Kempston Joystick 2, Joystick I/O

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Number $xx37
Decimal 55
Short desc. Kempston interface second joystick variant and controls joystick I/O.
Bit Mask
Readable Yes
Writable Yes
Subsystem Input

For READ functionality the details are identical with Kempston Joystick ($xx1F / 31)

Since core3.1.5 there is option to do I/O operations through joystick ports.

The I/O mode should be set by writing this port first followed by enabling I/O mode on the joysticks with a write to Peripheral 1 Register ($05).

The WRITE functionality:

Bit Function
7-6 select I/O mode: %00 = bit bang, %01 = clock, %10 = uart
5 Reserved must be 0
4 0 to read "input" part of I/O from left joystick socket, 1 to read from right joystick socket

The program can still change the active joystick via bit 4 of port 0x37 at any time but for uart mode this should be done during a quiet time in communication. Any output always appears on pin 7 of both joystick connectors.

3-1 Reserved must be 0
0 parameter bit controls state of pin 7 on both joystick connectors:

bit bang: bit 0 is copied to pin 7
clock: 0 = place slow clock on pin 7 (Fsys/2048 = 13.672kHz), 1 = place fast clock on pin 7 (Fsys/8 = 3.5MHz) †
uart: 0 = redirect esp uart to joystick connector, 1 = redirect pi uart to joystick connector (Tx appears on pin 7, Rx taken from pin 9)

† A runt clock pulse may appear in the first cycle, minimum pulse width is 1/Fsys = 35.7ns