wel97459 / comx-35f

FPGA comx-35 project

COMX-35F

This is a cycle-accurate COMX-35 computer that has been implemented on an FPGA, with all the hardware is written in SpinalHDL.

Video

The system currently supports NTSC video. Everything needed for PAL is available in the hardware.

For NTCS color you will need a clock source of 14.318MHz, currently, I'm using a si5351 breakout board. Both Burst and Luma use a 3-bit R2R ladder DAC. That gets mixed the same as on the COMIX-35 with a NPN transistor driving the video output.

Inputs

For the keyboard, I'm using a stranded PS2 interface.

there is a serial interface available for reading and writing programs to memory, which also allows for halting the CPU and resetting the system. This interface also can press keys for type in programs. The baud rate for this interface is 57600.

Using an Op-amp as a comparator for the input pin of the tape interface works well for the loading of programs.

Simulation

You can also simulate the core with an emulated NTSC monitor capable of displaying the output of the video hardware. This is extremely useful for obtaining a comprehensive hardware trace of the system as it runs. However, the simulator only runs at 2 FPS.

TODOs & Problems

It appears that unintentional improvements were made to the video hardware of this implementation. As a consequence, it is possible that any code developed on the system may not function correctly on the original COMX-35 hardware. Similarly, EMMA will also run the same code without error.