34 Neuromorphic Spike Codec (GF180) :: Quicker, easier and cheaper to make your own chip!

How it works

A compact 1×1-tile neuromorphic spike codec for TinyTapeout GF 26a that combines three encoders and three symmetric decoders in a single SPI-programmable block.

The block operates on a shared 8-bit window counter (WINDOW_N, 1..255). Encoder and decoder modes are selected independently via the CTRL register; both sides may be enabled simultaneously, e.g. an encoder upstream of an external interconnect under test and a decoder on the return path.

Encoders (enc_mode)

Rate (2'b00) — each cycle of the active window, an 8-bit maximal Fibonacci LFSR (taps 7, 5, 4, 3) is compared against enc_in; if lfsr < enc_in, uo_out[0] = spike_pos fires for that cycle. Produces a Bernoulli rate code whose expected spike count is WINDOW_N × enc_in / 256.
Latency (2'b01) — on enc_in write (or external start_ext), the block loads a countdown of 255 − enc_in cycles. A single spike_pos fires when the countdown reaches zero, yielding a time-to-first-spike code where larger values fire earlier.
Delta (2'b10) — maintains a running 8-bit reference. Writing enc_in triggers a comparison: if the signed difference exceeds +DELTA_THR, a spike_pos fires and the reference moves up by one threshold; if it is below −DELTA_THR, a spike_neg fires and the reference moves down.

Decoders (dec_mode)

Rate (2'b00) — counts rising edges of ui_in[0] over the window, saturates at 255, latches into DEC_OUT at window rollover.
Latency (2'b01) — records the cycle index of the first ui_in[0] rising edge in the window and publishes 255 − index (0 if no spike seen).
Delta (2'b10) — maintains a signed 16-bit accumulator driven by spike_pos = ui_in[0] and spike_neg = uio_in[6], adding / subtracting DELTA_STEP per spike and applying DELTA_LEAK per cycle toward zero. DEC_OUT is the offset-binary saturated high byte of the accumulator.

Both encoder and decoder provide sticky status latches (W1C on STATUS) plus a combined irq output so a small host can service spikes only at window boundaries.

How to test

Reset (rst_n = 0 for ≥ 10 clocks).
Program over SPI (16-bit frame {R/Wn, 7-bit addr, 8-bit data}, mode 0):
- WINDOW_N (0x01): desired window size.
- DELTA_THR (0x02), DELTA_STEP (0x03), DELTA_LEAK (0x04) as needed for delta modes.
- LFSR_SEED (0x05) for a deterministic rate code (0 clamps to 1).
Write CTRL (0x00) with enc_en=1 / dec_en=1 and the desired mode bits to start the block.
Encode: stream values into ENC_IN (0x06). Observe spikes on uo_out[0] (pos) and uo_out[1] (neg). uo_out[2] = enc_valid pulses at window end or latency fire.
Decode: feed spikes into ui_in[0] (and uio_in[6] for delta neg). Poll DEC_OUT (0x13) after uo_out[3] = dec_valid pulses, or freeze the decoder by writing dec_en = 0 to snapshot the in-flight state into DEC_OUT.
Poll STATUS (0x20) for sticky spike / valid flags and write 1 to a bit to clear it (W1C semantics).

External hardware

SPI master (Raspberry Pi / microcontroller) for register programming and readback of DEC_OUT / STATUS.
Any spike source on ui_in[0] (and optionally uio_in[6] for bipolar delta decoding) — logic analyser, function generator, or another TTGF tile.
Optional: pair with tt_um_santhosh_snn_wta_gf (tile 4) — route spike-codec encoder outputs into the SNN inference core, decode the winner spikes back into an 8-bit signed value.

Register summary

Addr	Name	R/W
0x00	CTRL	W
0x01	WINDOW_N	W
0x02	DELTA_THR	W
0x03	DELTA_STEP	W
0x04	DELTA_LEAK	W
0x05	LFSR_SEED	W
0x06	ENC_IN	W
0x07	DEC_RESET	W
0x10	ENC_PULSE	R
0x11	ENC_STATE	R
0x12	ENC_WIN_CNT	R
0x13	DEC_OUT	R
0x14	DEC_STATE_L	R
0x15	DEC_STATE_H	R
0x20	STATUS	R/W1C

CTRL bits

Bit	Name	Meaning
0	enc_en	Enable encoder
1	dec_en	Enable decoder (falling edge snapshots DEC_OUT)
3:2	enc_mode	00=rate, 01=latency, 10=delta, 11=reserved
5:4	dec_mode	00=rate, 01=latency, 10=delta, 11=reserved
6	start_enc	Software re-arm of latency / delta encoder

#	Input	Output	Bidirectional
0	spike_in (decoder external spike)	spike_pos	spi_cs_n
1	start_ext (external encoder re-arm pulse)	spike_neg	spi_mosi
2	reserved	enc_valid	spi_miso
3	reserved	dec_valid	spi_sck
4	reserved	busy	overflow_ever
5	reserved	any_spike	irq
6	reserved	window_tick	spike_neg_in (delta decoder negative channel)
7	reserved	lfsr_msb	reserved

34 Neuromorphic Spike Codec (GF180)