Re: [uCsimm] rtc and processor maintained clock

From: Michael Durrant (mdurrant@rt-control.com)
Date: Wed Jan 26 2000 - 17:38:12 EST


> We have an application which requires accurate timing in the short term,
> little on no drift within say 5 minutes. The ucsimm seems a good
> starting point and we have one which is working fine, I think. We have
> been doing some timing tests and it appears that the processor
> maintained clock is running approx 7% slower than real time and the rtc
> on the ucsimm. That is the processor maintained clock is loosing 7
> seconds in every 100.

Their can be many reasons for this possibly dram refresh and or
dma for the lcd. Both of these steal bus time from the cpu.
 
... stuff deleted
 
> We have looked around all the init code and the various timers appear to
> be setup ok. We are running a pre1 kernel. One interesting fact come
> to light is the cpuspeed reported in /proc/cpuinfo is only 15.2MHz
> rather than the 16.5ishMHz that is supposed to be running. We have not
> changed any of the PLL settings. This difference is in the order of the
> 7% we are looking for, so somewhere in the kernel timing code or timer
> setup stuff something must be awry. The fact that the onboard rtc is
> maintaining real time indicates that the 32kHz xtal is near enough the
> correct frequency.

The /proc/cpuinfo has not been verified to be accurate.

--
Michael Durrant
RT-CONTROL Inc.
mdurrant@rt-control.com 

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