From 523c4c6dea8ef9831f2b792e75b6d8d5066b6445 Mon Sep 17 00:00:00 2001
From: Gregor Riepl <onitake@gmail.com>
Date: Sat, 20 Aug 2022 11:11:19 +0200
Subject: [PATCH] Implement RX

---
 driver.a65 | 70 +++++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 62 insertions(+), 8 deletions(-)

diff --git a/driver.a65 b/driver.a65
index 9846f47..2212a57 100644
--- a/driver.a65
+++ b/driver.a65
@@ -385,7 +385,7 @@ BAUD_RATE = 300
 	@handleflip:
 		; read PIA1 CRA, interrupt flag is automatically cleared
 		lda PIA1_CRA
-		; save for later, so we don't need to fetch it again
+		; save CRA for later, so we don't need to fetch it again
 		tax
 		; bit 7 is the CA1 interrupt flag - fired?
 		and #%10000000
@@ -404,6 +404,14 @@ BAUD_RATE = 300
 		and #%01111111
 		sta PIA1_CRA
 
+		; check if we're already in the middle of a reception
+		lda inshift
+		; yes, don't do anything
+		bne @handletimer
+		; nope, initiate reception
+		lda #10
+		sta inshift
+
 	@handletimer:
 		; VIA timer 2 fired?
 		lda VIA_IFR
@@ -460,14 +468,14 @@ BAUD_RATE = 300
 		; load shift register bit 0
 		lda outbuf
 		and #%00000001
-		; set or clear?
-		beq @shiftoutclear
-		; set output
+		; mark or blank?
+		beq @shiftoutblank
+		; set output high
 		lda #%00001000
 		ora VIA_PB
 		bvc @shiftoutstore
-	@shiftoutclear:
-		; clear output
+	@shiftoutblank:
+		; set output low
 		lda #%11110111
 		and VIA_PB
 	@shiftoutstore:
@@ -483,9 +491,55 @@ BAUD_RATE = 300
 		dec outshift
 
 	@shiftin:
-		; TODO process input bit-bang
+		; FIXME we should check the start and stop bits, to synchronize RS232 transmissions
+
+		; check first if a reception is in progress
+		lda inshift
+		; nope, skip input processing
+		beq @irqreturn
+
+		; for unconditional branch
+		clv
+		; pick up the current CA1 state
+		lda ca1state
+		and #%00000100
+		; mark or blank?
+		beq @shiftinblank
+		; shift in high bit
+		sec
+		bvc @shiftinstore
+	@shiftinblank:
+		; shift in low bit
+		clc
+	@shiftinstore:
+		; rotate carry bit into buffer
+		rol inbuf
+		rol inbuf+1
+
+		; decrement counter
+		dec inshift
+
 	@storein:
-		; TODO process input fifo
+		; did we complete a transmission?
+		bne @irqreturn
+
+		; check if we have space available in the buffer
+		lda inqlen
+		cmp #16
+		; nope, drop this byte
+		bcc @irqreturn
+
+		; yes, store it
+		tax
+		lda inbuf+1
+		lsr
+		lda inbuf
+		ror
+		sta inq, x
+
+		; and update the buffer length
+		dex
+		stx inqlen
 
 	@irqreturn:
 		; restore registers