[ntp:questions] Re: linux system clock egregiously slow!
dave-gnus at bfnet.com
Wed Jan 21 21:10:26 UTC 2004
Me> 0. turn off interrupts
Me> 1. make sure the MIPS real-time clock is running
Me> 2. store the 32-bit number "270,000,000" (ten seconds) into
Me> countdown timer #5
Me> 3. store zero into the program cycle counter
Me> 4. loop while timer #5 > 270,000,000 minus 2,700,000 (.1 seconds)
Me> 5. read back the program cycle counter: that is the number of
Me> cycles in .1 seconds
Me> mips_counter_frequency = (number of cycles in .1 seconds) * 10
Hal> That looks like a resonable recipe to me, but that doesn't mean
Hal> it works as expected. What rate is timer #5 running at? Is
Hal> there some roundoff in setting it up?
Timer #5 is running off a 27MHz crystal with a spec of +/-30PPM.
Since the MIPS core is running at 300MHz, I expect that the error in
setting timer #5 is on the order of ten times smaller than the crystal
Hal> Print out the value and see if it makes sense.
It says about 149MHz, which is what I expect.
Hal> There have been round off errors when HZ was a strange value. I
Hal> forget the details.
Hmm... I'll look into that, but HZ is 100, which is about un-strange
as it gets.
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