Path: utzoo!dciem!dretor!bpd
From: bpd@dretor.dciem.dnd.ca (Brian P. Dickson)
Newsgroups: comp.sys.amiga.tech
Subject: Re: ciatimer.c ...
Message-ID: <1366@dretor.dciem.dnd.ca>
Date: 13 Jan 89 15:35:29 GMT
Article-I.D.: dretor.1366
References: <3236@sugar.uu.net>
Reply-To: bpd@dretor.dciem.dnd.ca (Brian P. Dickson)
Organization: NTT Systems, Inc., Toronto, Canada
Lines: 47

In article <3236@sugar.uu.net> karl@sugar.uu.net (Karl Lehenbauer) writes:
[most of souce code to very handy routines deleted]

>/* TIMER - Amiga CIA Timer Control Software
>
>	The routine ElapsedTime gets the time by getting the number of
>	65536 microsecond ticks that the handler has seen and retrieving
>	the 0-46911 number of 1.397 microsecond ticks from the CIA timer
>	registers, scaling them to 1.000 microsecond ticks and returning
>	the shifted-and-ored result.
>*/

After checking the hardware manual, I calculate the actual ticks to be
1.396825 microseconds (clock interval is 279.365 nanoseconds). While
the difference is small, it WILL cause clock drift, if you use these
routines as a realtime clock as opposed to just a timer. To get a much
more accurate clock, the following changes to the code should be made:

>
>/* timeslice is 46911 intervals.  Each interval is 1.397 microseconds,
> * this should correspond to a timing interval of 65536 microseconds */
>#define CIA_TIME_SLICE ((unsigned short) 46911)

/* timeslice is 40000 intervals. Each interval is 1.396825 microseconds,
 * this should correspond to a timing interval of 55833 microseconds,
 * almost exactly */
#define CIA_TIME_SLICE ((unsigned short) 40000)
#define CIA_TIME_QUANTITY ((unsigned short) 55833)

/* and now replace all occurrences of 65536 with CIA_TIME_QUANTITY */

>
>VOID CIAInterrupt()
>	CIA_Microseconds += 65536;
	CIA_Microseconds += CIA_TIME_QUANTITY;

/* the approximation for the scaling in ElapsedTime is acceptable, since
 * it will not affect the clock itself */

>-- 
>-- uunet!sugar!karl  | "We've been following your progress with considerable 
>-- karl@sugar.uu.net |  interest, not to say contempt."  -- Zaphod Beeblebrox
>-- Usenet BBS (713) 438-5018

--
Brian Dickson
bpd@dretor.dciem.dnd.ca