425 4-Tap Signed MAC Unit :: Quicker, easier and cheaper to make your own chip!

425 : 4-Tap Signed MAC Unit

Author: Mohammed Abdul Mujeeb

Description: Four-tap signed multiply-accumulate unit for DSP and dot-product operations.

Clock: 50000000 Hz

How it works

The 4-Tap Signed MAC Unit performs four signed 4-bit multiplication operations and combines their results into a multiply-accumulate result.

The operation is:

Y = (W0 × X0) + (W1 × X1) + (W2 × X2) + (W3 × X3)

Four signed 4-bit weights and four signed 4-bit samples are stored internally. Each corresponding weight and sample are multiplied to produce an 8-bit signed product. The four products are then added together.

The design also supports a running accumulation mode. When ACCUMULATE is asserted, the newly calculated MAC sum is added to the previously stored accumulator value. Otherwise, the current MAC operation replaces the accumulator value.

Input and control signals

Pin	Function
`ui_in[1:0]`	Selects one of the four MAC taps
`ui_in[2]`	`LOAD_WEIGHT` — loads a weight into the selected tap
`ui_in[3]`	`LOAD_SAMPLE` — loads a sample into the selected tap
`ui_in[4]`	`START_MAC` — starts the MAC calculation
`ui_in[5]`	`ACCUMULATE` — adds the new result to the existing accumulator
`ui_in[6]`	`CLEAR_ACC` — clears the accumulator and result
`ui_in[7]`	Unused
`uio_in[3:0]`	Signed 4-bit data input
`uio_in[7:4]`	Unused

The calculated result is available on uo_out[7:0]. The internal accumulator is 12 bits wide, while the lower 8 bits are provided at the output.

The design uses an active-low reset through rst_n and operates synchronously with the Tiny Tapeout clk input.

How to test

First apply reset to initialize the four weights, four samples, accumulator, and output.

Loading a weight

Select the desired tap using ui_in[1:0].
Place the signed 4-bit weight on uio_in[3:0].
Assert ui_in[2] (LOAD_WEIGHT) for one clock cycle.

Loading a sample

Select the desired tap using ui_in[1:0].
Place the signed 4-bit sample on uio_in[3:0].
Assert ui_in[3] (LOAD_SAMPLE) for one clock cycle.

Repeat these operations for all four taps.

After loading the weights and samples, assert ui_in[4] (START_MAC) to perform the calculation.

For example:

Weights = [1, 2, 3, 4]
Samples = [1, -1, 2, -2]

The expected MAC result is:

(1 × 1) + (2 × -1) + (3 × 2) + (4 × -2)
= 1 - 2 + 6 - 8
= -3

The signed result -3 is represented in two's complement. Its lower 8 bits can be observed on uo_out[7:0].

To perform another MAC operation while retaining the previous result, assert ui_in[5] (ACCUMULATE). To clear the stored result before a new calculation, assert ui_in[6] (CLEAR_ACC).

External hardware

No external hardware is required for simulation.

For a physical demonstration, the ui_in control signals and uio_in[3:0] data bus can be driven by an FPGA, microcontroller, or other digital controller.

The uo_out[7:0] bus can be connected to a logic analyzer or other digital interface to observe the MAC result.

#	Input	Output	Bidirectional
0	TAP_SEL[0]	MAC_RESULT[0]	DATA_IN[0]
1	TAP_SEL[1]	MAC_RESULT[1]	DATA_IN[1]
2	LOAD_WEIGHT	MAC_RESULT[2]	DATA_IN[2]
3	LOAD_SAMPLE	MAC_RESULT[3]	DATA_IN[3]
4	START_MAC	MAC_RESULT[4]	UNUSED
5	ACCUMULATE	MAC_RESULT[5]	UNUSED
6	CLEAR_ACC	MAC_RESULT[6]	UNUSED
7	UNUSED	MAC_RESULT[7]	UNUSED

Input

Output

Bidirectional

TAP_SEL[0]

MAC_RESULT[0]

DATA_IN[0]

TAP_SEL[1]

MAC_RESULT[1]

DATA_IN[1]

LOAD_WEIGHT

MAC_RESULT[2]

DATA_IN[2]

LOAD_SAMPLE

MAC_RESULT[3]

DATA_IN[3]

START_MAC

MAC_RESULT[4]

UNUSED

ACCUMULATE

MAC_RESULT[5]

UNUSED

CLEAR_ACC

MAC_RESULT[6]

UNUSED

MAC_RESULT[7]

UNUSED

Chip location

425 4-Tap Signed MAC Unit