[ECOS] Drift in Real Time Clock

Andrew Lunn andrew@lunn.ch
Tue Jun 5 18:31:00 GMT 2007

On Tue, Jun 05, 2007 at 09:59:21AM +0500, M Arshad Khan wrote:
> Hello
> we r running eCos on simple intel based PC. we want to find the drift
> in the real time colck, for this we simply capture a 100Sec signal
> using 20mSec interrupt. the signal was captured using a counter by
> triggering the channel of the counter using parallel port of the PC.
> the code is given below.
> from the results we found that  the drift was 7mili Sec in 100 Sec.
> This drift is very large to fitt for real time colck. we also tried
> this code on different PC's but app same results. can any body tell me
> why i am getting so much drift...
> any suggestions..

I believe the PC clock ticks at a base rate of 1193182Hz. The eCos
clock is usually configured for 10ms. That would be 11931.82
ticks. Obviously you cannot have fractional ticks, so you end up with
11932 ticks per 10ms. The error is then 0.0015%. 

0.0015% of 100Sec = 0.0015seconds i.e. 1.5ms. So something is not
right somewhere, but i think you get the idea. 

Check i have the clock frequency correct and perform your own
calculation about what accuracy is achievable.


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