CircuitPython asyncio on RP2350B with 2 busio UARTS + 6 PIO UARTs

code.py

# SPDX-FileCopyrightText: 2025 anecdata
# SPDX-License-Identifier: MIT

import time
import random
import storage
import board
import busio
import asyncio
import adafruit_pio_uart

# RP2xx0 UART test
# Pimoroni pico lipo 2XL W w/ Pimoroni PGA2350 firmware

# busio sw buffer 8 bytes
# busio default receiver_buffer_size 64 bytes (max 65535 bytes)
# raspberrypi hw uart rx buffer 32 bytes
# PIO uart rx buffer 8 bytes
MAXBUF = 8
MIN_INTERVAL = 2.5
label =  storage.getmount("/").label

def write(t, u):
    m = bytearray()

    # construct MAXBUF-byte chunk (≥8)
    m.extend(str(t))  # 1-byte UART number
    m.extend(label[2:5])  # 3-bytes from unique CIRCUITPY re-name
    m.extend(f"{str(round(time.monotonic()) % 10_000):0>4}")  # 4-byte monotonic time
    m.extend(f"{'x' * (MAXBUF - 8)}")

    print(f"{time.monotonic():.3f}s uart={t} writing {bytes(m)}", end=" ")
    written = u.write(m)
    print(f"{time.monotonic():.3f}s uart={t} {written=} {bytes(m)}")

async def awrite(t, u):
    print(f"WRITE uart={t} started")
    while True:
        write(t, u)
        await asyncio.sleep(MIN_INTERVAL + random.random())

def read(t, u):
    print(f"{time.monotonic():.3f}s uart={t} {u.in_waiting=}", end=" ")
    m = u.read(MAXBUF)
    print(f"{time.monotonic():.3f}s uart={t} read {bytes(m)}")

async def aread(t, u):
    print(f"READ uart={t} started")
    while True:
        if u.in_waiting:
            read(t, u)
        await asyncio.sleep(0)

async def main():
    # RP2350B non-wifi has enough state machines & pins for:
        # 6 PIO UARTs with flow control
        # and
        # 2 busio UARTs with [optional] flow control
    uarts = []

    # PIO UART
    # RP2040: 8 PIO state machines; 3.5 UARTs w/ wifi, 4 UARTs w/o wifi UF2
    # RP2350: 12 PIO state machines; 5.5 UARTs w/ wifi, 6 UARTs w/o wifi UF2
    # use hardware flow control b/c PIO buffers are very small
    uarts.append(adafruit_pio_uart.UART(board.GP0, board.GP1, 9600, 8, None, 1, 1, board.GP2, board.GP3))
    uarts.append(adafruit_pio_uart.UART(board.GP4, board.GP5, 9600, 8, None, 1, 1, board.GP6, board.GP7))
    uarts.append(adafruit_pio_uart.UART(board.GP8, board.GP9, 9600, 8, None, 1, 1, board.GP10, board.GP11))
    uarts.append(adafruit_pio_uart.UART(board.GP12, board.GP13, 9600, 8, None, 1, 1, board.GP14, board.GP15))
    uarts.append(adafruit_pio_uart.UART(board.GP16, board.GP17, 9600, 8, None, 1, 1, board.GP18, board.GP19))
    uarts.append(adafruit_pio_uart.UART(board.GP20, board.GP21, 9600, 8, None, 1, 1, board.GP26, board.GP27))
    # GP >=32: ValueError: program[6] waits on input outside of count
    # https://github.com/adafruit/circuitpython/issues/10619

    # busio UART
    # 2 UART peripherals; hardware flow control not typically needed because of software receiver buffer
    # peripheral 0
    uarts.append(busio.UART(board.GP32, board.GP33))  # cts=board.GP34, rts=board.GP35, receiver_buffer_size=MAXBUF))
    # peripheral 1
    uarts.append(busio.UART(board.GP36, board.GP37))  # rts=board.GP38, rts=board.GP39, receiver_buffer_size=MAXBUF))

    tasks = []
    for taskno in range(len(uarts)):
        uarts[taskno].reset_input_buffer()
        tasks.append(asyncio.create_task(aread(taskno, uarts[taskno])))
        tasks.append(asyncio.create_task(awrite(taskno, uarts[taskno])))
    print(f"created {len(tasks)} tasks")

    while True:
        await asyncio.sleep(0)

time.sleep(3)  # wait for serial after reset
asyncio.run(main())

With hardware flow control, writes will block indefinitely, so make sure the reads keep up. Especially important for PIO since the receive buffers are only 8 bytes, and there is no software buffering in the library currently. More experiments with UARTs with and without flow control here and here.

Choosing pins for PIO can be challenging. Each PIO block (3 in RP2350) has 4 state machines. A bi-directional UART uses 2 state machines. Each PIO block can connect to a set of 30 GPIO, which must be within a GPIO 'bank' of 32 consecutive pins. So with multiple UARTs, any 2 UARTs in the same PIO block (presumably init'd sequentially) should use pins that are relatively close together (not in different 'banks'. See: adafruit/circuitpython#10627