Electronic – Building a simple manual Intel 8085 study kit

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I am toying with the idea of building an Intel 8085 demonstration/study kit which would consist of an Intel 8085 microprocessor on a PCB, where all input nodes (including the clock X1, X2) would be connected to switches and the output nodes would be connected to LEDs. The user would then manually simulate the memory step-by-step by using the switches and storing the data on paper.

My question is whether this would be feasible. In particular, can the 8085 handle a "clock" signal that is activated by hand, i.e. very very low and irregular frequency? Moreover for the bidirectional bus, what would be a good design to keep the read and write modes? And is there anything that I should be especially aware of, or that may easily go wrong?

Best Answer

No. The 8085 uses dynamic logic internally, which means that it has a minimum required clock frequency for operation. The maximum clock period is 2000 ns, which corresponds to a minimum frequency of 500 kHz.

You need to look for a processor that's rated for "static" operation, whose clock frequency specification goes all the way to 0 Hz. (There might be CMOS 8085 "clones" from other manufacturers that meet this requirement.1) Even then, you'll have to add circuitry to "debounce" the switches you use for the clock and certain other critical (i.e., edge-sensitive) signals.

1 Maybe not. I just checked, and the OKI 80C85 used in my TRS80 Model 100 still has the maximum clock period of 2000 ns.

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