357 Three Channel RGB PWM Controller :: Quicker, easier and cheaper to make your own chip!

How it works

This project is a three-channel RGB Pulse Width Modulation (PWM) controller designed for TinyTapeout.

The design stores independent brightness values for red, green, and blue LED channels and continuously generates PWM waveforms on the outputs. The PWM duty cycle determines the effective brightness of each color channel.

The 8-bit input bus is divided into:

Bits	Purpose
ui_in[7:6]	Mode selection
ui_in[5:0]	Value data

The mode bits determine which internal register is updated.

Mode	Function
00	Set RED brightness
01	Set GREEN brightness
10	Set BLUE brightness
11	Set PWM clock divider

The PWM engine operates using:

a programmable clock divider
a 6-bit PWM counter
comparator-based PWM generation

Each output compares the PWM counter against its stored duty-cycle register:

PWM output HIGH when:
counter < duty_cycle

This creates variable duty-cycle square waves suitable for LED brightness control.

Internal Architecture

                ui_in[7:0]
                     │
                     ▼
              ┌────────────┐
              │   Decoder  │
              └─────┬──────┘
                    │
      ┌─────────────┼─────────────┐
      ▼             ▼             ▼
   RED REG      GREEN REG      BLUE REG
      │             │             │
      └─────────────┼─────────────┘
                    ▼
              PWM Comparators
                    │
                    ▼
                RGB Outputs

PWM Generation

A programmable clock divider slows the incoming TinyTapeout clock to generate a visible PWM frequency.

A 6-bit counter continuously ramps:

0 → 1 → 2 → ... → 63 → repeat

The RGB outputs turn on whenever the counter value is below the stored duty-cycle value.

Example:

Duty = 32

counter < 32 → HIGH
counter ≥ 32 → LOW

This creates a 50% duty cycle.

How to test

The design is tested using cocotb and Icarus Verilog simulation.

Simulation Inputs

The user controls the design through the ui_in[7:0] bus.

Example Commands

Set RED brightness to maximum

mode  = 00
value = 63

ui_in = 00111111

Set GREEN brightness to medium brightness

mode  = 01
value = 32

ui_in = 01100000

Set BLUE brightness to low brightness

mode  = 10
value = 8

ui_in = 10001000

Change PWM speed

mode  = 11
value = 4

ui_in = 11000100

Running Simulation

Simulation is performed using:

cd test
make

Waveforms can then be viewed using GTKWave or Surfer.

Verification Goals

The testbench verifies:

register updates
PWM activity
RGB output generation
clock divider operation
reset behavior

External hardware

The outputs are intended to drive:

RGB LEDs
transistor driver stages
logic-level PWM-compatible devices

For direct LED use:

connect current-limiting resistors in series with each LED channel
connect PWM outputs to the LED control pins
ensure current limits are respected

Typical usage:

uo_out[0] → Red LED
uo_out[1] → Green LED
uo_out[2] → Blue LED

The design may also interface with:

MOSFET drivers
LED strips
external power stages
digital lighting systems

#	Input	Output
0	value bit 0	PWM Red
1	value bit 1	PWM Green
2	value bit 2	PWM Blue
3	value bit 3
4	value bit 4
5	value bit 5
6	mode bit 0
7	mode bit 1

357 Three Channel RGB PWM Controller