On 08/10/2011 05:18 PM, Philip Ha wrote:
>
>> The question remains: are you sure the UART does not introduce the
>> delay, have you checked the datasheet?
>>
> Hi Gilles,
>
> I apologize for the delay in responding the last message.
>
> I measured the latency of the UART by pulsing the DTR pin on the
> serial port as soon as the ISR is called. Using an oscilloscope
> I monitored the DSR input signal from the function generator and
> the DTR signal from the serial port. The time difference between
> signals was 6-9 us. Therefore the latency of the UART is < 10 us
> at all times.
>
> As long as the UART latency is consistent the size of it does not
> matter for the test I am performing. An input signal that comes in
> every 1000000 us should result in the 16550 ISR being called every
> 1000000 us.
>
> The function generator is accurate to 1us, the UART ISR latency
> jitter is approximately 3 us. Therefore I would not expect to see
> 100 us of timing measurement inacurracy using OS time stamps. It
> is also strange that this timing measurement inaccuracy changes
> every time the system is rebooted.
>
> Any thoughts?
Could you try measuring the jitter in cpu cycles with rdtscll, in
kernel-space? If the inaccuracy persists, check that cpufreq is not
enabled, and that if running on a multicore system, tscs are
synchronized. Apart from that no, no idea.
--
Gilles.
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