David Taylor david-taylor at blueyonder.co.uk.invalid
Thu Nov 15 12:43:15 UTC 2012

On 15/11/2012 08:23, unruh wrote:
> On 2012-11-15, David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
>> If you search for "latency" here:
>>     http://www.raspberrypi.org/phpBB3/viewtopic.php?f=44&t=13883
>> "jbeale" says: "My experiments with NTP-PPS timing in the 3.1.9 kernel
>> with a relatively unloaded system indicate that my R-Pi has an interrupt
>> latency ranging between 10 and 80 microseconds, and occasionally gets as
>> long as 150 microseconds (see below)"
> Hm. Looks pretty bad.
> I once did tests with a 300MHz Linux machine, where I would send out a
> signal on one of the output pins on the parallel port, reading the time
> at which I did so,  and then read in the time on the Ack interrupt driven pin
> and found delays typically of 1-2 microseconds (timing only to the
> microsecond due to the clock reading function I used). That he seems to
> be betting up to 150us delay is really bad.

Yes, it was rather worse than I expected.  Perhaps someone has already 
tried a similar test to yours.  I have neither the skill nor the time to 
write such a modified driver to measure the latency .... but the 
Raspberry Pi would be an ideal unit to try that test.
Web: http://www.satsignal.eu

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