425 8-bit Multiply-Accumulate (MAC) with 2-Cycle Serial Interface :: Quicker, easier and cheaper to make your own chip!

How it works

This project is an 8×8→16‑bit Multiply–Accumulate (MAC) peripheral designed for the TinyTapeout platform. It is ideal for DSP applications or any design requiring efficient, repeated multiplication and addition.

To fit within the limited I/O of TinyTapeout, the MAC core uses a 2‑cycle 8‑bit serial interface. This allows two 8-bit operands to be sent to the core and a full 16‑bit result to be read back, all through a standard 8-bit data bus. The module also supports configurable signed/unsigned arithmetic and provides overflow detection.

Key Features

Compact MAC Core: Provides a full 8x8 MAC unit with a 17-bit accumulator and overflow detection.
2-Cycle Serial Interface: A simple 2‑cycle input/output protocol allows full 16-bit operations using only 8-bit data ports, making it easy to integrate with microcontrollers or other hosts.
Signed/Unsigned Support: A dedicated control pin (signed_mode) allows switching between signed and unsigned arithmetic.
High-Speed Operation: Maintains full 50 MHz operation with a deterministic 4‑cycle pipeline latency.

Architecture

The peripheral's architecture is a 4-stage pipeline designed for stable, high-speed operation:

Input Stage & Serial Interface: Captures and assembles the two 8‑bit operands and control signals over two clock cycles. A change detector triggers the pipeline only when new, stable data is available.
Pipeline Register Stage: Registers the inputs for timing closure and passes the signed_mode setting to the multiplier and accumulator.
Multiplier Stage: A configurable 8×8 block that produces a 16‑bit product, supporting both signed and unsigned modes.
Accumulator Stage: A 17‑bit adder with clear_and_mult control. It accumulates results and sets an overflow flag if the result exceeds the 16-bit range.

Block Diagram

Pinout

Pin	Direction	Function
`ui_in[7:0]`	Input	8-bit Data Bus. Used for Data A (cycle 1) and Data B (cycle 2).
`uio_in[0]`	Input	`clear_and_mult` (0 = accumulate, 1 = clear before multiplying)
`uio_in[1]`	Input	`enable` (Must be high during the 2-cycle input phase)
`uio_in[2]`	Input	`signed_mode` (0 = unsigned, 1 = signed)
`uo_out[7:0]`	Output	8-bit Data Bus. Cycles between the high and low bytes of the 16-bit result.
`uio_out[0]`	Output	`overflow` flag (High when an arithmetic overflow occurs)
`uio_out[1]`	Output	`data_ready` flag (High when a new result is available)

How to Use the MAC

Operating the MAC involves sending two 8-bit operands and control signals over two clock cycles, waiting for the pipeline to process, and then reading the 16-bit result over two subsequent cycles.

Data Transmission Protocol

Input Protocol (2 cycles):

Cycle 1:
- Place 8-bit Data A on ui_in[7:0].
- Set uio_in[1] to 1 to enable the interface.
- Set uio_in[0] (clear_and_mult) and uio_in[2] (signed_mode) as needed. These values are only captured on the first cycle.
Cycle 2:
- Place 8-bit Data B on ui_in[7:0].
- Keep uio_in[1] (enable) at 1.
After Cycle 2, set uio_in[1] (enable) to 0 to complete the input operation.

Output Protocol (2 cycles):

After an operation is complete (approx. 4-6 cycles), the data_ready flag (uio_out[1]) will go high.
The 16-bit result is available on uo_out[7:0] and cycles between the high and low bytes on every clock edge.
Read Cycle 1: Capture the High Byte (bits 15:8) of the result.
Read Cycle 2: Capture the Low Byte (bits 7:0) of the result.
The overflow flag is available on uio_out[0].

Usage Examples

Example 1: Basic Multiplication (`5 * 6`)

// 1. Send data to calculate 5 * 6 = 30
// Cycle 1: Send Data A (5) and control signals (clear=1, signed=0)
ui_in <= 8'h05;
uio_in <= 3'b011; // {signed_mode, enable, clear_and_mult}

// Cycle 2: Send Data B (6)
ui_in <= 8'h06;
uio_in <= 3'b010; // {signed_mode, enable, clear_and_mult} - only enable matters

// 2. Wait ~4-6 cycles for the pipeline.

// 3. Read the result (30 = 0x001E)
// Read Cycle 1: uo_out will be 0x00 (High Byte)
// Read Cycle 2: uo_out will be 0x1E (Low Byte)

Example 2: Accumulation (`100 + 25`)

// 1. First, calculate 10 * 10 = 100 with clear_and_mult = 1
// ... send 10 and 10 ...

// 2. Wait for the operation to complete.

// 3. Next, calculate 5 * 5 = 25 with clear_and_mult = 0 to accumulate
// Cycle 1: Send Data A (5) and control signals (clear=0, signed=0)
ui_in <= 8'h05;
uio_in <= 3'b010; // {signed_mode, enable, clear_and_mult}

// Cycle 2: Send Data B (5)
ui_in <= 8'h05;
uio_in <= 3'b010;

// 4. Wait for the pipeline.

// 5. Read the result (125 = 0x007D)
// Read Cycle 1: uo_out will be 0x00 (High Byte)
// Read Cycle 2: uo_out will be 0x7D (Low Byte)

Example 3: Signed Multiplication (`10 * -5`)

// 1. Send data for 10 * -5 = -50, with signed_mode = 1
//    -5 in 8-bit two's complement is 0xFB (251)
// Cycle 1: Send Data A (10) and control signals (clear=1, signed=1)
ui_in <= 8'h0A;
uio_in <= 3'b111; // {signed_mode, enable, clear_and_mult}

// Cycle 2: Send Data B (251)
ui_in <= 8'hFB;
uio_in <= 3'b110;

// 2. Wait for the pipeline.

// 3. Read the result (-50 = 0xFFCE in 16-bit two's complement)
// Read Cycle 1: uo_out will be 0xFF (High Byte)
// Read Cycle 2: uo_out will be 0xCE (Low Byte)

External hardware

No external hardware is required. This is a purely digital design that operates with:

Clock: A 50MHz system clock from the TinyTapeout board.
Reset: An active-low reset signal.
Digital I/O: The standard TinyTapeout pin interface for sending operands and control signals.

#	Input	Output	Bidirectional
0	Data[7:0] - 8-bit data input (Cycle 1: Data A, Cycle 2: Data B)	Result[7:0] - 8-bit data output (cycles between low/high bytes)	Clear_and_Mult (IN) / Overflow (OUT) - Control input or overflow flag output
1	Data[7:0] - 8-bit data input (Cycle 1: Data A, Cycle 2: Data B)	Result[7:0] - 8-bit data output (cycles between low/high bytes)	Enable (IN) / Data_Ready (OUT) - Interface enable input or data ready output
2	Data[7:0] - 8-bit data input (Cycle 1: Data A, Cycle 2: Data B)	Result[7:0] - 8-bit data output (cycles between low/high bytes)	Signed_Mode (IN) - Signed mode control (0=unsigned, 1=signed)
3	Data[7:0] - 8-bit data input (Cycle 1: Data A, Cycle 2: Data B)	Result[7:0] - 8-bit data output (cycles between low/high bytes)
4	Data[7:0] - 8-bit data input (Cycle 1: Data A, Cycle 2: Data B)	Result[7:0] - 8-bit data output (cycles between low/high bytes)
5	Data[7:0] - 8-bit data input (Cycle 1: Data A, Cycle 2: Data B)	Result[7:0] - 8-bit data output (cycles between low/high bytes)
6	Data[7:0] - 8-bit data input (Cycle 1: Data A, Cycle 2: Data B)	Result[7:0] - 8-bit data output (cycles between low/high bytes)
7	Data[7:0] - 8-bit data input (Cycle 1: Data A, Cycle 2: Data B)	Result[7:0] - 8-bit data output (cycles between low/high bytes)

425 8-bit Multiply-Accumulate (MAC) with 2-Cycle Serial Interface