more work on IEC, fixed command transmission

c64/kernal/iec_tx_command.s

@@ -0,0 +1,153 @@
+
+
+;; XXX - TEMPORARY IMPLEMENTATION
+
+;; https://www.pagetable.com/?p=1135, https://github.com/mist64/cbmbus_doc
+;; https://www.c64-wiki.com/wiki/CIA#CIA_2
+
+.alias CI2PRA $DD00
+
+.alias BIT_IEC_ATN_OUT $08    ; 1 - low (pulled), 0 - high (released)
+.alias BIT_IEC_CLK_OUT $10    ; 1 - low (pulled), 0 - high (released)
+.alias BIT_IEC_DAT_OUT $20    ; 1 - low (pulled), 0 - high (released)
+.alias BIT_IEC_CLK_IN  $40    ; 0 - low (pulled), 1 - high (released)
+.alias BIT_IEC_DAT_IN  $80    ; 0 - low (pulled), 1 - high (released)
+
+
+; XXX optimize these below
+
+xxx_to_idle_state:
+	lda CI2PRA
+	ora #BIT_IEC_CLK_OUT                            ; pull
+	and #$FF - BIT_IEC_DAT_OUT - BIT_IEC_ATN_OUT    ; release
+	sta CI2PRA
+	rts
+
+xxx_pull_atn_clk_release_data:
+	lda CI2PRA
+	ora #BIT_IEC_ATN_OUT + BIT_IEC_CLK_OUT    ; pull
+	and #$FF - BIT_IEC_DAT_OUT                ; release
+	sta CI2PRA
+	rts
+
+xxx_pull_clk_release_data:
+	lda CI2PRA
+	ora #BIT_IEC_CLK_OUT          ; pull
+	and #$FF - BIT_IEC_DAT_OUT    ; release
+	sta CI2PRA
+	rts
+
+xxx_release_clk_data:
+	lda CI2PRA
+	and #$FF - BIT_IEC_CLK_OUT - BIT_IEC_DAT_OUT    ; release
+	sta CI2PRA
+	rts
+
+xxx_release_clk_pull_data:
+	lda CI2PRA
+	and #$FF - BIT_IEC_CLK_OUT    ; release
+	ora #BIT_IEC_DAT_OUT          ; pull
+	sta CI2PRA
+	rts
+
+xxx_wait_for_data_release:
+	lda CI2PRA
+	;; Check the highest bit, which is DATA IN,
+	;; (highest bit set = negative value)
+	bpl xxx_wait_for_data_release
+	rts
+
+
+
+iec_tx_command:
+
+	pha
+
+	;; Notify all devices that we are going to send a byte
+	;; and it is going to be a command (pulled ATN)
+	jsr xxx_pull_atn_clk_release_data
+
+	;; Give devices time to respond (response is mandatory!)
+	jsr wait1ms
+
+	;; Did at least one device respond by pulling DATA?
+	lda CI2PRA
+	and #BIT_IEC_DAT_IN
+	beq +
+
+	;; No devices present on the bus, so we can immediately return with device not found
+	pla
+
+	jsr printf
+	.byte "DBG: NO DEVICES AT ALL", $0D, 0
+
+	jsr xxx_to_idle_state
+	jmp kernalerror_DEVICE_NOT_FOUND
+*
+	;; At least one device responded, but they are still allowed to stall
+	;; (can be busy processing something), we have to wait till they are all
+	;; ready (or bored with DOS attack...)
+	;; - release back CLK, keep DATA released
+	;; - afterwards wait till all devices also release DATA
+
+	jsr xxx_release_clk_data
+	jsr xxx_wait_for_data_release
+
+	;; Pull CLK back to indicate that DATA is not valid, keep it for 60us
+	;; Don't wait too long, as 200us or more is considered EOI
+
+	jsr xxx_pull_clk_release_data
+	jsr iec_wait60us
+
+	;; Now, we can start transmission of 8 bits of data
+	ldx #7
+	pla
+
+iec_tx_command_nextbit:
+	;; Is next bit 0 or 1?
+	lsr
+	pha
+	bcs +
+
+	;; Bit is 0
+	jsr xxx_release_clk_pull_data
+	jmp iec_tx_command_bitset
+*
+	;; Bit is 1
+	jsr xxx_release_clk_data
+	
+iec_tx_command_bitset:
+
+	;; Wait 20us, so that device(s) can pick DATA
+	jsr iec_wait20us
+
+	;; Pull CLK for 20us again
+	jsr xxx_pull_clk_release_data
+	jsr iec_wait20us
+
+	;; More bits to send?
+	pla
+	dex
+	bpl iec_tx_command_nextbit
+
+	;; XXX the flow below is REALLY dangerous, as if there are multiple devices,
+	;; one can signal that it's busy much earlier (more than 100ms) than the other.
+	;; Can we do something about it?
+
+	;; All done - give devices time to tell if they are busy by pulling DATA
+	;; They should do it within 1ms
+	ldx #$FF
+*	lda CI2PRA
+	;; BPL here is checking that bit 7 of $DD00 clears,
+	;; i.e, that the DATA line is pulled by drive
+	bpl +
+	dex
+	bne -
+	bpl iec_tx_command_done
+*
+	;; XXX wait 100ms
+	
+iec_tx_command_done:
+
+	clc
+	rts