Choice of Clock Speed?

[Anthony Drake]

Spencer,
How did you decide what clock speed to design to?  I know there are several versions of the Z80 that are the same except for the max speed they can run, from 2.5 MHz for the first one, to 4 MHz for the Z80A, 6 MHz for the Z80B, and 8 MHz for the Z80H.  When I had a Z80-based Kaypro II back in the '80s, there was a magazine called Microcornucopia that published instructions on how to double the factory speed of the Kaypro.  I did the mod on my own machine and that of a friend's, and the results were marvelous.  I wish I could put my hands on that article to see what the criteria were.  Of course everything on the bus needs to be compatible with the clock speed, but how did you choose what you did?
--Tony

[Spencer Owen]

Hi Tony

The key thing is that the 68B50 is driven from the same clock as the CPU. So, with a 7.3728 Mhz crystal, the baud rate is 115200 bps.

As a general rule, this is good and what you want. The Z80 I supply is rated up to 10 MHz and pretty much any serial device and handle 115200.

However, if you have a lower rated CPU (or a Russian Z80 clone as one RC2014 owner has) or are using a 6850 instead of a 68B50, or have a terminal that can't manage that speed (like a Cambridge Z88), then it's worth going for the lower 3.6864 MHz crystal which gives 57600 baud rate.

Of course, if you want flashy blinkenlights, or serial running even lower, you can just go lower and lower speed.

Hope that helps

Spencer

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[Anthony Drake]

Ah yes; I understand.  Thanks for the insight!

--Tony

[Anthony Drake]

I got curious about how I was able to increase the processor clock speed on the Kaypro without affecting the speed of the RS-232 port.  I found schematics and a theory of operation for the Kaypro II online.  The machine used Zilog programmable SIO and PIO chips, and the SIO had its own clock.  The baud rate could be set by outputting a hex value to a certain port address.  The theory of operations manual said the SIO chip was a more complex chip than the Z80, and programming it was a bear!

--Tony

[Ken Boak]

Hi

I have managed to run the RC2014 at 7.3728MHz successfully using an old Z80A (Zilog original) and a 6850 (ST Microelectronics). There seems to be sufficient unspecified additional clock frequency tolerance in these parts to allow you to get away with it.

The system clock frequency could possibly be raised (doubled?) , if a 20MHz Z80 were fitted - then the limiting factor would be the access times of the ROM (45nS) and RAM (55nS).  We might need the series termination resistors on some of the address and data lines.

Ken