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class StateMachine – access to the RP2040’s programmable I/O interface

The StateMachine class gives access to the RP2040’s PIO (programmable I/O) interface.

For assembling PIO programs, see rp2.asm_pio().


class rp2.StateMachine(id[, program, ...])

Get the state machine numbered id. The RP2040 has two identical PIO instances, each with 4 state machines: so there are 8 state machines in total, numbered 0 to 7.

Optionally initialize it with the given program program: see StateMachine.init.


StateMachine.init(program, freq=-1, *, in_base=None, out_base=None, set_base=None, jmp_pin=None, sideset_base=None, in_shiftdir=None, out_shiftdir=None, push_thresh=None, pull_thresh=None)

Configure the state machine instance to run the given program.

The program is added to the instruction memory of this PIO instance. If the instruction memory already contains this program, then its offset is reused so as to save on instruction memory.

  • freq is the frequency in Hz to run the state machine at. Defaults to the system clock frequency.

    The clock divider is computed as system clock frequency / freq, so there can be slight rounding errors.

    The minimum possible clock divider is one 65536th of the system clock: so at the default system clock frequency of 125MHz, the minimum value of freq is 1908. To run state machines at slower frequencies, you’ll need to reduce the system clock speed with machine.freq().

  • in_base is the first pin to use for in() instructions.

  • out_base is the first pin to use for out() instructions.

  • set_base is the first pin to use for set() instructions.

  • jmp_pin is the first pin to use for jmp(pin, ...) instructions.

  • sideset_base is the first pin to use for side-setting.

  • in_shiftdir is the direction the ISR will shift, either PIO.SHIFT_LEFT or PIO.SHIFT_RIGHT.

  • out_shiftdir is the direction the OSR will shift, either PIO.SHIFT_LEFT or PIO.SHIFT_RIGHT.

  • push_thresh is the threshold in bits before auto-push or conditional re-pushing is triggered.

  • pull_thresh is the threshold in bits before auto-pull or conditional re-pulling is triggered.[value])

Gets or sets whether the state machine is currently running.


Restarts the state machine and jumps to the beginning of the program.

This method clears the state machine’s internal state using the RP2040’s SM_RESTART register. This includes:

  • input and output shift counters

  • the contents of the input shift register

  • the delay counter

  • the waiting-on-IRQ state

  • a stalled instruction run using StateMachine.exec()


Execute a single PIO instruction.

If instr is a string then uses asm_pio_encode to encode the instruction from the given string.

>>> sm.exec("set(0, 1)")

If instr is an integer then it is treated as an already encoded PIO machine code instruction to be executed.

>>> sm.exec(rp2.asm_pio_encode("out(y, 8)", 0))
StateMachine.get(buf=None, shift=0)

Pull a word from the state machine’s RX FIFO.

If the FIFO is empty, it blocks until data arrives (i.e. the state machine pushes a word).

The value is shifted right by shift bits before returning, i.e. the return value is word >> shift.

StateMachine.put(value, shift=0)

Push words onto the state machine’s TX FIFO.

value can be an integer, an array of type B, H or I, or a bytearray.

This method will block until all words have been written to the FIFO. If the FIFO is, or becomes, full, the method will block until the state machine pulls enough words to complete the write.

Each word is first shifted left by shift bits, i.e. the state machine receives word << shift.


Returns the number of words in the state machine’s RX FIFO. A value of 0 indicates the FIFO is empty.

Useful for checking if data is waiting to be read, before calling StateMachine.get().


Returns the number of words in the state machine’s TX FIFO. A value of 0 indicates the FIFO is empty.

Useful for checking if there is space to push another word using StateMachine.put().

StateMachine.irq(handler=None, trigger=0 | 1, hard=False)

Returns the IRQ object for the given StateMachine.

Optionally configure it.

Buffer protocol

The StateMachine class supports the buffer protocol, allowing direct access to the transmit and receive FIFOs for each state machine. This is primarily in order to allow StateMachine objects to be passed directly as the read or write parameters when configuring a rp2.DMA() channel.