Editing POKEY (section)

===SKCTL $D20F Write===
Serial Port Control

{| class="wikitable"
|-
! Bit 7 !! Bit 6 !! Bit 5 !! Bit 4 !! Bit 3 !! Bit 2 !! Bit 1 !! Bit 0
|-
| Serial Break || Serial Mode2 || Serial Mode1 || Serial Mode0 || Serial Two-Tone || Fast Pot Scan || Enable KB Scan || KB debounce 
|}

Bit 0: Enable "debounce" scanning which is intended to eliminate noise or jitter from mechanical switches. A value of 1 enables POKEY to use an internal comparison register while scanning keys. A key must be detected in two simultaneous scans before it is identified as pressed, and it must be seen released for two consecutive scans to be considered released. This should be enabled to maintain normal keyboard handling with the Operating System.

Bit 1: Set to 1 to enable keyboard scanning. This should be enabled to maintain normal keyboard handling with the Operating System.

Bit 2: Set to 1 to enable fast, though less accurate Potentiometer scanning. Fast Pot scanning increments the counter on every cycle and returns a usable result within two scan lines. The Operating System uses the slow Pot Scanning which increments the counter once every 114 cycles (scan line) taking a frame (1/60th second) to produce a result. The OS reads the Pot values during its Vertical Blank Interrupt (VBI) and copies the result to the potentiometer Shadow registers in RAM. It then resets POTGO for the next read during the next VBI.

Bit 3: Enable Serial port two-tone mode. When enabled, 1 and 0 bits output to the SIO bus are replaced by tones set by timers 1 and 2. This is ordinarily used for writing analog tones representing digital data to cassette tape.

Bit 4-6: Clock Timing Control for serial port operation. Bit values described below:

{| class="wikitable"
|-
! Port Control [6:4] !! Bits Value !! Input Clock !! Output Clock !! Bidirectional Clock
|-
| 0 0 0 ||  $00 || External || External || Input
|-
| 0 0 1 ||  $10 || Channels 3+4 (async) || External || Input 
|-
| 0 1 0 ||  $20 || Channel 4 || Channel 4 || Output Channel 4
|-
| 0 1 1 ||  $30 || Channel 3+4 (async) || Channel 4 (async) || Input
|-
| 1 0 0 ||  $40 || External || Channel 4 || Input
|-
| 1 0 1 ||  $50 || Channel 3+4 (async) || Channel 4 (async) || Input
|-
| 1 1 0 ||  $60 || Channel 4 || Channel 2 || Output Channel 4
|-
| 1 1 1 ||  $70 || Channel 3+4 (async) || Channel 2 || Input
|}

Bit 7: Forces a known 0 output, so that timer 2 can reset timer 1 in two-tone serial output mode.