558 TCD Foxworks PicoRV32
558 : TCD Foxworks PicoRV32
- Author: Michael John Flynn
- Description: Modified open source PicoRV32 core to target TinyTapeout
- GitHub repository
- Open in 3D viewer
- Open in Tiny Tapeout Explorer
- Clock: 25000000 Hz
How it works
A small RISC-V microcontroller: a PicoRV32 core (RV32I) that executes in place from an external SPI flash, with 128 bytes of on-chip SRAM for stack and data, a UART, and an 8-in/8-out GPIO port. There is no on-chip program memory — every instruction is fetched over SPI as it runs, so the design fits in a small area at the cost of speed.
Memory map: SRAM at 0x0000_0000; flash execute-in-place window from 0x80 (reset vector 0x400); UART at 0x0200_0004/8; GPIO at 0x0300_0000.
Timing closed at 50MHz, however testing conducted at 25MHz clock with SPI-flash at clk/2.
How to test
Attach an SPI flash (or an RP2040 emulating one) holding a RISC-V
program binary compiled according to the rulesets defined in the fw directory, hold rst_n low while it powers up, raise ena, then release
rst_n. The chip boots and runs. A UART console at 115200 8N1 reports
what it's doing; the sample firmware either:
- BOUNCE: prints a banner and bounces a lit bit across the 8 output pins, with speed set by the input pins.
- SELFTEST: runs a system diagnostic and outputs the results over UART. retrigerrable by writing a
'?'to the UART.
External hardware
An SPI flash on uio[3:0], or an RP2040 running the Tiny Tapeout spi-ram-emu firmware to emulate one. A USB-serial adapter on the UART pins (uio[6]/uio[7]) for the console. Optionally switches on the input PMOD and LEDs on the output PMOD.
Pinout
| Pin | Function |
|---|---|
| ui[7:0] | GPIO in (readable at 0x0300_0000) |
| uo[7:0] | GPIO out |
| uio[0] | FLASH_SCK (out) |
| uio[1] | FLASH_CSB (out) |
| uio[2] | FLASH_IO0 / MOSI (bidir) |
| uio[3] | FLASH_IO1 / MISO (bidir) |
| uio[4] | FLASH_IO2 (driven 1 = /WP) |
| uio[5] | FLASH_IO3 (driven 1 = /HOLD) |
| uio[6] | SERIAL_TX (115200) |
| uio[7] | SERIAL_RX |
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | GPIO in 0 | GPIO out 0 | FLASH_SCK |
| 1 | GPIO in 1 | GPIO out 1 | FLASH_CSB |
| 2 | GPIO in 2 | GPIO out 2 | FLASH_IO0 (MOSI) |
| 3 | GPIO in 3 | GPIO out 3 | FLASH_IO1 (MISO) |
| 4 | GPIO in 4 | GPIO out 4 | FLASH_IO2 (/WP, driven 1) |
| 5 | GPIO in 5 | GPIO out 5 | FLASH_IO3 (/HOLD, driven 1) |
| 6 | GPIO in 6 | GPIO out 6 | SERIAL_TX |
| 7 | GPIO in 7 | GPIO out 7 | SERIAL_RX |