Zilog Z80 freezes while binary counting up to 65 536 (2^16)

Question

A few days ago, I got Z80A CPU from eBay. So I tried to run it with classic 555 timer as a clock signal generator. I connected the 8-bit data bus of the CPU to the ground to "simulate ROM" (00 for NOP). I wired up all missing inputs: WAIT pin 24, pin 16, 17, 25, 26 to the 5V. And all address bus pins to LEDs to see the NOP, how it behaves. Normally it should count up to 2^16 without any problems, but it randomly freezes somewhere. I also tested my 2nd Zilog Z80C version with the same circuit and it works just fine.

What could cause the freezing of the CPU? Is the CPU defective?

Edit (solution): The problem was solved with blocking capacitor between + and - on breadboard (row 2), decoupling capacitor between +5V and GND on the Z80 and with the reset button. Thank you for all the answers.

Answer 1

There are many issues here.

1. As it is already said in comments, decoupling capacitor is a must!

2. 555 (non-CMOS) timer output is very much like the output of TTL ICs, however Z80 requires a firm logic one. When feeding Z80 clock pin from a TTL output, you should use pullup resistor of 200..500 Ohm.

3. NMOS Z80 uses dynamic logic, that means it has some minimal clocking frequency, and it won't work at all if your clock is much less than datasheet's minimal frequency (or a datasheet's maximal clock period).

4. You should also initialize Z80 with the /RESET signal lasting several clocks before ever hoping to get anything sane from the CPU.

The easiest way to get Z80 working in your configuration is to swap it to CMOS version, which is fully static (i.e., it has no limit to how slow the clock might be). Additional attention must be paid to get clock levels right (swap TTL 555 to CMOS one like TS555 or add a pullup resistor) and decoupling capacitors to both 555 and Z80. Finally, add a reset button and keep it pressed for several clocks in the beginning.

Answer 2

I'd just like to expand on a couple of points in lvd's excellent answer.

Reset Circuit

You might get by with just using a jumper wire to short the reset pin to ground for a brief moment after you've powered up the CPU. It's worked for me, but if you're having problems it's best to build a proper reset circuit.

Many CPUs have a minimum length for the reset pulse. If your reset switch isn't debounced, it might generate a perfectly fine result pulse but then bounce on release, leaving you with a too-short pulse (and possibly a confused CPU) after that.

A 555, of which you clearly already have at least one, works well to generate a pulse of a specific length; just wire it up as a one-shot. Gates with Schmitt-trigger inputs, such as a 74LS14, are also great for generating pulses and debouncing switches, and you can use an spares as regular gates (for address decoding or whatever) as well.

Some examples of the above circuits, and a good introduction to reset circuits, are given in the Reset Circuits section of Wilson Mines Co. 6502 Primer. I strongly encourage you to read through the relevant pages in that primer as you build sections of a computer, such as the clock, reset circuits, address decoding, etc. etc. It will give you a general understanding of the problems and solutions in each area and also give a lot of specific advice on how to avoid pitfalls.

Decoupling Capacitor

Regarding the decoupling capacitor, 0.1 μF is a typical value; just drop one lead of the cap into the the breadboard column for the Vcc pin and the other into the breadboard column for the ground pin of that chip. (This goes right over the top of the chip for the Z80.) If the two pins are too far apart, and you don't want to take the time to extend the cap with a wire, even just using an adjacent ground rail will still help enormously.

You should do this for all your chips, not just the CPU.

If you have an oscilloscope, run your system with a 1 MHz clock and, with and without the decoupling capacitor, have a look at the waveforms of your clock input and an output such as A0. Freqently you will get a pretty dramatic demonstration of what a decoupling capacitor does for you.