I've decided to take some time to refresh my electronics knowledge after 30 years out of college. I've run into a small problem that I can't quite figure out and need a pointer.
TLDR: The Z80 initially starts (watching M1 strobing) but halts after just two or three, sometimes more, sometimes fewer, machine cycles.
I've built an ~ 5hz 555 astable circuit to drive the clock line of a Z80B. The Z80B has all of the data lines pulled down to ground. The INT, NMI, HALT, BUSRQ, and WAIT are all pulled high. I have the RESET on an RC circuit to go high after 1 second, plus a push-button to pull it low for a manual reset (pictured below... Ignore the crystal oscillator circuit; that isn't connected to anything).
The 555 is putting out a nice steady square pulse at about a 50% duty cycle. M1 will go high during reset and then cycle between one and five times, but then stops (seeming to indicate that the processor has stopped). With the data lines pulled to zero, I'm expecting it to continue to run, executing NOPs.
Can someone see anything obvious that I've done wrongly here? The thing that keeps running through my head (and that I can't seem to find any data on anywhere) is that the clock might be too slow if the Z80B uses dynamic registers internally. I can't seem to locate any authoritative datasheet for the Z80B.