805 Interactive XOR Plasma (Spade HDL) :: Quicker, easier and cheaper to make your own chip!

How it works

This project renders a full-screen animated XOR 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 Spade HDL and compiled to SystemVerilog via swim build before synthesis.

Plasma formula

Each active pixel is coloured by XOR-combining screen coordinates with an 8-bit frame counter:

animated = pattern(hcount[7:0], vcount[7:0]) XOR frame_ctr

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

`ui_in[2:1]`	Pattern	Formula
`00`	Grid (default)	`hcount XOR vcount`
`01`	Diagonal	`hcount + vcount`
`10`	Interference	`hcount XOR (hcount + vcount)`
`11`	Plasma	`hcount XOR vcount XOR (hcount + vcount)`

Colour mapping

The 8-bit animated value is split into three 2-bit pairs and assigned to the R, G, B channels. The colour rotation control (ui_in[7:6]) shifts which bits of animated feed each channel, cycling through four assignments and producing distinct colour palettes.

With invert (ui_in[5]) high, all colour bits are complemented before output.

Animation

frame_ctr advances by 1 (or by ×2, ×4, ×8 depending on ui_in[4:3]) once per frame. Asserting ui_in[0] freezes frame_ctr — pattern, invert, and colour rotation 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
        | rot  |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` = grid · `01` = diagonal · `10` = interference · `11` = plasma
`ui_in[4:3]`	Speed	Frame counter step: `00`=×1 · `01`=×2 · `10`=×4 · `11`=×8
`ui_in[5]`	Invert	1 = complement all colour bits
`ui_in[7:6]`	Colour rotation	Rotate R/G/B channel assignment through four 2-bit pairs of `animated`

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: grid XOR pattern, speed ×1, standard colours
`0x01`	Freeze the animation at the current frame
`0x06`	Switch to diagonal pattern (`ui_in[2:1] = 01`)
`0x0E`	Interference pattern at speed ×2
`0x18`	Speed ×4 (`ui_in[4:3] = 10`)
`0x20`	Invert all colours
`0x40`	Colour rotation mode 1 (`ui_in[7:6] = 01`)
`0xFF`	All controls: plasma pattern, speed ×8, inverted, rotation 3, frozen

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

To test in simulation:

cd spade && swim build && cp build/spade.sv ../src/design.v && cd ..
cd test && make clean && make

A passing run reports TESTS=8 PASS=8 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)

805 Interactive XOR Plasma (Spade HDL)