79 Hey FlexCompute-130 :: Quicker, easier and cheaper to make your own chip!

Hey FlexCompute-130

How it works

The Hey FlexCompute-130 is a hybrid computational accelerator designed for the Tiny Tapeout 1x1 tile. It combines two powerful mathematical engines in a single design: a resource-optimized INT8 Neural Processing Unit (NPU) and a 12-bit CORDIC Processor.

1. NPU Core (Sequential MAC)

To fit within the strict ~1,000 gate limit, the NPU uses a Folded MAC Architecture.

Sequential Multiplier: Replaced the area-heavy combinational multiplier with an 8-cycle shift-and-add unit.
Signed Arithmetic: Full 2's complement support for both weights and inputs.
SPI Weight Streaming: Features a custom SPI receiver with CDC synchronization to stream weights from external memory.
Zero-Cost ReLU: Integrated activation function directly in the output stage.

2. CORDIC Core (Trigonometric/Hyperbolic)

A bit-iterative hardware engine that computes complex functions using only shifts and additions.

12-bit Precision: Uses a Q2.10 fixed-point format (1 sign, 1 integer, 10 fractional bits).
Versatile Modes:
- Circular: Computes Sine and Cosine.
- Hyperbolic: Computes Sinh and Cosh (includes the $i=4, 13 \dots$ iteration repeats for convergence).
- Vectoring: Computes the Arctangent (Atan) of an input coordinate.
Pre-computed LUT: Uses a high-precision atan_lut for rotation angles.

How to test

The chip's behavior is controlled by the Mode Pins (uio[7:6]).

Pin Mapping Overview

Pin	Function	Mode dependency
`uio[7:6]`	Mode Select	00=NPU, 01=Circular, 10=Hyperbolic, 11=Atan
`uio[4]`	Byte Select	0=Output Low Byte [7:0], 1=Output High Byte [15:8]
`uio[3]`	Selector / Reset	NPU: Clear Acc. CORDIC: 0=X (Cos), 1=Y (Sin)
`uio[5]`	Done Pulse	Pulses high when a calculation is finished

Testing the NPU (Mode 00)

Clear: Pulse uio[3] high to reset the accumulator.
Input: Provide an 8-bit feature on ui_in.
Weight: Stream an 8-bit weight via SPI (uio[0]=SCLK, uio[1]=MOSI, uio[2]=CS).
Read: Toggle uio[4] to read the 16-bit result from uo_out.

Testing CORDIC (Modes 01, 10, 11)

Mode: Set uio[7:6] to 01 (Sin/Cos) or 10 (Sinh/Cosh).
Input: Provide the angle $\theta$ on ui_in (Scale: $1.0 \text{ rad} \approx 128$ in decimal).
Wait: Wait for the uio[5] pulse (approx 12-15 cycles).
Select Result:
- Set uio[3] = 0 for Cosine / Cosh.
- Set uio[3] = 1 for Sine / Sinh.
Read: Use uio[4] to read the 16-bit result in two bytes.

IO Connections

Port	Pin	Name	Description
Input	`ui[7:0]`	`data_in`	8-bit Data/Angle Input
Output	`uo[7:0]`	`acc_out`	8-bit Multiplexed Output
Bidirectional	`uio[0]`	`sclk`	SPI Clock
	`uio[1]`	`mosi`	SPI Data
	`uio[2]`	`cs`	SPI Chip Select
	`uio[3]`	`ctrl`	Reset Acc / Function Select
	`uio[4]`	`sel`	Byte Selector (Low/High)
	`uio[5]`	`done`	Done Flag
	`uio[7:6]`	`mode`	Mode Selection

#	Input	Output	Bidirectional
0	data_in[0]	acc_out[0]	spi_sclk
1	data_in[1]	acc_out[1]	spi_mosi
2	data_in[2]	acc_out[2]	spi_cs
3	data_in[3]	acc_out[3]	reset_acc
4	data_in[4]	acc_out[4]	byte_sel
5	data_in[5]	acc_out[5]	layer_done
6	data_in[6]	acc_out[6]	Active Mode[0]
7	data_in[7]	acc_out[7]	Active Mode[1]

79 Hey FlexCompute-130