Add core serial IO implementation

Author: quinnyo

unbricked/serial-link/sio.asm

@@ -0,0 +1,231 @@
+INCLUDE "hardware.inc"
+
+; ::::::::::::::::::::::::::::::::::::::
+; ::                                  ::
+; ::                       ______.    ::
+; ::       _              |````` ||   ::
+; ::     _/ \__@_         |[- - ]||   ::
+; ::    /        `--<[|]= |[ m  ]||   ::
+; ::    \      .______    | ```` ||   ::
+; ::    /     !| `````|   | +  oo||   ::
+; ::   (      ||[ ^u^]|   |  .. #||   ::
+; ::    `-<[|]=|[    ]|   `______//   ::
+; ::          || ```` |               ::
+; ::          || +  oo|               ::
+; ::          ||  .. #|               ::
+; ::          !|______/               ::
+; ::                                  ::
+; ::                                  ::
+; ::::::::::::::::::::::::::::::::::::::
+;
+; This concludes the physical portion of the tutorial.
+; (I'm sure we can all agree that this was worth whatever it cost.)
+
+
+; Duration of timeout period in ticks. (for externally clocked device)
+DEF SIO_TIMEOUT_TICKS EQU 240
+; Duration of 'catchup' delay period in ticks. (for internally clocked device)
+DEF SIO_CATCHUP_TICKS EQU 1
+
+DEF SIO_CONFIG_INTCLK  EQU SCF_SOURCE
+DEF SIO_CONFIG_DEFAULT EQU 0
+EXPORT SIO_CONFIG_INTCLK
+
+; SioStatus transfer state enum
+RSRESET
+DEF SIO_IDLE           RB 1
+DEF SIO_XFER_START     RB 1
+DEF SIO_XFER_STARTED   RB 1
+DEF SIO_XFER_COMPLETED RB 1
+DEF SIO_XFER_FAILED    RB 1
+EXPORT SIO_IDLE, SIO_XFER_START, SIO_XFER_STARTED, SIO_XFER_COMPLETED, SIO_XFER_FAILED
+
+
+SECTION "SioCore State", WRAM0
+; Sio config flags
+wSioConfig:: db
+; Sio state machine current state
+wSioState:: db
+; Source address of next value to transmit
+wSioTxPtr:: dw
+; Destination address of next received value
+wSioRxPtr:: dw
+; Number of transfers to perform (bytes to transfer)
+wSioCount:: db
+
+; Timer state (as ticks remaining, expires at zero) for timeouts and delays.
+wTimer: db
+
+
+SECTION "SioCore Impl", ROM0
+
+SioInit::
+	ld a, SIO_CONFIG_DEFAULT
+	ld [wSioConfig], a
+	ld a, SIO_IDLE
+	ld [wSioState], a
+	xor a
+	ld [wTimer], a
+	ld a, $FF ; using FFFF as 'null'/unset
+	ld [wSioTxPtr + 0], a
+	ld [wSioTxPtr + 1], a
+	ld [wSioRxPtr + 0], a
+	ld [wSioRxPtr + 1], a
+	xor a
+	ld [wSioCount], a
+
+	; enable serial interrupt
+	ldh a, [rIE]
+	or IEF_SERIAL
+	ldh [rIE], a
+	ret
+
+
+SioTick::
+	ld a, [wSioState]
+	cp SIO_XFER_START
+	jr z, SioProcessQueue
+	cp SIO_XFER_STARTED
+	jr z, .xfer_started
+	cp SIO_XFER_COMPLETED
+	jr z, SioProcessQueue
+	; treat anything else as failed/error and do nothing
+	ret
+.xfer_started:
+	; update timeout on external clock
+	ldh a, [rSC]
+	and SCF_SOURCE
+	ret nz
+	ld a, [wTimer]
+	and a
+	ret z ; timer == 0, timeout disabled
+	dec a
+	ld [wTimer], a
+	jr z, SioAbortTransfer
+	ret
+
+
+SioProcessQueue:
+	; if SioCount > 0: start next transfer
+	ld a, [wSioCount]
+	and a
+	ret z
+
+	; FALLTHROUGH
+
+SioStartNextTransfer:
+	; delay on internal clock device
+	ldh a, [rSC]
+	bit SCB_SOURCE, a
+	jr z, .no_delay
+		ld a, [wTimer]
+		and a
+		jr z, .no_delay
+			dec a
+			ld [wTimer], a
+			ret
+.no_delay
+
+	; get next value and start transfer
+	ld hl, wSioTxPtr
+	ld a, [hl+]
+	ld h, [hl]
+	ld l, a
+	ld b, [hl]
+	call SioStartTransfer
+
+	ret
+
+
+; Start a serial port transfer immediately.
+; @param B: data byte to send
+; @mut: AF
+SioStartTransfer::
+	; set the clock source (do this first & separately from starting the transfer!)
+	ld a, [wSioConfig]
+	and SCF_SOURCE ; the sio config byte uses the same bit for the clock source
+	ldh [rSC], a
+	; load the value to send
+	ld a, b
+	ldh [rSB], a
+	; start the transfer
+	ldh a, [rSC]
+	or SCF_START
+	ldh [rSC], a
+
+	; reset timeout (on externally clocked device)
+	bit SCB_SOURCE, a
+	jr nz, :+
+	ld a, SIO_TIMEOUT_TICKS
+	ld [wTimer], a
+:
+	ld a, SIO_XFER_STARTED
+	ld [wSioState], a
+	ret
+
+
+SioAbortTransfer::
+	ld a, SIO_XFER_FAILED
+	ld [wSioState], a
+	ldh a, [rSC]
+	res SCB_START, a
+	ldh [rSC], a
+	ret
+
+
+SioSerialInterruptHandler:
+	push af
+	push hl
+
+	; check that we were expecting a transfer
+	ld a, [wSioState]
+	cp SIO_XFER_STARTED
+	ret nz
+
+	; store the received value
+	ld hl, wSioRxPtr
+	ld a, [hl+]
+	ld h, [hl]
+	ld l, a
+	ldh a, [rSB]
+	ld [hl+], a
+	; store the updated Rx pointer
+	ld a, l
+	ld [wSioRxPtr + 0], a
+	ld a, h
+	ld [wSioRxPtr + 1], a
+
+	; update the Tx pointer
+	ld hl, wSioTxPtr
+	inc [hl] ; inc low byte
+	jr nz, :+
+	; inc high byte if overflow
+	inc hl
+	inc [hl]
+:
+
+	; update transfer count
+	ld hl, wSioCount
+	dec [hl]
+
+	; set transfer state to 'completed'
+	ld a, SIO_XFER_COMPLETED
+	ld [wSioState], a
+
+	ldh a, [rSC]
+	and SCF_SOURCE
+	jr z, :+
+	; reset delay timer on internal clock
+	ld a, SIO_CATCHUP_TICKS
+	ld [wTimer], a
+:
+
+	pop hl
+	pop af
+	reti
+
+
+section "Serial Interrupt", rom0[$58]
+SerialInterrupt:
+	jp SioSerialInterruptHandler
+