Datasheet

Autopull (see Section 3.5.4) allows the hardware to automatically refill the OSR in the majority of cases, with the state
machine stalling if it tries to OUT from an empty OSR. This has two benefits:
•
No instructions spent on explicitly pulling from FIFO at the right time
•
Higher throughput: can output up to 32 bits on every single clock cycle, if the FIFO stays topped up
After configuring autopull, the above program can be simplified to the following, which behaves identically:
1 .program pull_example2
2
3 loop:
4 out pins, 8
5 .extern entry_point
6 jmp loop
Program wrapping (section Section 3.5.2) allows further simplification and, if desired, an output of 1 byte every system
clock cycle.
1 .program pull_example3
2
3 .extern entry_point
4 .wrap_target
5 out pins, 8 [1]
6 .wrap
3.2.3.2. Input Shift Register (ISR)
Figure 39. Input Shift
Register (ISR). Data
enters 1…32 bits at a
time, and current
contents is shifted left
or right to make room.
Once full, contents is
written to the RX FIFO.
•
IN instructions shift 1…32 bits at a time into the register.
•
PUSH instructions write the ISR contents to the RX FIFO.
•
The ISR is cleared to all-zeroes when pushed.
•
The state machine will automatically push the ISR on an IN instruction, once some shift threshold is reached, if
autopush is enabled.
•
Shift direction is configurable by the processor via configuration registers
Some peripherals, like UARTs, must shift in from the left to get correct bit order, since the wire order is LSB-first; however,
the processor may expect the resulting byte to be right-aligned. This is solved by the special null input source, which
allows the programmer to shift some number of zeroes into the ISR, following the data.
3.2.3.3. Shift Counters
State machines remember how many bits, in total, have been shifted out of the OSR via OUT instructions, and into the ISR
via IN instructions. This information is tracked at all times by a pair of hardware counters, capable of holding values from
0 to 32 inclusive (the width of a shift register). The state machine can be configured to perform certain actions when the
IN or OUT count reaches a configurable threshold:
•
The OSR can be automatically refilled once some number of bits have been shifted out. See Section 3.5.4
RP2040 Datasheet
3.2. Programmer’s Model 310