Datasheet


CAUTION
Changing FJOIN discards any data present in the state machine’s FIFOs. If this data is irreplaceable, it must be drained
beforehand.
3.5.4. Autopush and Autopull
With each OUT instruction, the OSR gradually empties, as data is shifted out. Once empty, it must be refilled: for example, a
PULL transfers one word of data from the TX FIFO to the OSR. Similarly, the ISR must be emptied once full. One approach
to this is a loop which performs a PULL after an appropriate amount of data has been shifted:
Ê1 .program manual_pull
Ê2 .side_set 1 opt
Ê3
Ê4 .wrap_target
Ê5 set x, 2 ; X = bit count - 2
Ê6 pull side 1 [1] ; Stall here if no TX data
Ê7 bitloop:
Ê8 out pins, 1 side 0 [1] ; Shift out data bit and toggle clock low
Ê9 jmp x-- bitloop side 1 [1] ; Loop runs 3 times
10 out pins, 1 side 0 ; Shift out last bit before reloading X
11 .wrap
This program shifts out 4 bits from each FIFO word, with an accompanying bit clock, at a constant rate of 1 bit per 4
cycles. When the TX FIFO is empty, it stalls with the clock high (noting that side-set still takes place on cycles where the
instruction stalls). Figure 41 shows how a state machine would execute this program.
System Clock
0 2 3 432
2 1 0 2-1
1
Instruction
Scratch X
Clock pin (side -set)
OSR shift count
PULLSET OUT JMP OUT
JMP OUT JMP SETOUT PULL
Bit 0 Bit 1 Bit 2 Bit 3
Data pin (OUT)
Figure 41. Execution
of manual_pull
program. X is used as
a loop counter. On
each iteration, one
data bit is shifted out,
and the clock is
asserted low, then
high. A delay cycle on
each instruction
brings the total up to
four cycles per
iteration. After the
third loop, a fourth bit
is shifted out, and the
state machine
immediately returns to
the start of the
program to reload the
loop counter and pull
fresh data, while
maintaining the 4
cycles/bit cadence.
This program has some limitations:
•
It occupies 5 instruction slots, but only 2 of these are immediately useful (out pins, 1 set 0 and … set 1), for
outputting serial data and a clock.
•
Its throughput is limited to system clock over 4, due to the extra cycles required to pull in new data, and reload the
loop counter
This is a common type of problem for PIO, so each state machine has some extra hardware to handle it. State machines
keep track of the total shift count OUT of the OSR and IN to the ISR, and trigger certain actions once these counters reach a
programmable threshold.
•
On an OUT instruction which reaches or exceeds the pull threshold, the state machine can simultaneously refill the
OSR from the TX FIFO, if data is available.
•
On an IN instruction which reaches or exceeds the push threshold, the state machine can write the shift result
directly to the RX FIFO, and clear the ISR.
The manual_pull example can be rewritten to take advantage of automatic pull (autopull):
RP2040 Datasheet
3.5. Functional Details 335