Z80 Telescope Controller (was Homebrew newsgroups / forums?)

[Dwight K. Elvey]

>From: gtulloch at shaw.ca
>
>Well, ok, in that case ...
>
>I'm building a Z80 based microcontroller as a telescope controller, to 
>run a few stepper motors, calculate and correct for periodic errors as 
>well as XYZ errors in the mount. The driver circuit is done 
>(http://www.bbastrodesigns.com/cot/steppercircuit.html), and 
>currently runs off a DOS app which uses the parallel port to pulse 
>the motor windings. However, I prefer not to use a dedicated
>DOS PC so I'd like to put a Z80 in front of the driver with an RC 
>circuit connected to the NMI so I know how long the duration between 
>interrupts is and can thus calc the steps required to track. For historic
>reasons the code will be written in FORTH - a PC connected via serial
>will do all the heavy lifting as far as telling the Z80 where to point 
>and how. Comments on the veracity of this architecture welcome!
>
>I have a few questions I'm hoping someone can answer for me, starting 
>with the following: I have a Z80A and a 4 mhz TTL clock 
>oscillator - I'm wondering if it would be ok to connect the clock 
>directly to the CPU or is it wise to connect it via a parallel or 
>serial resonant oscillator circuit like those described here:
>
>http://www.z80.info/uexosc.htm
>
>Why be so elaborate? The data sheets for the oscillator doesn't say 
>"Make sure you connect via a parallel or serial resonent oscillator 
>circuit!"
>
>Thanks for any light you can shed on this, Google hasn't been helpful.
>I have references that do it both ways without explanation so I'm 
>confused!

Hi
 I think you are confusing a crystal oscillator with a crystal.
The circuits shown are for the crystal alone and you provide
the oscillator.
 If you are using a crystal, one needs to match the type of
crystal, parallel/series, to get it to work at the stated
frequency. This has to do with the phase shift that the crystal
provides for the oscillator. In one case, the phase is 0 degrees
while the other, it is 180 degrees. Even so, using ttl parts
for oscillators can have issues. If the crystal is over driven,
it will age quickly and drift in frequence or even fail to
oscillate. Also issues like startup time need to be addressed
in some applications. You'd want the oscillator running well
before releasing reset on a CPU.
Dwight

>
>Regards,
>  Gord
>
>cctalk-bounces at classiccmp.org wrote on 26/10/2005 09:25:58 PM:
>
>> Gord wrote...
>> > I'm building a Z80 based computer as a stepper motor controller, and 
>it
>> > would be nice if there was a homebrew newsgroup to ask questions on 
>since
>> > I don't think this is the group to do so - any recommendations?
>> 
>> Um... I would certainly think developing a new system around a classic 
>> processor (Z-80) is certainly ok on this group :)
>> 
>> Jay 
>> 
>