more work on IEC, some refactoring and temporary breakage

c64/kernal/iec_tx_byte.s

@@ -1,87 +1,10 @@
-iec_tx_byte:	
+
+;; Implemented based on https://www.pagetable.com/?p=1135, https://github.com/mist64/cbmbus_doc
+
+iec_tx_byte:
 
 	pha
 
-	jsr iec_assert_clk_release_data
-
-	;; Give devices time to respond
-	jsr wait1ms
-
-	;; Did a device respond? (DATA will be pulled if so)
-	lda $DD00
-	bpl +
-
-	;; No devices present, so we can immediately return with device not found
-	pla
-	jmp kernalerror_DEVICE_NOT_FOUND
-*
-
-	;; Release CLK and wait for DATA to release.
-	jsr iec_release_clk_and_data
-	
-	;; Wait for DATA to be released
-	;; Listener is allowed to hold DATA for as long as they want.
-	;; i.e., this is the part of the protocol where the listener indicates
-	;; that they are busy doing something, like processing the previous
-	;; character.
-	jsr iec_wait_for_data_release
-
-	;; We then wait for 60 usec (less than 200usec so we don't
-	;; signal EOI), before asserting CLK again.
-	jsr iec_wait60us
-
-	;; Do the actual transmission of 8 bits of data
-	ldx #7
-
-	pla
-	
-iec_tx_byte_nextbit:	
-	;; Is next bit 0 or 1?
-	lsr
-	pha
-	bcs +
-	
-	;; Setup 0
-	jsr iec_assert_clk_and_data
-	jmp iec_tx_byte_l1
-*
-	;; Setup 1
-	jsr iec_assert_clk_release_data
-iec_tx_byte_l1:
-	;; Clock bit out
-	jsr iec_wait20us
-
-	jsr iec_release_clk
-
-	jsr iec_wait20us
-
-	;; More bits to send?
-	pla
-	dex
-	bpl iec_tx_byte_nextbit
-
-	;; Done sending bits. Assert CLK and wait for acknowledgement
-	jsr iec_assert_clk_release_data
-
-	;; Acknowledgement must happen within 1ms, else it
-	;; is DEVICE NOT PRESENT
-	;; Acknowledgement consists of the DATA line being pulled
-	;; by the listener.
-	;; We can't easily count rasters here, so we just
-	;; try a set number of times
-	ldx #$ff
-*	lda $DD00
-	;; BPL here is checking that bit 7 of $DD00 clears,
-	;; i.e, that the DATA line is asserted.
-	bpl +
-	dex
-	bne -
-
-	;; Timeout - DEVICE NOT PRESENT
-	jmp kernalerror_DEVICE_NOT_FOUND
-*
-	;; All done.
-	;; End state is TALKER is asserting CLK, and listener is asserting DATA
-	;; until they are ready for the next byte.
-	clc
+;; XXX to be filled-in
 	rts
+