;
; Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
;
; SPDX-License-Identifier: BSD-3-Clause
;

; These programs implement full-duplex SPI, with a SCK period of 4 clock
; cycles. A different program is provided for each value of CPHA, and CPOL is
; achieved using the hardware GPIO inversion available in the IO controls.
;
; Transmit-only SPI can go twice as fast -- see the ST7789 example!


.program spi_cpha0
.side_set 1

; Pin assignments:
; - SCK is side-set pin 0
; - MOSI is OUT pin 0
; - MISO is IN pin 0
;
; Autopush and autopull must be enabled, and the serial frame size is set by
; configuring the push/pull threshold. Shift left/right is fine, but you must
; justify the data yourself. This is done most conveniently for frame sizes of
; 8 or 16 bits by using the narrow store replication and narrow load byte
; picking behaviour of RP2040's IO fabric.

; Clock phase = 0: data is captured on the leading edge of each SCK pulse, and
; transitions on the trailing edge, or some time before the first leading edge.

    out pins, 1 side 0 [1] ; Stall here on empty (sideset proceeds even if
    in pins, 1  side 1 [1] ; instruction stalls, so we stall with SCK low)


.program spi_cpha1
.side_set 1

; Clock phase = 1: data transitions on the leading edge of each SCK pulse, and
; is captured on the trailing edge.

    out x, 1    side 0     ; Stall here on empty (keep SCK deasserted)
    mov pins, x side 1 [1] ; Output data, assert SCK (mov pins uses OUT mapping)
    in pins, 1  side 0     ; Input data, deassert SCK