704 Tiny Canvas :: Quicker, easier and cheaper to make your own chip!

How it works

Tiny Canvas is a hardware implementation of an MS Paint-style drawing application. It receives input from an SNES-compatible gamepad controller via the Gamepad PMOD and streams canvas coordinates and color data over I2C to a host device.

Core Functionality

Color Mixing: The A, Y, and X buttons toggle Red, Green, and Blue color channels respectively. These combine additively to produce 8 colors:

R+G = Yellow, R+B = Magenta, G+B = Cyan, R+G+B = White
When no color is selected, the cursor moves without painting

Brush Controls:

L/R shoulder buttons adjust brush size from 1×1 to 8×8 pixels
Start button cycles through symmetry modes: Off → Horizontal → Vertical → 4-Way

Fill Rectangle: Select button toggles fill mode. Press B to set corner A, move cursor, press B again to set corner B and fill the rectangle.

Undo/Redo: L+R together undoes, Select+Start together redoes (4-operation buffer).

Architecture

The design consists of modular Verilog components:

gamepad_pmod_single: Decodes SNES controller serial protocol
colour: RGB mixing logic with smart paint-enable
position: 8-bit X/Y cursor tracking
brush_settings: Size (0-7) and symmetry mode (0-3) control
packet_generator: Expands single pixels to brush footprint with symmetry
fill_mode / fill_draw: Fill rectangle state machine and pixel generator
undo_redo: 4-entry circular buffer for undo/redo operations
i2c_slave: Read-only I2C slave at address 0x64

I2C Protocol

Reading 3 bytes from address 0x64 returns:

Byte	Content
0	X position (0-255)
1	Y position (0-255)
2	Status: `[7:4]=D-pad, [3]=BrushMode, [2:0]=RGB`

How to test

Using the Interactive Emulator

Install dependencies: pip install pygame cocotb
Run the emulator: python test/interactive_emulator.py
Use keyboard controls:
- Arrow keys: Move cursor
- R/G/B: Toggle color channels
- +/-: Adjust brush size
- S: Cycle symmetry mode
- F: Toggle fill mode
- Space: Set fill corner
- Z/Y: Undo/Redo

On Hardware

Connect the Gamepad PMOD to ui[0:2] (data, clock, latch)
Connect I2C: SDA to uio[1], SCL to uio[2]
Poll the I2C slave at address 0x64 to read canvas state
Use controller buttons as documented to draw

Running Cocotb Tests

cd test
make

External hardware

Required:

Gamepad PMOD - SNES controller interface
SNES-compatible game controller

For canvas display:

I2C master device (e.g., RP2040 on the Tiny Tapeout demoboard)
Display or computer running host software to render the canvas

The I2C interface streams X, Y, and color data at each paint event, allowing a host device to render the 256×256 canvas in real-time.

#	Input	Output	Bidirectional
0		i2c_state[0]
1		i2c_state[1]	SDA
2		i2c_state[2]	SCL
3
4	pmod_latch
5	pmod_clk
6	pmod_data
7

704 Tiny Canvas

704 : Tiny Canvas