The Unreleased PC Engine Keyboard
I have written a short program in HuC which will properly scan the keyboard and display when keys are pressed or released.
The program source can be found in this repository in the Software/Test_ROM directory. The RELEASE subdirectory conatins a pre-compiled executable.
As of the time of writing, this requires the most recent version of HuC (development version as at March 25, 2024 - not even published in a proper release yet) in order to build correctly.
The reason for this is because programs written for HuC automatically call the embedded joypad reading routine at every Vertical Sync interval (1/60 second). In truth, we don't want that older jopyad routine to be used at all, replacing it with our own. The HuC library needed a small modification in order to make this possible.
A trivial shell script (build.sh) was included with the appropriate command-line options to build the ROM.
The joypad routine triggers a read sequence (as a normal joypad read sequence would), but rather than reading up to 5 bytes of input, this continues to read 18 bytes in total.
The first byte, as per the protocol, is a 0x02 - this demonstrates that it could not be a normal joypad, because all 4 direction arrows would be depressed (the upper 4 of the '0' bits).
The next 16 bytes contain the keypress information. This is encoded in a similar way to the original PC-88 keyboard data, exept that the most significant bit is always a '1' value, and has no corresponding keys. Keys which are pressed are represented by '0' bit values, and keys which are not pressed are represented as '1' bit values.
The 18th byte is also a 0x02, to indicate end of sequence.