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Path: utzoo!linus!philabs!cmcl2!seismo!hao!noao!terak!doug
From: doug@terak.UUCP (Doug Pardee)
Newsgroups: net.micro.cbm
Subject: Re: Flaky TOD Alarm Interrupts
Message-ID: <571@terak.UUCP>
Date: Wed, 22-May-85 12:34:35 EDT
Article-I.D.: terak.571
Posted: Wed May 22 12:34:35 1985
Date-Received: Fri, 24-May-85 08:20:21 EDT
References: <1755GMW@psuvm>
Organization: Terak Corporation, Scottsdale, AZ, USA
Lines: 32

I have never seen anything definitive from Commodore/MOS Technology,
but I believe that the Time-Of-Day feature is "not supported" in the
CIA chips.  My guess is that MOS Technology had to "steal" some gates
from TOD in order to fix problems in other sections of the CIA.

Consider that the C-64 kernal goes to an awful lot of trouble to avoid
using the TOD.  The resulting software TOD is much less reliable than
the CIA TOD should be.  Conclusion:  the hardware TOD is actually less
reliable than the software version.

The TOD features are not mentioned anywhere in the official Commodore
64 Programmer's Guide, except in the reproduction of the CIA's data
sheet.

Various problems have been noticed in the CIA's TOD.  Mine, for example,
both run at about 2.4 times the correct rate (those seconds really
click off!).

If we presume that in your case the CIA TOD *is* reliable, I can think
of two areas where there might be trouble.  First, remember that the
registers are in BCD.  They count 0-1-2-3-4-5-6-7-8-9-16-17-18-19...
And the "hours" register has a flag in bit 7 for PM, so it counts
0-1-2-3-4-5-6-7-8-9-16-17-128-129-...-136-137-144-145

Another problem area would be interfering interrupts.  You should turn
off the 60-Hz "jiffy" timer while playing with the TOD interrupts.  But
remember that this will also disable your keyboard.

Good luck!
-- 
Doug Pardee -- Terak Corp. -- !{ihnp4,seismo,decvax}!noao!terak!doug
               ^^^^^--- soon to be CalComp