488 pio-ram-emulator example: Julia fractal :: Quicker, easier and cheaper to make your own chip!

How it works

This is an example of the using the https://github.com/toivoh/pio-ram-emulator RAM emulator for Tiny Tapeout. The RAM is used to store a frame buffer, 320x480 at 2 bits/pixel. The frame buffer is continuously read to output a 640x480 @60 Hz VGA signal. At the same time, the logic computes a Julia fractal, writing 16 bits to the frame buffer for every 8 pixels computed. After about a second, the whole frame buffer is filled in.

For more info about the RAM emulator, see https://github.com/toivoh/pio-ram-emulator/blob/main/docs/pio-ram-emulator.md.

The project contains some helper code for working with with the RAM emulator:

pio_ram_emulator.v and pio_ram_emulator.vh (sb_io.v is also need) contain the modules pio_ram_emu_transmitter and pio_ram_emu_receiver
- These are used to transmit and receive messages using the RAM emulator's message format
- The design still has to follow the rules in https://github.com/toivoh/pio-ram-emulator/blob/main/docs/pio-ram-emulator.md about which messages can be sent when
- See julia_top.v for an example of how to use these modules
test/pio_ram_emulator_model.v contains a simulation model of the RAM emulator
- See test/tb.v for an example of how to use the simulation model in a test
- See verilator/vtop.v for an example of how to use the simulation model in a verilator setup
- The model will try to detect behavior that violates the rules in https://github.com/toivoh/pio-ram-emulator/blob/main/docs/pio-ram-emulator.md, in which case it will set an error flag and stop responding (see the ERROR_RESPONSE parameter)
- The simulation model is helpful, but might not capture the ful behavior of the RAM emulator. Please try to run your design on an FPGA against the actual RAM emulator as well.

How to test

Plug in a TinyVGA VGA Pmod to the output Pmod. The https://github.com/toivoh/pio-ram-emulator RAM emulator must be running on the RP2040. TODO: Instructions for how to set up. Start the project.

Controls

The appearance of the Julia fractal is controlled by the C parameter, which can be seen as a complex value or 2d vector. The C paramter can be changed using the ui_in port:

button_up / button_down / button_left / button_right move the C value.
button_incstep doubles the step length.
button_decstep halves the step length.

A new ui_in[5:0] value must be stable for 2^19 cycles, or approximately 10 ms (at a 50.4 MHz clock rate), before it is accepted. The use_both_button_dirs input changes how the input is interpreted:

When use_both_button_dirs = 0, an input is triggered when one of the button_ signals goes from high to low (and is stable for 10 ms). Recommended if the inputs are connected to buttons.
When use_both_button_dirs = 1, an input is triggered when one of the button_ signals goes from high to low or low to high (and is stable for 10 ms). Recommended if the inputs are connected to toggle switches.

External hardware

This project needs a TinyVGA VGA Pmod.

#	Input	Output	Bidirectional
0	button_up	R1
1	button_down	G1
2	button_right	B1
3	button_left	vsync
4	button_incstep	R0	tx_out[0]
5	button_decstep	G0	tx_out[1]
6		B0	rx_in[0]
7	use_both_button_dirs	hsync	rx_in[1]

488 pio-ram-emulator example: Julia fractal