Testing a Mostek z80 CPU found in a Walker Sat Nav 801.
Max clock = 2.5MHz
3 the same Z80PIO chips - can't check these now.
1 x z80 MK3880N-41RL -- binary counting the same as the circuit above, but you need a min of 4 LEDs to start this and can go to all 16 buss pins 40-29 next to pin 28 ground and 1-5.
You NEED to unplug completely to restart the power an arbitrary- amount of times due to an unknown factor, to restart the counting operation as it seems to latch some/all of the LEDs on most times - possibly saved state or refresh cycle counts etc? That makes building this as a tester circuit invalid as is! Need to make it reset to the count state at power on somehow or have a reset type button to force it to count state? Can't find that info in the PDF or make sense of it.
When the unit counts with 8+ LEDs, the LEDs from 8-11 flash rather than stay on to mark the power of 2 - power saving?:
16 LEDs all lit = 2^16-1 count = 65535
This Mostek unit doesn't get hot driving these white LEDs with no limiting resistors either.
I added the remaining 5 blue LEDs, pins 1-5, which should give the total count to 2^16 = 65536 for this 16 bit CPU.
Found the reset pin 26 by trial and error shorts to gnd:
It's now consistent so useful as a tester so worth building for the experience.