RGB Mixer Chip¶

Why this project¶

I work at a FabLab with high school students. I wanted a project that is concrete enough for a beginner — turn a knob, see an LED change — but technical enough to go deep: encoder decoding, debouncing, PWM, synthesis, timing closure, tapeout. RGB Mixer works on both levels.

This was also personally hard. I made many mistakes throughout this course — wrong PDK paths, broken netlists, failed GDS exports — and had to work through each one. That process is documented in the session pages. The chip design itself is not complex, but understanding why each step works took real effort.

Reference design: tt05 RGB Mixer by Matt Venn — reimplemented from scratch.

What it does¶

Three rotary encoders control the brightness of Red, Green, and Blue LEDs independently via PWM. Turn clockwise to increase brightness, counter-clockwise to decrease. Three identical pwm_channel modules run in parallel inside rgb_top.

Block Diagram¶

RGB Mixer block diagram

Each channel: encoder input → debounce → 8-bit counter → PWM generator → LED output. Clock: 10 MHz.

Simulation¶

GTKWave — pwm_r, pwm_g, pwm_b

All three PWM channels verified with an iverilog testbench. At reset each channel defaults to 50% duty cycle.

Synthesis¶

Yosys — 270 cells

270 Sky130 standard cells after Yosys synthesis. 48× dfrtp_1 flip-flops (the three 8-bit counters). Zero latches.

Place & Route¶

OpenROAD layout

OpenROAD timing report

270 cells placed in 2658 µm² at 11% utilization. Timing closed with 98.29 ns slack against a 100 ns clock period.

Layout¶

KLayout — dfxtp_1 poly layer

dfxtp_1 D flip-flop from the Sky130 standard cell library — the building block of the counters.

KLayout full render

Numbers¶

Parameter	Value
Process	Sky130 — 130nm
Standard cells	270
Design area	2658 µm²
Utilization	11%
Total power	54.3 µW
Timing slack	98.29 ns (MET ✓)
Critical path	1.58 ns
Supply voltage	1.8V

Tools¶

Stage	Tool
RTL Simulation	iverilog + GTKWave
Linting	Verilator
Synthesis	Yosys
Place & Route	OpenROAD
Layout Viewer	KLayout
Analog Verification	ngspice
PDK	SkyWater Sky130