; RP2040 PIO program for accelerating SCSI communication
; Run "pioasm scsi_accel.pio scsi_accel.pio.h" to regenerate the C header from this.
; GPIO mapping:
; - 0-7: DB0-DB7
; -   8: DBP
; Side set is REQ pin

.define REQ 9
.define ACK 10

; Delay from data setup to REQ assertion.
; deskew delay + cable skew delay = 55 ns minimum
; One clock cycle is 8 ns => delay 7 clocks
.define REQ_DLY 7

; Write to SCSI bus using asynchronous handshake.
; Data is written as 16-bit words that contain the 8 data bits + 1 parity bit.
; 7 bits in each word are discarded.
; Number of bytes to send must be multiple of 2.
.program scsi_accel_async_write
    .side_set 1

    pull ifempty block          side 1  ; Get data from TX FIFO
    out pins, 9                 side 1  ; Write data and parity bit
    out null, 7 [REQ_DLY-2]     side 1  ; Discard unused bits, wait for data preset time
    wait 1 gpio ACK             side 1  ; Wait for ACK to be inactive
    wait 0 gpio ACK             side 0  ; Assert REQ, wait for ACK low

; 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_accel_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 1 gpio ACK             side 1  ; Wait for ACK high
    wait 0 gpio ACK             side 0  ; Assert REQ, wait for ACK low
    in pins, 9                  side 1  ; Deassert REQ, read GPIO
    in null, 7                  side 1  ; Padding bits
    push iffull block           side 1  ; Put data to RX FIFO
    jmp x-- start               side 1  ; Decrement byte count and jump to start