more IEC work - the turnaround seems to work now

c64/kernal/iec_release_atn.s

@@ -0,0 +1,8 @@
+
+iec_release_atn:
+
+	lda CI2PRA
+	and #$FF - BIT_CI2PRA_ATN_OUT    ; release
+	sta CI2PRA
+	rts
+

c64/kernal/iec_tx_command.s

@@ -57,9 +57,12 @@ iec_tx_command:
 	jsr iec_set_idle
 	jmp kernalerror_DEVICE_NOT_FOUND
 *
-	;; All done - before exiting wait a moment and restore bus to idle state
+	;; All done
-	jsr iec_wait20us
+	
-	jsr iec_set_idle
+	;; XXX All the documentation says we should release ATN at this point, but when I do so
+	;; the turnaround mechanism does not work. Why???
+	;; jsr iec_wait20us
+	;; jsr iec_set_idle
 
 	clc
 	rts

c64/kernal/iec_tx_common.s

@@ -43,7 +43,7 @@ iec_tx_common_bit_is_sent:
 	;; Pull CLK for 20us again, before sending the next bit
 	;; or performing any other action
 	jsr iec_pull_clk_release_data
-	jsr iec_wait20us
+	jsr iec_wait20us ; XXX this might be an overkill, there is a lot of code besides this wait here - few NOPs might be enough
 
 	;; More bits to send?
 	dex

c64/kernal/iec_tx_eoi.s

@@ -1,6 +1,9 @@
 
 iec_tx_eoi:
 
+	;; Releasing ATN needed due to current iec_tx_command implementation
+	jsr iec_release_atn
+
 	;; Wait at least 256 usec
 	jsr iec_wait100us
 	jsr iec_wait100us