights             │
│ biases              │
│ parameters          │
│ buffers             │
└─────────────────────┘

This provides an important separation between hardware specialization and network data.

---

4. Application-Specific Neural Networks

The Neural Network Engine is not intended to implement one fixed neural network.

Instead, each application can define its own network.

For example:

Application A
    16 inputs
    8 neurons
    1 output

Application B
    32 inputs
    16 neurons
    4 outputs

Application C
    64 inputs
    multiple layers
    custom parallelism

The FPGA hardware can then be generated specifically for the required architecture.

This allows the design to exploit the FPGA resources efficiently rather than implementing a completely generic and potentially inefficient neural-network processor.

---

5. Parametric Compute Engine

The current implementation contains a parametric neural-network layer.

A validated configuration is:

N_INPUTS  = 32
N_NEURONS = 4
PARALLEL  = 8

The datapath processes the inputs in parallel groups.

Conceptually:

32 inputs
    │
    ├── 8 parallel MACs
    │
    ├── 8 parallel MACs
    │
    ├── 8 parallel MACs
    │
    └── 8 parallel MACs
             │
             ▼
        Accumulation
             │
             ▼
           Bias
             │
             ▼
        Activation
             │
             ▼
          Output

The architecture is intended to scale by changing the synthesis parameters.

---

6. Current Functional Validation

The 32 × 4 / PARALLEL = 8 configuration has been successfully simulated.

Test output:

========================================
PARAMETRIC LAYER TEST
N_INPUTS  = 32
N_NEURONS = 4
PARALLEL  = 8
========================================

PASS N0: 8192
PASS N1: 4096
PASS N2: 0 (ReLU)
PASS N3: 16384

========================================
PARAMETRIC TEST PASSED
========================================

Validated functionality:

• multiple inputs;
• multiple neurons;
• parallel MAC processing;
• accumulation across multiple input groups;
• bias handling;
• independent neuron outputs;
• ReLU activation;
• parametric layer architecture.

The corresponding test has been committed to the repository.

Commit:

test: validate parametric 32x4 layer with parallelism 8

---

7. FPGA Boot and Initialization

The FPGA is configured during system initialization using its normal FPGA configuration mechanism.

The FPGA bitstream defines the hardware architecture of the Neural Network Engine.

Conceptually:

Power-on
   │
   ▼
FPGA configuration
   │
   │ bitstream
   ▼
Neural Network Engine available
   │
   ▼
Host initialization
   │
   │ SPI
   ▼
Load network parameters
   │
   ▼
Load weights / biases
   │
   ▼
Engine ready

This means that the host does not dynamically construct the FPGA datapath during normal operation.

The datapath already exists in hardware.

The host configures the network data that the datapath operates on.

---

8. Host Interface

The primary external interface is intended to be:

SPI

with possible future support for:

Dual SPI

The interface must remain independent of the host operating system.

The same hardware protocol should therefore be usable from:

Linux
ESP32
MCU
PC

The host interface should provide access to:

• control registers;
• status;
• network configuration;
• RAM;
• input data;
• output data;
• start/stop control;
• completion status.

A conceptual command sequence is:

RESET
  │
  ▼
CONFIGURE
  │
  ▼
LOAD NETWORK PARAMETERS
  │
  ▼
LOAD WEIGHTS
  │
  ▼
LOAD BIASES
  │
  ▼
LOAD INPUT
  │
  ▼
START
  │
  ▼
WAIT FOR DONE
  │
  ▼
READ OUTPUT

8.1 SPI Protocol v1 (draft, 2026-09-02)

Concrete opcode-level draft of the section above, written before any Phase 4 RTL. Opcode values and the exact set of commands are illustrative/example at this stage, not frozen — the framing rules (MSB-first, explicit length, sticky STATUS.done) and the two decisions already made (explicit length field over CS-delimited streaming; a runtime READ_CONFIG command) are the parts intended to stick; the opcode table itself is expected to be revised as Phase 4 RTL work starts.

Physical layer: SPI Mode 0 (CPOL=0, CPHA=0), MSB-first, single SPI for v1 (Dual SPI is a future extension per §8, not addressed here). The FPGA is always SPI slave. One command per CS-low period; byte 0 of every transaction is the opcode.

Multi-byte fields are big-endian (most significant byte first). Byte addresses are ADDR_WIDTH-bit (22 bits today, from rtl/neuron_memory.v), carried in a 3-byte field with the top 2 bits reserved as 0.

Length is explicit, not CS-edge-delimited: WRITE_RAM/READ_RAM carry a 2-byte length field, so the SPI controller only needs a byte counter, not CS-edge detection mid-transfer.

Opcode table

Opcode	Name	Payload (host → FPGA)	Response (FPGA → host)	Function
0x00	NOP	—	—	No operation (idle/dummy clocking)
0x01	WRITE_RAM	addr(3B) + len(2B) + len data bytes	—	Write a block into PSRAM (X, weights, bias, network params)
0x02	READ_RAM	addr(3B) + len(2B)	len data bytes	Read a block back from PSRAM
0x0F	RESET	—	—	Synchronous reset pulse to the compute engine (neuron_memory) and clears the STATUS latch below. Does not erase PSRAM contents. Kept as a distinct opcode from NOP.
0x10	SET_BASE	sel(1B) + addr(3B)	—	Sets x_base(sel=0) / w_base(sel=1) / bias_addr(sel=2) / table_base(sel=3) / buf_a_base(sel=4) / buf_b_base(sel=5) / activation(sel=6) / n_inputs_real(sel=7) / n_neurons_real(sel=8) — sel=6..8 are single-layer/manual-path registers only (RUN_NETWORK reads the equivalent fields per-layer from the descriptor table instead). sel=6 uses only the low 2 bits of the low address byte (see neuron_parallel.v's ACT_* localparams; reset default ACT_RELU). sel=7/8 use the low 2 of the 3 address bytes as a 16-bit BE value (reset default = this bitstream's build-time N_INPUTS/N_NEURONS) — see "Runtime network width" below.
0x20	START	—	—	Pulses start on neuron_memory directly (single-layer/manual path); ignored (no-op) if the engine is busy in any form (single-layer or a RUN_NETWORK job)
0x21	STATUS	—	1 byte	bit0=busy (live, OR of the single-layer and RUN_NETWORK busy signals), bit1=done (sticky, clear-on-read; latches on the final layer's completion for a RUN_NETWORK job, not each intermediate layer), bits7:2 reserved=0
0x22	READ_OUTPUT	—	N_NEURONS bytes	y_bus, neuron-major (byte 0 = neuron 0)
0x23	RUN_NETWORK	num_layers(1B)	—	Phase 5: pulses layer_sequencer's run_start to chain num_layers (1..N_LAYERS) runs of neuron_memory using the descriptor table at table_base and the ping-pong buffers at buf_a_base/buf_b_base; layer 0 reads from x_base. Each layer's activation (see below) is read per-layer from the descriptor table, independent of the single-layer path's activation register. Ignored (no-op) if the engine is already busy.
0x30	READ_CONFIG	—	8 bytes	Hardware config record, see below

Why STATUS.done is sticky / clear-on-read: in rtl/neuron_memory.v done is a single-cycle pulse (asserted for exactly one clock in STATE_WAIT_N, deasserted the next cycle). A host polling over SPI — orders of magnitude slower than the FPGA clock — would almost certainly miss a raw one-cycle pulse. The SPI register bank must therefore latch done into a sticky bit on the pulse, and clear it when the host issues STATUS (or RESET), not sample the raw neuron_memory.done signal directly. busy has no such problem (it is level-held for the whole computation) and can be read live.

READ_CONFIG payload (fixed 8 bytes, lets one host firmware build work across different bitstreams without recompiling):

Byte(s)	Field	Source
0	ADDR_WIDTH (bits)	neuron_memory.ADDR_WIDTH
1–2	N_INPUTS (16-bit BE)	neuron_memory.N_INPUTS
3	N_NEURONS	neuron_memory.N_NEURONS
4	PARALLEL	neuron_memory.PARALLEL
5	DATA_WIDTH (bits)	neuron_memory.DATA_WIDTH
6–7	protocol version (16-bit BE)	0x0001 for this spec

Runtime network width — one bitstream, any topology up to its build-time max (2026-09-02): READ_CONFIG's N_INPUTS/N_NEURONS report this bitstream's build-time maximum width (neuron_memory's own synthesis parameters) — the ceiling, not necessarily what the currently-loaded network actually uses. The real per-run width is a separate, host-set value:

• Single-layer/manual path: SET_BASE sel=7 (n_inputs_real) / sel=8 (n_neurons_real), 16-bit BE, defaulting to the build-time max on reset.
• RUN_NETWORK (multi-layer) path: each layer's own n_inputs_real/n_neurons_real in its descriptor table entry (see the RUN_NETWORK row above and the table format below) — independent per layer, so a network can taper (e.g. 256 -> 64 -> 16 -> 4) inside one chained run.

n_inputs_real must be a multiple of PARALLEL (the same constraint N_INPUTS itself is held to at elaboration time — see rtl/neuron_parallel.v's parameter guard — now the host's runtime responsibility instead of a build-time check). Both fields directly bound the hardware's own loops (neuron_memory's X/W RAM reads, neuron_parallel's MAC group count, and — for RUN_NETWORK — the ping-pong copy-out length): a layer that only uses part of the bitstream's max width genuinely computes and copies out faster, not just "as if" narrower. No zero-padding of RAM is needed for the unused tail (contrast with the old padding-only convention this replaced) — data beyond n_inputs_real/n_neurons_real is simply never read.

Example session (fills in the conceptual sequence above with concrete opcodes):

RESET                 -> 0x0F
READ_CONFIG           -> 0x30           (host learns N_INPUTS/N_NEURONS/...)
WRITE_RAM (weights)   -> 0x01 ...
WRITE_RAM (biases)    -> 0x01 ...
SET_BASE (X/W/BIAS)   -> 0x10 x3
WRITE_RAM (input X)   -> 0x01 ...
START                 -> 0x20
poll STATUS           -> 0x21           (until done bit set; clears on this read)
READ_OUTPUT           -> 0x22

RUN_NETWORK (Phase 5) example session:

WRITE_RAM (layer descriptor table) -> 0x01 ...   (N_LAYERS entries of w_base(3B)+bias_addr(3B)+activation(1B)+n_inputs_real(2B)+n_neurons_real(2B), MSB-first)
WRITE_RAM (weights/biases per layer, X for layer 0) -> 0x01 ...
SET_BASE (X/TABLE/BUF_A/BUF_B)     -> 0x10 x4
RUN_NETWORK(num_layers)            -> 0x23 <num_layers>
poll STATUS                        -> 0x21   (until done bit set; clears on this read)
READ_OUTPUT                        -> 0x22   (final layer's y_bus)

Not yet decided / explicitly out of scope for v1: Dual SPI framing, a CRC/checksum on transfers (SPI is assumed reliable for a board-level trace in v1). Multi-layer sequencing itself (RUN_NETWORK, opcode 0x23) is implemented per rtl/layer_sequencer.v and the table above.

---

9. Dedicated FPGA RAM

The RAM is considered part of the Neural Network Engine.

It is not intended to be supplied by the host system.

Depending on the final architecture, RAM may contain:

Weights
Biases
Input buffers
Intermediate layer buffers
Output buffers
Network parameters

The memory architecture must be designed according to:

• number of parallel MAC units;
• data width;
• required bandwidth;
• number of layers;
• buffering requirements;
• FPGA block-RAM resources;
• possible external RAM requirements.

The preferred architecture is that the FPGA directly controls this memory.

---

10. Training

Training and inference are conceptually separated.

The first implementation does not require the FPGA to perform the complete training process.

Training can be performed externally:

PC / Linux / other host
        │
        │ training
        ▼
Network weights
        │
        │ SPI
        ▼
FPGA RAM

The FPGA then performs inference using the resulting parameters.

This approach greatly reduces the complexity of the initial hardware implementation.

However, the architecture should not prevent future implementation of hardware-assisted or fully hardware-based training.

---

11. Inference

During inference, the host only supplies input data and retrieves the result.

              HOST
                │
             Input
                │
                │ SPI
                ▼
        ┌───────────────┐
        │      FPGA     │
        │               │
        │ Neural Network│
        │    Engine     │
        │               │
        └───────┬───────┘
                │
             Output
                │
                │ SPI
                ▼
              HOST

The host is not involved in the individual MAC operations.

This provides:

• deterministic computation;
• reduced host workload;
• hardware parallelism;
• predictable latency;
• independence from the host CPU architecture.

---

12. Multi-Layer Architecture

The current implementation starts from a single parametrized layer.

The intended architecture is eventually:

Input
  │
  ▼
┌──────────────┐
│   Layer 0    │
└──────┬───────┘
       │
       ▼
┌──────────────┐
│   Layer 1    │
└──────┬───────┘
       │
       ▼
┌──────────────┐
│   Layer 2    │
└──────┬───────┘
       │
       ▼
    Output

Intermediate data will be stored in FPGA-controlled memory buffers.

The number and size of layers should ultimately be part of the hardware generation process.

---

13. Reusability

The main purpose of the architecture is reuse.

A future project should be able to use the same general Neural Network Engine architecture with a different hardware configuration.

For example:

Project A
    N_INPUTS  = 32
    N_NEURONS = 8
    PARALLEL  = 8

Project B
    N_INPUTS  = 64
    N_NEURONS = 16
    PARALLEL  = 16

Project C
    N_INPUTS  = 128
    N_NEURONS = 32
    PARALLEL  = 32

The HDL architecture remains conceptually the same while synthesis parameters generate an implementation appropriate for the target application.

---

14. Design Philosophy

The project should be considered a:

Reusable FPGA Neural Network Accelerator Platform

rather than a single neural-network implementation.

The application determines:

Input size
Network topology
Number of layers
Number of neurons
Parallelism
Numerical precision
Activation functions
Memory requirements
Performance requirements

The hardware generator then produces the corresponding FPGA implementation.

---

15. Development Roadmap

Phase 1 — Parametric Layer

• Parametric inputs
• Parametric neurons
• Parametric parallelism
• Accumulation
• Bias
• ReLU
• 32×4 / P=8 functional test

Phase 2 — Parameter Sweep

Validate multiple combinations of:

N_INPUTS
N_NEURONS
PARALLEL

including configurations where the number of inputs is not an exact multiple of the parallelism.

• Exact-multiple sanity configs (32×8, 64×32)
• Non-exact-multiple configs (30×8, 20×16)
• Degenerate config, PARALLEL > N_INPUTS (4×8)
• Sweep testbench: sim/parameter_sweep_tb.v

Findings — FIXED (2026-09-02):

• neuron_parallel.v computed GROUPS = N_INPUTS / PARALLEL with integer division. When N_INPUTS was not an exact multiple of PARALLEL, only the first GROUPS * PARALLEL inputs were ever read by the accumulator — the remainder was silently dropped (no error, no warning). Confirmed for 30×8 → only 24 of 30 inputs summed, and 20×16 → only 16 of 20 inputs summed.
• If PARALLEL > N_INPUTS, GROUPS = 0 and the controller's group_index == GROUPS-1 terminal condition was never satisfied: the neuron entered busy and never asserted done (confirmed hang, 500-cycle watchdog in the sweep bench).
• Both share the same root cause (N_INPUTS % PARALLEL != 0, degenerate PARALLEL > N_INPUTS included) and both are now rejected at elaboration time, in simulation and synthesis alike, by a generate guard added to neuron_parallel.v (instantiates a deliberately undefined module when the parameter combination is invalid — zero cost, zero behavior change for any valid configuration). The validated datapath itself (mac8/mac_unit/accumulation/ReLU/saturation) was not modified. See sim/neuron_parallel_guard_negative_nonmultiple_tb.v and sim/neuron_parallel_guard_negative_degenerate_tb.v for the negative-test proof, and sim/parameter_sweep_tb.v for the updated positive sweep (now valid-configs-only, including PARALLEL=2 and PARALLEL=4, the two best-performing values from docs/FPGA-Neural-Datapatch-Benchmark.md).

Phase 3 — Memory Architecture

Define:

• weight memory;

• bias memory;

• input buffers;

• output buffers;

• intermediate buffers;

• memory addressing;

• bandwidth requirements.

• neuron_memory.v: single-neuron memory integration (N_NEURONS=1)

• neuron_memory.v: multi-neuron memory integration (N_NEURONS>1)

• Intermediate/multi-layer buffers (deferred to Phase 5)

• Bandwidth analysis against PSRAM timing (deferred to Phase 7)

Multi-neuron design (2026-09-02): neuron_memory.v now takes an N_NEURONS parameter and loops over neurons in memory: X is read once (shared input vector), and for each neuron in turn W and bias are re-read from PSRAM and fed to a single, reused neuron_parallel instance — memory-bound by design, one neuron computed at a time, no change to the validated compute datapath. Addressing follows the same neuron-major convention as layer.v: neuron n's weights live at w_base + n*N_INPUTS bytes, its bias at bias_addr + n. Output is now y_bus (packed, DATA_WIDTH*N_NEURONS bits, neuron-major), replacing the old single-neuron y port. Validated end to end through the full memory stack (memory_interface + psram_controller + psram_model) in sim/neuron_memory_multi_tb.v (N_NEURONS=3: scale, larger value, ReLU). sim/neuron_memory_tb.v (N_NEURONS=1) still passes unchanged, confirming backward compatibility.

Phase 4 — SPI Interface

Implement:

• SPI controller;

• register map;

• RAM access;

• configuration protocol;

• input/output protocol;

• status and control.

• Protocol/opcode set drafted — see §8.1 SPI Protocol v1

• SPI controller RTL — rtl/spi_slave.v (physical layer: Mode 0, MSB-first, 3-stage CDC synchronizer for SCLK/MOSI/CS_N)

• Register bank RTL — rtl/spi_engine.v (all 8 opcodes: NOP, WRITE_RAM, READ_RAM, RESET, SET_BASE, START, STATUS, READ_OUTPUT, READ_CONFIG; sticky clear-on-read STATUS.done)

• RAM access passthrough RTL — rtl/mem_arbiter.v (fixed-priority arbiter, neuron_memory > spi_engine) + shared int8_memory_access instance in rtl/spi_neuron_top.v

• Testbenches: sim/spi_slave_tb.v (4 tests), sim/spi_engine_tb.v (10 tests, synthetic RAM), sim/spi_neuron_top_tb.v (end-to-end, real psram_model.v, no synthetic mock — RESET/READ_CONFIG/WRITE_RAM/READ_RAM/SET_BASE/START/STATUS/READ_OUTPUT all exercised purely over simulated SPI)

Real-toolchain verification (Yosys + nextpnr-ecp5 + ecppack, 2026-09-02):

• spi_slave.v alone: PASS, Fmax 403.23 MHz.
• spi_engine.v alone: PASS, Fmax 191.31 MHz. Neither uses any DSP.
• spi_neuron_top.v (full integration: SPI + arbiter + neuron_memory
  • PSRAM chain), N_NEURONS=1: FAIL at 80 MHz — Fmax ~52.58 MHz (PARALLEL=8) / ~55.85 MHz (PARALLEL=2, the benchmark's own 80 MHz-passing config in isolation). The critical path in both cases is entirely inside neuron_parallel.v's saturation comparator (> 127, rtl/neuron_parallel.v:127) — zero contribution from the new SPI/arbiter logic — but its routed delay is ~57% worse than in the isolated benchmark (17.91 ns vs. 11.38 ns) due to placement/routing congestion once the SPI + PSRAM logic shares the fabric with it, not resource exhaustion (DSP utilization only 2%). This means the current auto-placed full system would need to run around 50-56 MHz to stay within timing margin at speed grade -8, not the 80 MHz target — a system-level floorplanning/pipelining problem, out of scope here and left for Phase 7 (Optimization: "pipeline depth", "FPGA resource utilization"). It does not affect functional correctness (verified independently, in simulation, against real PSRAM timing) or synthesizability (0 CHECK-pass problems, no latches).

Phase 5 — Multi-Layer Network

Implement:

• multiple layers;

• intermediate buffers;

• layer sequencing;

• configurable activation functions.

• layer_sequencer.v: chains up to N_LAYERS runs of a single, reused neuron_memory instance, reading each layer's w_base/bias_addr/activation/n_inputs_real/n_neurons_real from a host-written descriptor table (11 bytes/layer) and ping-ponging each layer's output between two RAM buffers (layer 0 reads the external x_base; layer k>0 reads the buffer layer k-1 wrote)

• Configurable activation functions — neuron_parallel.v gains a 2-bit activation port (ACT_NONE=linear+two-sided saturate, ACT_RELU=the original hardwired behavior, kept as the default so every pre-existing caller/testbench is unaffected), threaded through neuron_memory.v at runtime and settable per-layer via the descriptor table (Phase 5 path) or per-run via SET_BASE sel=6 (legacy single-layer path). Unit-tested directly in neuron_parallel_tb.v (linear pass-through + negative saturation to -128) and end-to-end in spi_neuron_top_runnetwork_tb.v (a real negative accumulator that ACT_RELU would have clamped to 0 comes through unclamped under ACT_NONE, verified over real SPI/RAM)

• Runtime network width — one synthesized bitstream serves any topology up to its build-time max: neuron_parallel.v gains a runtime n_inputs_real (bounding its MAC group loop) and neuron_memory.v gains n_inputs_real/n_neurons_real (bounding its X/W RAM-read loop and its neuron loop), all defaulting to the build-time max so every pre-existing caller is unaffected. Settable per-layer via the descriptor table (Phase 5 path) or per-run via SET_BASE sel=7/8 (legacy single-layer path). This is a real early termination, not just address bookkeeping — no RAM zero-padding needed for the unused tail, and it measurably runs faster: neuron_parallel_tb.v TEST 7 shows 3 cycles vs. 6 for a reduced-vs-full run (with garbage loaded in the skipped lanes, proving they're never read), and neuron_memory_tb.v TEST 5 shows the same effect through the real PSRAM stack: 209 cycles vs. 788 for an 8-of-32 vs. full-32 run. layer_sequencer_tb.v additionally proves a reduced n_neurons_real shortens the ping-pong copy-out itself (untouched RAM bytes beyond the real count, not just a differing value)

• mem_arbiter.v: extended to a third port (Port C, priority B > C > A) for the sequencer's own RAM master access

• spi_engine.v: RUN_NETWORK opcode (0x23) + SET_BASE selectors for table_base/buf_a_base/buf_b_base; STATUS.busy/STATUS.done extended to track the sequencer (seq_busy/seq_done) as well as neuron_memory directly, so done latches on the last layer only, not each intermediate one — see §8.1

• spi_neuron_top.v: instantiates the sequencer and muxes neuron_memory's control inputs between it (while seq_busy) and spi_engine's direct-drive path (legacy single-layer mode)

• Testbenches: sim/layer_sequencer_tb.v (2-layer run: descriptor table, ping-pong addressing verified by address not just value, byte-exact output-buffer copy, seq_busy held across the layer boundary, seq_done fires exactly once, plus a reduced n_neurons_real proven to shorten the copy-out itself); sim/spi_engine_tb.v gained tests K/L/M/N (new SET_BASE selectors incl. activation and runtime width, RUN_NETWORK accept/busy-ignore gating, STATUS semantics) — both use a mocked neuron_memory, same pattern as Phase 4's spi_engine_tb.v; sim/neuron_parallel_tb.v and sim/neuron_memory_tb.v each gained a dedicated runtime-width test against the real datapath/real PSRAM stack (see the runtime-width bullet above for the cycle counts)

• Real-toolchain (Yosys + nextpnr-ecp5) synthesis/Fmax check of the extended spi_neuron_top.v — worse than Phase 4 alone in both configs checked, speed grade -8: N_INPUTS=32/N_NEURONS=1/PARALLEL=8 gives 40.57 MHz, FAIL (Phase 4 alone: ~52.58 MHz); PARALLEL=2 gives 42.54 MHz, FAIL (Phase 4 alone: ~55.85 MHz). The Phase 5 wiring (sequencer + mux + arbiter Port C) measurably cost 12-13 MHz of headroom on top of Phase 4's own placement-congestion problem, and the critical path itself shifted from Phase 4's saturation-comparator finding to neuron_memory's x_mem/w_mem LUT-RAM read-mux tree — see the Phase 7 critical-path analysis below.

• sim/spi_neuron_top_runnetwork_tb.v: real end-to-end test, RUN_NETWORK driven purely over simulated SPI against the real neuron_memory + PSRAM chain (N_INPUTS=N_NEURONS=4, PARALLEL=2, 2 layers, hand-computed expected output verified via both READ_OUTPUT and a READ_RAM of buf_b_base, plus buf_a_base's intermediate layer-0 output) — all PASS, and confirms the mux correctly hands neuron_memory back to the legacy single-layer START path afterward

Bug found and fixed while writing the above test (2026-09-02): a real race in the STATUS.done sticky/clear-on-read mechanism (§8.1), present since Phase 4 and not specific to RUN_NETWORK — it only needed continuous STATUS polling racing a done transition to surface, which the new end-to-end test's wait_done polling loop finally did. tx_byte's OP_STATUS case read status_done_sticky/busy live/combinationally for the whole ST_RESP window, while the sticky bit was cleared unconditionally on any STATUS read (status_read_now). If done_event landed while a STATUS response byte was already mid-transmission, the byte actually shifted out to the host could still be the stale pre-done value while the engine simultaneously treated the read as having delivered done and cleared it — silently dropping the transition forever, hanging any host polling STATUS in a tight loop. Fixed in rtl/spi_engine.v by latching a status_snapshot register once, at OP_STATUS opcode-accept time, and gating the sticky clear on status_snapshot[1] (i.e. only clear if the byte actually transmitted showed done=1) instead of clearing unconditionally on every STATUS read. A done_event that arrives too late for one snapshot is now reported on the next poll instead of being lost. All existing testbenches (spi_slave_tb.v, spi_engine_tb.v, spi_neuron_top_tb.v, layer_sequencer_tb.v) still pass unchanged.

Phase 6 — Host Software

Develop host-side drivers for:

• Linux;
• ESP32.

The same FPGA protocol should be usable by both.

Phase 7 — Optimization

Evaluate:

• pipeline depth;
• MAC parallelism;
• memory bandwidth;
• numerical precision;
• FPGA resource utilization;
• latency;
• throughput.

Critical-path analysis (real Yosys + nextpnr-ecp5, 2026-09-02): both spi_neuron_top.v builds checked so far miss the 80 MHz target, and the shortfall grew with Phase 5 wired in:

Build	N_INPUTS/N_NEURONS/PARALLEL	Fmax	vs. 80 MHz
Phase 4 only (documented above)	32/1/8	~52.58 MHz	-34%
Phase 4 only (documented above)	32/1/2	~55.85 MHz	-30%
Phase 5 (this session)	32/1/8	40.57 MHz	-49%
Phase 5 (this session)	32/1/2	42.54 MHz	-47%

Resource utilization is not the cause in either case: MULT18X18D (DSP) and DP16KD (block RAM) both sit at 0% in the Phase 5 builds — neuron_memory.v's x_mem/w_mem arrays (32 x INT8 each) are being inferred as LUT-based distributed RAM, not the ECP5's dedicated DP16KD block RAM, which sits completely idle.

The critical path itself shifted between the two checks. Phase 4's finding (quoted above) pinned it on neuron_parallel.v's saturation comparator (> 127), with routing congestion as the driver, not logic depth. In this session's Phase-5 builds the reported critical path (posedge -> posedge, neuron_memory's accumulator register) instead runs through a long chain of nested PFUMX/L6MUX21 LUT multiplexers reading w_mem/x_mem before reaching acc — consistent with a 32-entry LUT-RAM read select tree, not the saturation logic. Whether this is a genuinely different bottleneck or an artifact of where Phase 5's extra logic pushed the placer is not yet determined; it would need a placement-seed sweep (same design, several nextpnr --seed values) to separate "real critical path" from "this particular placement's critical path" — neither Phase 4's nor Phase 5's single P2/P8 runs establish that.

Candidate directions (not implemented, need a decision before touching the validated core further):

Placement-seed sweep (2026-09-02) — resolved: it's structural, not noise. Re-ran nextpnr-ecp5 on the already-synthesized Phase 5 netlists (top.json reused, only placement re-seeded — no re-synth) at --seed 1/2/3 for both P8 and P2:

Build	seed (default)	seed 1	seed 2	seed 3	spread
P8	40.57 MHz	39.70 MHz	39.54 MHz	40.00 MHz	1.03 MHz (2.6%)
P2	42.54 MHz	42.82 MHz	42.72 MHz	45.01 MHz	2.47 MHz (5.8%)

Both builds land in a tight band regardless of seed — nothing close to the P2/P4/P8 same-tier benchmark's own non-monotonic swings (that benchmark's design uses <2% of the device, so its placer has enormous freedom; spi_neuron_top's fuller design does not). This confirms a real structural bottleneck, not a lucky/unlucky placement — a seed sweep or floorplan constraint will not close a ~2x gap to 80 MHz by itself; an RTL change is needed.

Candidate directions (not implemented, need a decision before touching the validated core further):

• Move x_mem/w_mem onto DP16KD block RAM (0% used) instead of LUT fabric — the leading candidate. The critical path runs through what looks like w_mem's write-address decode (neuron_memory loads one byte per cycle into a variable index during STATE_READ_X/STATE_READ_W, which needs a full-width mux/demux in LUT fabric to steer that byte to the right register) feeding forward into the accumulator's input path — not the arithmetic itself. A block-RAM read/write port replaces that scattered LUT-based decode with a single hard macro. This is a genuine redesign, not a drop-in swap: x_bus/w_bus are currently exposed to neuron_parallel as one fully-parallel N_INPUTS-wide combinational bus (built by unrolling every x_mem/w_mem element every cycle); block RAM has a registered, address-in/ data-out-next-cycle read port, so neuron_parallel's group processing would need to become RAM-latency-aware instead of assuming the whole bus is already valid. Real potential upside, real design effort — needs a go-ahead, not something to do opportunistically.
• A pipeline register between MAC accumulation and the activation/saturate stage (this session's original guess) does not target the actual Phase 5 bottleneck — that finding was Phase 4's critical path (the saturation comparator), and the path moved once Phase 5's logic was wired in (see above). Keeping this noted for Phase 4-only builds; not a fix for the current problem.

Phase 8 — Optional Hardware Training

Investigate:

• backpropagation;
• gradient calculation;
• weight updates;
• hardware-assisted training.

---

16. Long-Term Vision

The final objective is to create a reusable hardware block that can be integrated into different future MIKILAB projects.

                         APPLICATION
                              │
                ┌─────────────┴─────────────┐
                │                           │
             Linux                       ESP32
                │                           │
                └─────────────┬─────────────┘
                              │
                         SPI / Dual SPI
                              │
                              ▼
                 ┌────────────────────────┐
                 │          FPGA          │
                 │                        │
                 │ Neural Network Engine  │
                 │                        │
                 │ ┌────────────────────┐ │
                 │ │ Control            │ │
                 │ ├────────────────────┤ │
                 │ │ Input Interface    │ │
                 │ ├────────────────────┤ │
                 │ │ NN Compute Core    │ │
                 │ ├────────────────────┤ │
                 │ │ Activation         │ │
                 │ ├────────────────────┤ │
                 │ │ Output Interface   │ │
                 │ └────────────────────┘ │
                 │                        │
                 │ Dedicated RAM          │
                 │                        │
                 └────────────────────────┘

The host platform can change without changing the fundamental Neural Network Engine architecture.

The FPGA becomes a dedicated neural-computation peripheral, analogous to other hardware accelerators, but optimized specifically for the neural-network topology required by each application.

---

17. Current Status

Component	Status
Parametric neuron layer	OK Working
Parametric input count	OK
Parametric neuron count	OK
Parametric parallelism	OK
Accumulation	OK
Bias	OK
ReLU	OK
32×4 / P=8 validation	OK
Dedicated RAM architecture	OK (memory_interface + psram_controller + int8_memory_access, real-PSRAM tested)
SPI interface	OK (spi_slave + spi_engine, all 9 opcodes incl. RUN_NETWORK, real-toolchain Fmax checked in isolation)
Dual SPI	Future
Multi-layer engine	- RTL + unit tests + real end-to-end (simulated SPI) done; real toolchain checked (P8: FAIL 40.57 MHz, worse than Phase 4 alone)
Configurable activation functions	OK (ACT_NONE / ACT_RELU, per-layer via descriptor table or per-run via SET_BASE)
Runtime network width (one bitstream, any topology up to build max)	OK (per-layer or per-run n_inputs_real/n_neurons_real, real cycle savings measured end to end)
Linux host driver	Planned
ESP32 host driver	Planned
Hardware training	Future

---

Core architectural principle

The FPGA implements the neural-network machine.
The FPGA owns its RAM.
The host configures and uses the machine.
A build sets the machine's ceiling (max layers, max width, PARALLEL); the host configures the actual network — layer count, per-layer input/output width, per-layer activation, and trained parameters — entirely at runtime, over SPI, into FPGA-local memory. One bitstream serves any topology up to that ceiling (2026-09-02, see §17 "Runtime network width").

This separation is the foundation of the project.