More progress on the Z80 computer:
* The computer core (processor, memory, etc) was transferred piece by piece from the solderless breadboard to a wire-wrapped version.
* Termination of the data and address busses was added (1K resistors to ground)
* A 555-based internal clock was added; this will allow execution at speeds up to 1MHz.
* The virtual-control-panel circuit was pared down; it now consists of a PIC16F877A, a MAX232 chip, and a few resistors and capacitors. It’s still on solderless breadboard at this point; a more permanent control panel is planned, using three microcontrollers and having a lot more functionality.
After verifying that it all still worked, I decided to see how fast it would go (the idea being that if it ran well at speed, it should be very reliable at the slow speeds we’ll be using in class next term.) I connected it up to a signal generator and a mixed-signal Agilent scope. It proved to be stable at up to slightly more than 9MHz clock speed (18MHz into the J/K flip-flop).
Here’s a picture of the system in action.
Here is a trace of the system running a "Fibonacci" program. Execution starts at address 0x000A, with a JMP 0x0003 instruction. (The analog trace at the top is address line 12 — which goes high whenever the Z80 writes to memory location 0x1234. The reason that the data for this location is ambiguous is that, unlike the other steps in the program, this data is constantly changing.)
