Datasheet

The side-set data is encoded in the Delay/side-set field of each instruction. Any instruction can be combined with side-
set, including instructions which write to the pins, such as OUT PINS or SET PINS. Side-set’s pin mapping is independent
from OUT and SET mappings, though it may overlap. If side-set and an OUT or SET write to the same pin simultaneously, the
side-set data is used.

NOTE
If an instruction stalls, the side-set still takes effect immediately.
1 .program spi_tx_fast
2 .side_set 1
3
4 loop:
5 out pins, 1 side 0
6 jmp loop side 1
The spi_tx_fast example shows two benefits of this: data and clock transitions can be more precisely co-aligned, and
programs can be made faster overall, with an output of one bit per two system clock cycles in this case. Programs can
also be made smaller.
There are four things to configure when using side-set:
1.
The number of MSBs of the Delay/side-set field to use for side-set rather than delay. This is configured by
PINCTRL_SIDESET_COUNT. If this is set to 5, delay cycles are not available. If set to 0, no side-set will take place.
2. Whether to use the most significant of these bits as an enable. Side-set takes place on instructions where the
enable is high. If there is no enable bit, every instruction on that state machine will perform a side-set, if
SIDESET_COUNT is nonzero. This is configured by EXECCTRL_SIDE_EN.
3.
The GPIO number to map the least-significant side-set bit to. Configured by PINCTRL_SIDESET_BASE.
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:
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 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:
RP2040 Datasheet
3.5. Functional Details 351