MEGA65 R7 Change Request Marshalling Point

[gardners]

This issue is to collect suggested changes for any future MEGA65 R7 PCB.

(Note that this does not mean that an R7 is currently planned, or will necessarily ever be produced in the future. It's just so that we can collect things that we would like fixed, if and when that happens.)

[gardners]

Add FPGA-enableable pull-up to POT AX, POT AY, POT BX, POT BY on joystick ports to support middle and right button of Amiga mouses without requiring the addition of a pull-up resistor into the Amiga mouse, or the use of a joystick extension cable containing the pull-up resistors. See, for example, https://www.mssiah-forum.com/viewtopic.php?pid=13749 for a discussion of this. See also #751

[gardners]

Add read line for A14 of expansion port to better support C64 KERNAL cartridges that pull A14 to indirectly read HIMEM line. See, for example, http://skoe.de/kernal/kernal-cartridge.pdf

[gardners]

Modify J21 to provide disconnection ability from joystick port bi-di lines, e.g., using a latching buffer, so that the pins can be used for other functions, without causing mayhem on the joystick ports.

[gardners]

Modify cart R/W line to be open-collector, with a separate read sense line. F_CART_EN could be tied to GND, to free up the FPGA pin for reading R/W line. i.e., no extra FPGA pins required.