803 Interactive Triangle-Wave Plasma (Clash HDL) :: Quicker, easier and cheaper to make your own chip!

How it works

This project renders a full-screen animated triangle-wave plasma effect on a VGA display at 640×480 resolution, 60 Hz refresh rate, using a 25 MHz pixel clock. The design is authored in Clash HDL — a Haskell-based hardware description language — and compiled to Verilog via cabal exec clash before synthesis.

Plasma formula

The effect uses triangle-wave approximations of sine functions (no lookup table required). Three overlapping waves are evaluated at different spatial angles across the screen and summed into a single 8-bit plasma value. This value is mapped to 2-bit RGB with 120-degree phase offsets between the channels, producing 64 distinct colours per frame.

Four pattern variants are selectable via ui_in[2:1]:

`ui_in[2:1]`	Pattern
`00`	Three-wave plasma (default)
`01`	Two-wave (horizontal + vertical)
`10`	Diagonal waves
`11`	XOR plasma

Colour mapping

The plasma value is split into three segments with 120-degree phase offsets and mapped to the R, G, B channels. The palette control (ui_in[7:6]) selects one of four colour mappings:

`ui_in[7:6]`	Palette
`00`	RGB with 120° phase offsets (default)
`01`	Shifted hue
`10`	Fire
`11`	Greyscale

With invert (ui_in[5]) high, the plasma value is complemented before colour mapping.

Animation

An 8-bit frame counter advances once per frame, animating the plasma. The speed control (ui_in[4:3]) sets the counter step per frame. Asserting ui_in[0] freezes the counter — pattern, invert, and palette continue to render at the frozen frame.

Input controls

All eight input pins are active and take effect concurrently:

  bit:  7   6   5   4   3   2   1   0
        |palette|inv|  speed  |pattern|pause
         [7:6]   [5] [4:3]   [2:1]    [0]

Pin(s)	Field	Values
`ui_in[0]`	Pause	1 = freeze animation, 0 = run
`ui_in[2:1]`	Pattern	`00` = three-wave · `01` = two-wave · `10` = diagonal · `11` = XOR plasma
`ui_in[4:3]`	Speed	Frame counter step: `00`=×1 · `01`=×2 · `10`=×4 · `11`=×8
`ui_in[5]`	Invert	1 = complement plasma value before colour mapping
`ui_in[7:6]`	Palette	`00` = RGB 120° · `01` = shifted hue · `10` = fire · `11` = greyscale

VGA timing

Standard 640×480 @ 60 Hz:

HSYNC: active-low pulse during horizontal counts 656–751
VSYNC: active-low pulse during vertical counts 490–491

Output is mapped to the Tiny Tapeout VGA PMOD standard on uo_out[7:0]:

Bit	Signal	Bit	Signal
7	hsync	3	vsync
6	B0	2	B1
5	G0	1	G1
4	R0	0	R1

How to test

Connect a VGA monitor via the Tiny Tapeout VGA PMOD board plugged into the uo output port. Apply a 25 MHz clock to clk. Assert rst_n low briefly on power-up, then release it high.

You should see a full-screen animated colour plasma. Use the ui_in pins to interact with it in real time:

`ui_in` value	Effect
`0x00`	Default: three-wave plasma, speed ×1, RGB palette
`0x01`	Freeze the animation at the current frame
`0x06`	Switch to two-wave pattern (`ui_in[2:1] = 01`)
`0x18`	Speed ×4 (`ui_in[4:3] = 10`)
`0x20`	Invert the plasma value
`0x40`	Shifted-hue palette (`ui_in[7:6] = 01`)
`0x80`	Fire palette (`ui_in[7:6] = 10`)
`0xC0`	Greyscale palette (`ui_in[7:6] = 11`)
`0xFF`	All controls: XOR plasma, speed ×8, inverted, greyscale, frozen

Multiple pins may be set high simultaneously — all fields take effect concurrently.

To test in simulation:

scripts/build-clash.sh          # generate src/gvt_core.v
cd test && make clean && make

A passing run reports TESTS=3 PASS=3 FAIL=0. Waveforms are written to test/tb.fst and can be inspected with GTKWave or Surfer.

External hardware

Tiny Tapeout VGA PMOD (plugged into the uo output port) — provides the VGA connector and resistor-ladder DAC
VGA monitor — any standard monitor or capture card accepting 640×480 @ 60 Hz
Up to 8 push-buttons or DIP switches wired to ui_in[7:0] (optional, for interactive control)

803 Interactive Triangle-Wave Plasma (Clash HDL)