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   Crystal UNclear...
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starhawk
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Crystal UNclear...
« on: Dec 20th, 2019, 1:16am »
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Background:
 
Some time ago, I'd borked up the original springloaded mechanism for the battery contacts in my Org2, an XP-labeled LA. I forget now exactly what happened... I think the spring fell out and got lost in the carpet. Regardless, it was replaced later with a standard snap-contact set.
 
Just now, I finally got around to swapping the springloaded mechanism in from the dead Org2 that Mikesan gave me to reverse-engineer... works perfectly but while I was inside, I noticed something that struck me as a bit odd...
 
The discovery and question:
 
The timing crystal, which drives the HD6303X CPU directly, is rated at 3.6884 MHz, rather than an even 2, 3, or 4 MHz... "Wait, what?" is what my brain said upon noticing this. Does anyone here know the rationale for such an odd frequency...? There's no proper video out, and sound on this thing is *ahem* more primitive than an IBM 5150 PC, so... I dunno, serial something?
jaap
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Re: Crystal UNclear...
« Reply #1 on: Dec 20th, 2019, 4:58am »
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Apparently this has to do with baud rates.
 
The crystal runs at 3686400 Hz. This number is nicely divisible by all the standard baud rates of 300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 38400, 57600, and 115200 bits per second. These rates can be achieved by dividing 3686400 by powers of 2 and sometimes one factor of 3. Halving a frequency is trivial to do in binary hardware, and while division by 3 is a bit trickier, a circuit that does this division does not need that many transistors either.
 
In any machine that needs to do communications at standard baud rates, e.g. via RS232, a clock crystal with this frequency is the default component.
 
The Psion's processor actually runs at a quarter of the speed of the clock crystal, at 921600 Hz. It has built-in circuitry for doing communications at baud rates of up to 9600 bps, and this is what is used by the Comms Link. See table 7 in the hd6303 datasheet:
https://www.jaapsch.net/psion/pdffiles/hd6303X_datasheet.pdf

Jaap Scherphuis
https://www.jaapsch.net/psion/
starhawk
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Posts: 105
Re: Crystal UNclear...
« Reply #2 on: Dec 20th, 2019, 4:05pm »
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Ahhh... I was guessing it was something like that! (Kind of had to be, logically...) Alas, math and I have a profound and nasty relationship going back quite a while, so I would've had quite a time figuring that one out on my own.
 
Thank you, Jaap, for your sleuthing. Well done, indeed -- or, at least, not ba[u]d... <nervous chuckle>
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