From 83cfce2960ad8a4705c564dffc7a3d884dc58e86 Mon Sep 17 00:00:00 2001
From: Gregor Riepl <onitake@gmail.com>
Date: Fri, 19 Aug 2022 15:03:38 +0200
Subject: [PATCH] Use VIA T2, implment output

---
 driver.a65 | 118 +++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 93 insertions(+), 25 deletions(-)

diff --git a/driver.a65 b/driver.a65
index ea548bf..2e17dd6 100644
--- a/driver.a65
+++ b/driver.a65
@@ -24,6 +24,7 @@ BAUD_RATE = 300
 
 ; we expect to be loaded somewhere in the middle of user ram
 ; TODO verify if this is safe with the Editor/Basic 2.0 ROM
+; or if there is a way to limit memory usage by the ROM
 .org $7000
 
 ; entry points
@@ -67,7 +68,7 @@ BAUD_RATE = 300
 	initialized: .byte 0
 	; saved IRQ vector
 	oldvector: .word 0
-	; current CRA value on CA1 change (bit contains the state of CA1)
+	; state of the CA1 input: bit2=0 low level, bit2=1 high level
 	ca1state: .byte 0
 	; output buffer
 	; this is 16bit, because we need to process start and stop bits as well
@@ -115,14 +116,19 @@ BAUD_RATE = 300
 		sta IRQVec
 		lda #<irqhandler
 		sta IRQVec+1
-		
-		; the timer is only started when a transmission is actually initiated
-		; but we'll enable VIA timer 1 interrupt
-		lda VIA_IER
-		ora #%01000000
+
+		; VIA timer 2 cannot be enabled and disabled on the fly, so we'll
+		; configure it here and arm it later
+		; one-shot mode
+		lda VIA_CR
+		and #%11011111
+		sta VIA_CR
+		; enable interrupt
+		; interrupt enable is done by setting bit7 and the desired interrupt
+		; bit in IER to 1 (no need to read-modify-write)
+		lda #%10100000
 		sta VIA_IER
 
-		
 		; set up the PIA1 and VIA to process I/O
 		; TXD: VIA PB3, output
 		lda VIA_DDRB
@@ -186,9 +192,9 @@ BAUD_RATE = 300
 		; IRQ disable
 		and #%11111110
 		sta PIA1_CRA
-		; and the VIA timer 1
-		lda VIA_IER
-		and #%10111111
+		; and the VIA timer 2
+		; clearing is done by setting bit 7 to 0 and the desired interrupt to 1
+		lda #%00100000
 		sta VIA_IER
 
 		; we leave the I/O pins and the state alone
@@ -336,17 +342,13 @@ BAUD_RATE = 300
 		rts
 
 	; start the timer
-	@starttimer:
-		; set up the VIA timer 1 to fire periodically
+	starttimer:
+		; arm VIA timer 2 by latching the counter
 		period = PHI2_CLOCK/BAUD_RATE
 		lda #>period
 		sta VIA_T1CL
 		lda #<period
 		sta VIA_T1CH
-		; VIA timer 1 free-run mode
-		lda VIA_CR
-		ora #%01000000
-		sta VIA_CR
 		; return
 		rts
 	
@@ -398,20 +400,86 @@ BAUD_RATE = 300
 		sta PIA1_CRA
 
 	@handletimer:
-		; VIA timer 1 fired?
+		; VIA timer 2 fired?
 		lda VIA_IFR
-		and #%01000000
+		and #%00100000
 		; zero, no interrupt
 		beq @irqreturn
 
-		; handle timer interrupt
-		; reset timer 1 interrupt
-		; FIXME should we do this? perhaps someone else is also expecting a VIA interrupt
-		; if not, we *must* reset it
-		lda VIA_T1CL
+		; timer 2 fired: rearm (we're in one-shot mode!), and clear the interrupt flag
+		; FIXME this should be done earlier, for more accurate timing
+		; FIXME we could skip writing the low byte here, writing the high byte is enough
+		jsr starttimer
 
-	@processfifo:
-		; TODO process input and output
+	@loadout:
+		; do we have more bits to send?
+		lda outshift
+		; yes, skip loading next byte
+		bne @shiftout
+
+		; load next byte from FIFO
+
+		; do we have more bytes?
+		lda outqlen
+		; nope, skip ahead to input processing
+		beq @shiftin
+
+		; load length
+		ldx outqlen
+		; decrement, also gives the index
+		dex
+		; load next byte from fifo
+		lda outq,x
+		; store new queue length
+		stx outqlen
+
+		; prepare shift register
+		; bit0 = start bit = 0
+		; bit1..7 = data0..6
+		clc
+		rol
+		sta outbuf
+		; bit8 = data7
+		; bit9 = stop bit = 1
+		lda #%00000001
+		rol
+		sta outbuf+1
+		; set shift length
+		lda #10
+		sta outshift
+
+	@shiftout:
+		; make sure overflow flag is clear, so we can do unconditional branches
+		clv
+		; load shift register bit 0
+		lda outbuf
+		and #%00000001
+		; set or clear?
+		beq @shiftoutclear
+		; set output
+		lda #%00001000
+		ora VIA_PB
+		bvc @shiftoutstore
+	@shiftoutclear:
+		; clear output
+		lda #%11110111
+		and VIA_PB
+	@shiftoutstore:
+		sta VIA_PB
+
+		; shift the next bit in
+		; 0 -> high byte -> carry
+		lsr outbuf+1
+		; carry -> low byte
+		ror outbuf
+
+		; decrement counter
+		dec outshift
+
+	@shiftin:
+		; TODO process input bit-bang
+	@storein:
+		; TODO process input fifo
 
 	@irqreturn:
 		; restore registers