Datasheet

4.
Whether side-set writes to GPIO levels or GPIO directions. Configured by EXECCTRL_SIDE_PINDIR
In the above example, we have only one side-set data bit, and every instruction performs a side-set, so no enable bit is
required. SIDESET_COUNT would be 1, SIDE_EN would be false. SIDE_PINDIR would also be false, as we want to drive the clock
high and low, not high- and low-impedance. SIDESET_BASE would select the GPIO the clock is driven from.
3.5.2. Program Wrapping
PIO programs often have an "outer loop": they perform the same sequence of steps, repetitively, as they transfer a stream
of data between the FIFOs and the outside world. The square wave program from the introduction is a minimal example
of this:
1 .program squarewave
2 again:
3 set pins, 1 [1] ; Drive pin high and then delay for one cycle
4 set pins, 0 ; Drive pin low
5 jmp again ; Set PC to label `again`
The main body of the program drives a pin high, and then low, producing one period of a square wave. The entire program
then loops, driving a periodic output. The jump itself takes one cycle, as does each set instruction, so to keep the high and
low periods of the same duration, the set pins, 1 has a single delay cycle added, which makes the state machine idle for
one cycle before executing the set pins, 0 instruction. In total, each loop takes four cycles. There are two frustrations
here:
•
The JMP takes up space in the instruction memory that could be used for other programs
•
The extra cycle taken to execute the JMP ends up halving the maximum output rate
As the Program Counter (PC) naturally wraps to 0 when incremented past 31, we could solve the second of these by filling
the entire instruction memory with a repeating pattern of set pins, 1 and set pins, 0, but this is wasteful. State machines
have a hardware feature, configured via their EXECCTRL control register, which solves this common case.
1 .program squarewave_wrap
2
3 .wrap_target
4 set pins, 1 [1] ; Drive pin high and then delay for one cycle
5 set pins, 0 [1] ; Drive pin low and then delay for one cycle
6 .wrap
After executing an instruction from the program memory, state machines use the following logic to update PC:
1.
If the current instruction is a JMP, and the Condition is true, set PC to the Target
2.
Otherwise, if PC matches EXECCTRL_WRAP_TOP, set PC to EXECCTRL_WRAP_BOTTOM
3.
Otherwise, increment PC, or set to 0 if the current value is 31.
The .wrap_target and .wrap assembly directives are essentially labels. They export constants which can be written to the
WRAP_BOTTOM and WRAP_TOP control fields, respectively:
1 // --- squarewave_wrap ---
2
3 static const uint16_t squarewave_wrap_program[] = {
4 0xe101, // 00
5 0xe100, // 01
6 };
7
RP2040 Datasheet
3.5. Functional Details 333