Datasheet

The PIO has a total of nine instructions: JMP, WAIT, IN, OUT, PUSH, PULL, MOV, IRQ, and SET. See Section 3.4 for details on these
instructions.
Though the PIO only has a total of nine instructions, it would be difficult to edit PIO program binaries by hand. PIO
assembly is a textual format, describing a PIO program, where each command corresponds to one instruction in the
output binary. Below is an example program in PIO assembly:
Pico Examples: https://github.com/raspberrypi/pico-examples/tree/master/pio/squarewave/squarewave.pio Lines 7 - 12
Ê7 .program squarewave
Ê8 set pindirs, 1 ; Set pin to output
Ê9 again:
10 set pins, 1 [1] ; Drive pin high and then delay for one cycle
11 set pins, 0 ; Drive pin low
12 jmp again ; Set PC to label `again`
The PIO assembler is included with the SDK, and is called pioasm. This program processes a PIO assembly input text file,
which may contain multiple programs, and writes out the assembled programs ready for use. For the SDK these
assembled programs are emitted in form of C headers, containing constant arrays: For more information see Section
3.3
3.2.2. Control Flow
On every system clock cycle, each state machine fetches, decodes and executes one instruction. Each instruction takes
precisely one cycle, unless it explicitly stalls (such as the WAIT instruction). Instructions may also insert a delay of up to
31 cycles before the next instruction is executed to aid the writing of cycle-exact programs.
The program counter, or PC, points to the location in the instruction memory being executed on this cycle. Generally, PC
increments by one each cycle, wrapping at the end of the instruction memory. Jump instructions are an exception and
explicitly provide the next value that PC will take.
Our example assembly program (listed as .program squarewave above) shows both of these concepts in practice. It drives
a 50/50 duty cycle square wave onto a GPIO, with a period of four cycles. Using some other features (e.g. side-set) this
can be made as low as two cycles.

NOTE
Side-set is where a state machine drives a small number of GPIOs in addition to the main side effects of the
instruction it executes. It’s described fully in Section 3.5.1.
The system has write-only access to the instruction memory, which is used to load programs:
Pico Examples: https://github.com/raspberrypi/pico-examples/tree/master/pio/squarewave/squarewave.c Lines 34 - 38
34 // Load the assembled program directly into the PIO's instruction memory.
35 // Each PIO instance has a 32-slot instruction memory, which all 4 state
36 // machines can see. The system has write-only access.
37 for (int i = 0; i < count_of(squarewave_program_instructions); ++i)
38 pio->instr_mem[i] = squarewave_program_instructions[i];
The clock divider slows the state machine’s execution by a constant factor, represented as a 16.8 fixed-point fractional
number. Using the above example, if a clock division of 2.5 were programmed, the square wave would have a period of
cycles. This is useful for setting a precise baud rate for a serial interface, such as a UART.
RP2040 Datasheet
3.2. Programmer’s Model 332