Datasheet

Chip Select
SDO/SDI 7 6 5 4 3 2 1 0
CPHA=0 CPOL=0
CPHA=0 CPOL=1
Chip Select
SDO/SDI 7 6 5 4 3 2 1 0
CPHA=1 CPOL=0
CPHA=1 CPOL=1
Figure 48. In SPI, a
host and device
exchange data over a
bidirectional pair of
serial data lines,
synchronous with a
clock (SCK). Two
flags, CPOL and
CPHA, specify the
clock’s behaviour.
CPOL is the idle state
of the clock: 0 for low,
1 for high. The clock
pulses a number of
times, transferring one
bit in each direction
per pulse, but always
returns to its idle
state. CPHA
determines on which
edge of the clock data
is captured: 0 for
leading edge, and 1 for
trailing edge. The
arrows in the figure
show the clock edge
where data is captured
by both the host and
device.
SPI is a common serial interface with a twisty history. The following program implements full-duplex (i.e. transferring data
in both directions simultaneously) SPI, with a CPHA parameter of 0.
Pico Examples: https://github.com/raspberrypi/pico-examples/tree/pre_release/pio/spi/spi.pio Lines 14 - 32
14 .program spi_cpha0
15 .side_set 1
16
17 ; Pin assignments:
18 ; - SCK is side-set pin 0
19 ; - MOSI is OUT pin 0
20 ; - MISO is IN pin 0
21 ;
22 ; Autopush and autopull must be enabled, and the serial frame size is set by
23 ; configuring the push/pull threshold. Shift left/right is fine, but you must
24 ; justify the data yourself. This is done most conveniently for frame sizes of
25 ; 8 or 16 bits by using the narrow store replication and narrow load byte
26 ; picking behaviour of RP2040's IO fabric.
27
28 ; Clock phase = 0: data is captured on the leading edge of each SCK pulse, and
29 ; transitions on the trailing edge, or some time before the first leading edge.
30
31 out pins, 1 side 0 [1] ; Stall here on empty (sideset proceeds even if
32 in pins, 1 side 1 [1] ; instruction stalls, so we stall with SCK low)
This code uses autopush and autopull to continuously stream data from the FIFOs. The entire program runs once for
every bit that is transferred, and then loops. The state machine tracks how many bits have been shifted in/out, and
automatically pushes/pops the FIFOs at the correct point. A similar program handles the CPHA=1 case:
Pico Examples: https://github.com/raspberrypi/pico-examples/tree/pre_release/pio/spi/spi.pio Lines 34 - 42
34 .program spi_cpha1
35 .side_set 1
36
37 ; Clock phase = 1: data transitions on the leading edge of each SCK pulse, and
38 ; is captured on the trailing edge.
39
40 out x, 1 side 0 ; Stall here on empty (keep SCK deasserted)
41 mov pins, x side 1 [1] ; Output data, assert SCK (mov pins uses OUT mapping)
42 in pins, 1 side 0 ; Input data, deassert SCK
RP2040 Datasheet
3.6. Examples 344