PIO State Machines

State machines transfer data to/from other entities using First-In-First-Out (FIFO) buffers, which allow the state machine and main processor to work independently yet also synchronise their data. The FIFOs can be linked to the DMA to transfer larger amounts of data without extensive CPU usage.

Instructions

Each PIO block has 32 instructions of memory shared among its four state machines.

All PIO instructions follow a common pattern:

<instruction> .side(<side_set_value>) [<delay_value>]

The side-set .side(...) and delay [...] parts are both optional, and if specified allow the instruction to perform more than one operation. This keeps PIO programs small and efficient.

There are nine instructions which perform the following tasks:

• jmp() transfers control to a different part of the code

• wait() pauses until a particular action happens

• in_() shifts the bits from a source (scratch register or set of pins) to the input shift register

• out() shifts the bits from the output shift register to a destination

• push() sends data to the RX FIFO

• pull() receives data from the TX FIFO

• mov() moves data from a source to a destination

• irq() sets or clears an IRQ flag

• set() writes a literal value to a destination

The instruction modifiers are:

• .side() sets the side-set pins at the start of the instruction

• [] delays for a certain number of cycles after execution of the instruction

There are also directives:

• wrap_target() specifies where the program execution will get continued from

• wrap() specifies the instruction where the control flow of the program will get wrapped from

• label() sets a label for use with jmp() instructions

• word() emits a raw 16-bit value which acts as an instruction in the program

An example

Take the pio_1hz.py example for a simple understanding of how to use the PIO and state machines. Below is the code for reference.

# Example using PIO to blink an LED and raise an IRQ at 1Hz.

import time
from machine import Pin
import rp2


@rp2.asm_pio(set_init=rp2.PIO.OUT_LOW)
def blink_1hz():
    # Cycles: 1 + 1 + 6 + 32 * (30 + 1) = 1000
    irq(rel(0))
    set(pins, 1)
    set(x, 31)                  [5]
    label("delay_high")
    nop()                       [29]
    jmp(x_dec, "delay_high")

    # Cycles: 1 + 1 + 6 + 32 * (30 + 1) = 1000
    nop()
    set(pins, 0)
    set(x, 31)                  [5]
    label("delay_low")
    nop()                       [29]
    jmp(x_dec, "delay_low")


# Create the StateMachine with the blink_1hz program, outputting on Pin(25).
sm = rp2.StateMachine(0, blink_1hz, freq=2000, set_base=Pin(25))

# Set the IRQ handler to print the millisecond timestamp.
sm.irq(lambda p: print(time.ticks_ms()))

# Start the StateMachine.
sm.active(1)

This creates an instance of class rp2.StateMachine which runs the blink_1hz program at 2000Hz, and connects to pin 25. The blink_1hz program uses the PIO to blink an LED connected to this pin at 1Hz, and also raises an IRQ as the LED turns on. This IRQ then calls the lambda function which prints out a millisecond timestamp.

The blink_1hz program is a PIO assembler routine. It connects to a single pin which is configured as an output and starts out low. The instructions do the following:

• irq(rel(0)) raises the IRQ associated with the state machine.

• The LED is turned on via the set(pins, 1) instruction.

• The value 31 is put into register X, and then there is a delay for 5 more cycles, specified by the [5].

• The nop() [29] instruction waits for 30 cycles.

• The jmp(x_dec, "delay_high") will keep looping to the delay_high label as long as the register X is non-zero, and will also post-decrement X. Since X starts with the value 31 this jump will happen 31 times, so the nop() [29] runs 32 times in total (note there is also one instruction cycle taken by the jmp for each of these 32 loops).

• The single nop() correlates with the cycle used for IRQ raise, and ensures the same number of cycles are used for LED on and LED off.

• set(pins, 0) will turn the LED off by setting pin 25 low.

• Another 32 loops of nop() [29] and jmp(...) will execute.

• Because wrap_target() and wrap() are not specified, their default will be used and execution of the program will wrap around from the bottom to the top. This wrapping does not cost any execution cycles.

The entire routine takes exactly 2000 cycles of the state machine. Setting the frequency of the state machine to 2000Hz makes the LED blink at 1Hz.

This example works identically on both RP2040 and RP2350 (using StateMachine ID 0). On RP2350, higher freq values are possible due to the faster system clock.

Compatibility Notes

PIO programs using the basic instructions shown here are fully compatible between RP2040 and RP2350. The MicroPython rp2 module API (including @rp2.asm_pio, StateMachine, etc.) is the same, though RP2350 supports StateMachine IDs 0–11.

RP2350 offers additional advanced features (new instructions, FIFO modes, security, GPIO relocation) not covered in this introduction; see the RP2350 datasheet for details. Some programs may require minor adjustments due to differences in default GPIO reset states between the two chips.