Re: Z80 Softcard speed?

["David Wilson" <mcs6502@gmail.com>]

On Feb 4, 5:54 am, Linards Ticmanis <ticma...@gmx.de> wrote:
> Alex Freed wrote:
> >> My question is, at how many MHz does the Z80 run effectively? (as long
> >> as you don't switch back to the 6502, that is.
>
> >> And is this number a constant or does it depend on what instructions are
> >> executed?
>
> > That's an easy one. The z80 clock pulses twice during ph1 and stays low
> > during ph0.
> > So there are 2 clocks every 1 uS. This gives a 2 MHz clock. More accurately
> > 14.28/7
> > or about 2.04 MHz.
>
> Thanks!
>
> Hmmm... what about the requirement to let the NMOS 6502 run a cycle
> every 5 or so microseconds, so that it doesn't lose its register
> contents? How is that handled?

See page 2-33 of Microsoft Softcard Volume 1.

"The Z-80 dynamic refresh control lines are used to implement this
function. Therefore, the 6502 "refresh" occurs immediately after an op
code fetch, and is thus transparent to the SoftCard and the user. No
wait cycles have to be added to any Z-80 machine cycles, because the
6502 refresh time is used by the Z-80 to decode the op code. While the
6502 has control of the bus again, the SoftCard address and data
buffers are placed in tri-state mode."