Friday, 26 August 2011

17. Old Timer

Thinking about what to use as a clock source to tick over the Z80, I remembered I had a couple of 555 timers. These have been around since 1971 and not much has changed. There are several modes you can operate these 555s but I am going to get it to generate a squarish pulse using the 'astable' confuration. You can read up about how to do that elsewhere on the web. Suffice to say I'm using 2x 1k resistors and a 10nF capacitor. Hook these up to the 555 and you get a pusle from pin 3. Yay!

Here a pic of the setup:

Now to measure the pulse we have produced. The Xprotolab Oscilloscope comes into it own here. I connect pin3 from the 555 to the CH1 input. It seems we have a pulse operating at 42.55KHz. Ok its not the speediest rate to run a Z80 at but it'll do to start with.

Here's the output from the oscilloscope:

There is formula for working out what combination of resistors and capacitor give a particular frequency, however, as I found out using resistors of too low a value gave a very weak pulse, ie the high voltage was not enough to trigger a response on the CLK input to the Z80. However, just changing the capacitor and keeping the 1K resistors, does allow you to adjust the frequency. Changing it to a 470uF almost gets you a 1 seconds pulse. Its slightly over, and it does also depend on the tolerance of the resistors, but its close enough. This lead me to ponder if there was such a thing as a variable capacitor, and then I remembered from my Denshi Block days that is what was inside the tuner. With something like that we could have a variable speed computer. Something to look at later.

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