Datasheet

Here we load an example program into the state machine, which does two things:
•
Enters an infinite loop
•
Enters a loop which repeatedly pulls 32 bits of data from the TX FIFO, and executes the lower 16 bits as an
instruction
The C program sets the state machine running, at which point it enters the hang loop. While the state machine is still
running, the C program forces in a jmp instruction, which causes the state machine to break out of the loop.
When an instruction is written to the INSTR register, the state machine immediately decodes and executes that
instruction, rather than the instruction it would have fetched from the PIO’s instruction memory. The program counter
does not advance, so on the next cycle (assuming the instruction forced into the INSTR interface did not stall) the state
machine continues to execute its current program from the point where it left off, unless the written instruction itself
manipulated PC.
Delay cycles are ignored on instructions written to the INSTR register, and execute immediately, ignoring the state
machine clock divider. This interface is provided for performing initial setup and effecting control flow changes, so it
executes instructions in a timely manner, no matter how the state machine is configured.
Instructions written to the INSTR register are permitted to stall, in which case the state machine will latch this instruction
internally until it completes. This is signified by the EXECCTRL_EXEC_STALLED flag. This can be cleared by restarting the state
machine, or writing a NOP to INSTR.
In the second phase of the example state machine program, the OUT EXEC instruction is used. The OUT itself occupies one
execution cycle, and the instruction which the OUT executes is on the next execution cycle. Note that one of the
instructions we execute is also an OUT — the state machine is only capable of executing one OUT instruction on any given
cycle.
OUT EXEC works by writing the OUT shift data to an internal instruction latch. On the next cycle, the state machine
remembers it must execute from this latch rather than the instruction memory, and also knows to not advance PC on this
second cycle.
This program will print "12345678" when run.

CAUTION
If an instruction written to INSTR stalls, it is stored in the same instruction latch used by OUT EXEC and MOV EXEC, and will
overwrite an in-progress instruction there. If EXEC instructions are used, instructions written to INSTR must not stall.
3.6. Examples
These examples illustrate some of PIO’s hardware features, by implementing common I/O interfaces.
Looking to get started?
The Raspberry Pi Pico C/C++ SDK book has a comprehensive PIO chapter, which walks through writing
and building a first PIO application, and goes on to walk through some programs line-by-line. It also
covers broader topics such as using PIO with DMA, and goes into much more depth on how PIO can be
integrated into your software.
3.6.1. Duplex SPI
RP2040 Datasheet
3.6. Examples 363