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'6818 calendar IC'
1999\10\23@071942 by Jinx

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1999\10\23@100137 by paulb

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Jinx wrote:

> Member of Mensa NZ,

 Modest to boot...

> MC6818 calendar chip, as found in many PCs.  They aren't as common in
> new machines

 Because they were integrated into the 82C205 (IIRC) support chip, then
the later chipsets.  That said, I too have many 286 motherboards
sporting one.

 It is my understanding that the Motorola (Intel cloned it as an
82{x}18 or something similar) and Hitachi chip had a nasty bug and
gained a bad name in the original designs.

 I *think* the bug was that it used a single-inverter CMOS driver for
the crystal, and most 32,768 Hz crystals won't operate reliably in
parallel mode.  As a result, virtually *all* PCs but the very earliest
used, including *all* those using the 82C505, and many still do, an
external oscillator using a 4069 with two gates in the loop to excite
the crystal in series mode.

 I'm always open for someone who really knows to confirm/ deny this.
Of course, there may indeed be other bugs/ tricks in the (MC/HD)146818.
--
 Cheers,
       Paul B.

1999\10\23@164737 by steve

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>   I *think* the bug was that it used a single-inverter CMOS driver for
> the crystal, and most 32,768 Hz crystals won't operate reliably in
> parallel mode.  As a result, virtually *all* PCs but the very earliest
> used, including *all* those using the 82C505, and many still do, an
> external oscillator using a 4069 with two gates in the loop to excite
> the crystal in series mode.

I used a 6818T in a product some years ago. I think this is an SPI
version of the same thing. The problem I recall with it was that it
had software programmable gain but this defaulted to the value needed
for a 4MHz crystal. That meant that it would overdrive the 32kHz one
until corrected in software. This is not good for 32kHz crystals.
Just for entertainment value, a 32kHz crystal overdriven this much
makes many transitions. Several MHz worth. So it appears to be
working, sort of.

Steve.



======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: spam_OUTstevebTakeThisOuTspamtla.co.nz      fax +64 9 820-1929
======================================================

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