RP2040 PIO - set(pins, 1) works fine, but mov(pins, x) or out(pins, 1) does not

[electronich]

Hello, sorry for the very fundamental question.

I've started writing an I2C/I3C library using PIO for my own learning (and testing) purposes.

I'm using GPIO2 (SDA) and GPIO3 (SCL) - both declared as outputs for the time being. I found that I can "set" the pin states high or low, however I can't assign the pin state to scratch register x or shift a bit in from the OSR.

This command works fine:
sm_i2c.exec("set(pins, 0).side(0)") # both pins low

This command DOES NOT work:
sm_i2c.exec("out(pins, 1).side(0)") # output 1-bit from OSR --> SDA

I suspect my error is related to the setup/config of the PIO block?

Other information:

• Currently using MicroPython v1.25.0 on 2025-04-15
• Adafruit Feather RP2040 with RP2040

Here's my full code:

from machine import Pin
import rp2
from time import sleep_ms

# PIO Examples:
# Micropython - https://github.com/raspberrypi/pico-micropython-examples/tree/master/pio
# C++ - https://github.com/raspberrypi/pico-examples/tree/master/pio

# Reference:
# https://docs.micropython.org/en/latest/library/rp2.html#module-rp2
# https://docs.micropython.org/en/latest/library/rp2.StateMachine.html#rp2.StateMachine


@rp2.asm_pio(set_init=rp2.PIO.OUT_HIGH, sideset_init=rp2.PIO.OUT_HIGH)
def i2c_test():
    pull(block) # Pull 32-bit from the TX FIFO into OSR.. or wait forever!
    set(y, 7) # prepare to loop n+1 times

    set(pins, 0)                .side(1)
    
    label("transmit_addr")
    nop()                       .side(1)
    nop()                       .side(0)
    out(pins, 1)                .side(0)
    jmp(y_dec, "transmit_addr") .side(1)
    set(pins, 1)                .side(1)


def generate_sm_i2c(F_clk_target, PIN_SCL, PIN_SDA):
    sm_i2c = rp2.StateMachine(0)
    sm_i2c.init(i2c_test, freq=F_clk_target*4,
                in_base=PIN_SDA, # in command not used yet
                out_base=PIN_SDA,
                set_base=PIN_SDA,
                sideset_base=PIN_SCL,
                out_shiftdir=rp2.PIO.SHIFT_RIGHT,
                in_shiftdir=rp2.PIO.SHIFT_RIGHT)
    
    return sm_i2c


if __name__ == "__main__":
    # https://docs.micropython.org/en/latest/library/machine.Pin.html
    PIN_SCL = Pin(3, Pin.OUT) # RP2040 GPIO3, SCL
    PIN_SDA = Pin(2, Pin.OUT) # RP2040 GPIO2, SDA
    
    sm_i2c = generate_sm_i2c(100000, PIN_SCL, PIN_SDA)
    sm_i2c.restart()
    
    sleep_ms(200)
    
    # (works) test that all pins toggle as expected
    sm_i2c.exec("set(pins, 0).side(0)") # both pins low 
    sm_i2c.exec("set(pins, 1).side(0)") # SDA low
    sm_i2c.exec("set(pins, 0).side(1)") # SCL low
    sm_i2c.exec("set(pins, 1).side(1)")
    
    # (works) Check if PIO can be sent a value, and return the same value
    # TX FIFO --> OSR --> ISR --> RX FIFO
    sm_i2c.put(0xAAAAAAAA)
    sm_i2c.exec("pull(block)") # TX FIFO --> OSR
    sm_i2c.exec("out(isr, 32)") # OSR --> ISR
    sm_i2c.exec("push(block)") # ISR --> RX FIFO
    print(hex(sm_i2c.get())) # output is 0xaaaaaaaa
    
    
    # (doesn't work!!) Transmit a number, one bit at a time, on SDA
    # TX FIFO --> OSR --> repeat(1-bit to pins)
    sm_i2c.put(0xAAAAAAAA)
    sm_i2c.exec("pull(block)")
    for i in range(4):
        sleep_ms(250)
        sm_i2c.exec("out(pins, 1).side(0)") # output 1-bit from OSR --> SDA
        sleep_ms(250)
        sm_i2c.exec("out(pins, 1).side(1)") # output another 1-bit from OSR --> SDA
    sm_i2c.exec("out(isr, 32)") # shift whatever remains in OSR --> ISR
    sm_i2c.exec("push(block)") # push ISR --> RX_FIFO
    print(hex(sm_i2c.get())) # print remainder of RX_FIFO, expected to be 8-bits shorter than what we input (measured: 0xaaaaaa)
    
    sleep_ms(1000)

    # (doesn't work!!) Transmit a number, one bit at a time, on SDA
    # TX FIFO --> OSR --> X --> pins
    sm_i2c.put(0xAAAAAAAA)
    sm_i2c.exec("pull(block)")
    for i in range(4):
        sleep_ms(250)
        sm_i2c.exec("out(x, 1).side(0)")
        sm_i2c.exec("mov(pins, x).side(0)")
        sleep_ms(250)
        sm_i2c.exec("out(x, 1).side(1)")
        sm_i2c.exec("mov(pins, x).side(1)")
    sm_i2c.exec("out(isr, 32)") # shift whatever remains in OSR --> ISR
    sm_i2c.exec("push(block)") # push ISR --> RX_FIFO
    print(hex(sm_i2c.get())) # print remainder of RX_FIFO, expected to be 8-bits shorter than what we input (measured: 0xaaaaaa)
    
    sleep_ms(1000)
    
    # (doesn't work!!) See if pins can be set to value of x
    # value 0 or 1 --> x --> pins
    for i in range(4):
        sleep_ms(250)
        sm_i2c.exec("set(x, 1).side(0)")
        sm_i2c.exec("mov(pins, x).side(0)")
        sleep_ms(250)
        sm_i2c.exec("set(x, 1).side(1)")
        sm_i2c.exec("mov(pins, x).side(1)")

[GitHubsSilverBullet]

Could be that the StateMachine need to be active to handle out():
sm_i2c.active(True)

[electronich]

Thanks for taking a look Norbert. It appears I didn't configure the PIO properly... I needed to set out_init=rp2.PIO.OUT_HIGH in my @rp2.asm_pio decorator:

@rp2.asm_pio(out_init=rp2.PIO.OUT_HIGH, set_init=rp2.PIO.OUT_HIGH, sideset_init=rp2.PIO.OUT_HIGH)

Without that line, the PIO out(pins, 1) instruction didn't work. With that line, out(pins, 1) appears to work as expected. I will update this ticket once my code is working properly