; RP2040 PIO program for accelerating SCSI initiator / host function
;
; Copyright (c) 2022 Rabbit Hole Computing™
;
; Run "pioasm scsi_accel_host.pio scsi_accel_host.pio.h" to regenerate the C header from this.
; GPIO mapping:
; - 0-7: DB0-DB7
; -   8: DBP
; Side set is ACK pin

.define REQ 19
.define ACK 26

; Read from SCSI bus using asynchronous handshake.
; Data is returned as 16-bit words that contain the 8 data bits + 1 parity bit.
; Number of bytes to receive minus 1 should be written to TX fifo.
; Number of bytes to receive must be divisible by 2.
.program scsi_host_async_read
    .side_set 1

    pull block                  side 1  ; Get number of bytes to receive
    mov x, osr                  side 1  ; Store to counter X

start:
    wait 0 gpio REQ             side 1  ; Wait for REQ low
    nop                 [1]     side 1  ; Wait for signals to settle
    in pins, 9                  side 0  ; Assert ACK, read GPIO
    in null, 7                  side 0  ; Padding bits
    wait 1 gpio REQ             side 0  ; Wait for REQ high
    jmp x-- start               side 1  ; Deassert ACK, decrement byte count and jump to start