Re: Z80 its refresh pin

[zwsdotcom@gmail.com]

On Mar 27, 9:35 am, srk...@gmail.com wrote:

> microsoft's (with a few FPGA implementation tweaks), so i expect it is
> a timing issue. Without knowing what could invoke the Z80 into this
> state makes it hard to begin looking for it.

Is the CPU actually executing? Trigger on falling edge of _M1 and
verify that you see the correct waveform (_M1 and _RFSH should be out
of phase within the instruction fetch portion of the instruction
cycle).

Is _BUSRQ externally asserted? That will take the Z80 off-bus
permanently and float _RFSH.

And for the obvious one... Are you sure that the Z80 is actually
asserting _RFSH and it's not something else on your PCB... isolate
that pin of the chip, put a pullup on it, and see what it's doing.