tapeout/docs/info.md

How it works

A small hardware accelerator for single-bit operations on 64-bit words. It computes the bit's address and performs a read-modify-write when needed.

effective bit = (base * 8) + offset          (64-bit arithmetic)
word          = effective bit / 64
bit index     = effective bit mod 64

Opcode	Operation	Writes	Result bit
0	GET	no	the bit
1	TEST	no	the bit
2	SET	yes	1
3	CLEAR	yes	0
4	TOGGLE	yes	the new bit
5-7	undefined	no	error

The chip holds 4 words of RAM (byte addresses 0-31). An address outside that range ends the operation with an error and never writes.

Everything is driven over a UART (8 data bits, no parity, 1 stop bit). All multi-byte fields are little-endian:

Host sends	Chip replies
P	B	ping
W idx data[8]	K	write word idx (0-3)
R idx	data[8]	read word idx
X op base[8] offset[8]	0xA0 + 2·error + bit	run one operation
anything else	?

The design is the bit_accelerator core from https://github.com/SNAPKITTYAGENT9NOVA/bit-string-accelerator. Before hardening it was checked as follows:

• a self-checking testbench, with random backpressure, read latency and fault injection
• Why3 proofs of the address arithmetic
• the same RTL running on iCE40 and ECP5 FPGAs
• a formal equivalence proof of its sky130 netlist

How to test

1. Set the clock, and set the UART divisor pins to match it. The chip uses 4 × d clocks per bit, where d = {ui[7:4], ui[2:0]}. With all divisor pins low it uses 104 clocks per bit, which is 115200 baud at a 12 MHz clock. Other examples:

   Clock	Baud	d	ui[7:4]	ui[2:0]
   12 MHz	115200	0 (default)	0000	000
   25 MHz	115200	54 (216 clocks)	0110	110
   50 MHz	115200	108 (432 clocks)	1101	100

2. Connect a 3.3 V USB-UART adapter: adapter TX to ui[3] (RX), adapter RX to uo[4] (TX).

3. Reset the design. uo[0] is high while an operation runs. uo[1] is the last operation's error flag. uo[2] is its result bit.

4. Use the host tool from the repository (fpga/host, Rust):

   bitacc --port /dev/ttyUSB0 --words 4 ping                  # pong
   bitacc --port /dev/ttyUSB0 --words 4 write 0 0x8000000000000001
   bitacc --port /dev/ttyUSB0 --words 4 exec get 0 63         # 1
   bitacc --port /dev/ttyUSB0 --words 4 exec toggle 0 5       # 1
   bitacc --port /dev/ttyUSB0 --words 4 read 0                # 0x8000000000000021
   bitacc --port /dev/ttyUSB0 --words 4 exec get 32 0         # error (outside RAM)
   bitacc --port /dev/ttyUSB0 --words 4 selftest 10000        # 0 mismatches
   

selftest runs random operations, including faults and undefined opcodes, and checks every reply and the final RAM contents against a model of the design.

External hardware

A 3.3 V USB-UART adapter (for example an FTDI or CP2102 cable) on ui[3]/uo[4].