robustness improvements in IEC support

c64/kernal/iec_pull_clk_release_data.s

@@ -1,7 +1,13 @@
 
 iec_pull_clk_release_data:
+
+	;; Pulled CLK = DATA considered invalid; for extra robustness on non-emulated
+	;; hardware give some lines few more cycles to stabilize; see page 10 of
+	;; http://www.zimmers.net/anonftp/pub/cbm/programming/serial-bus.pdf
+
 	lda CI2PRA
 	ora #BIT_CI2PRA_CLK_OUT          ; pull
+	sta CI2PRA
 	and #$FF - BIT_CI2PRA_DAT_OUT    ; release
 	sta CI2PRA
 	rts