What attracted me to the latest offer from Lez was the inclusion of Z180 CPUs. The idea of building a fast system with a large amount of RAM (for a Z80) appealed.
What I am currently planning is:
- Z180 CPU clocked at 20MHz.
- 1 MByte RAM.
- Boot ROM. Not sure whether I will have the option to switch this out once booted.
- SD card for storage.
- Just because I have them amongst the first lot of chips from Lez:
- DS12887 Real Time Clock.
- AMD 9511 Floating Point Unit.
- The greater availability of SD cards.
- Just to avoid repeating what Martin has done.
I also need to take a look at what Andy did for Rememoriser.
For the first stage I intend to use one of the Z180 UARTs for user interface. For a later stage I will probably add a version of my Propeller VGA display driver. Alternately, I have a couple of 1980's 640x200 monochrome LCD displays which I know a Propeller can drive.
In terms of maximising performance of the main CPU there are advantages to continue to use a UART for keyboard input, and if what is received is keyboard press and release events rather than character codes it can be just as versatile as directly interfacing the keyboard. So another possible extension is a microcontroller keyboard interface. Various options for the actual keyboard:
- I have an existing matrix keyboard left over from an old project.
- PS/2 keyboards are not so common today, but are easy to interface.
- USB keyboards require a more powerful microcontroller.
- Keyboards are cheap. Take one apart and there is probably a matrix keyboard inside.
- The SD card interface.
- CPM/FAT. The idea is to produce a BDOS which has largely the same ABI as CP/M, but which produces a FAT file system on disk. The function calls will be the same, and the FCB will differ only in the contents of the allocation section (bytes 16-31). Low level utilities will have to be re-written, but I would expect most user programs to run unmodified.
- CP/M Plus. Hence the real-time clock.