> The question remains: are you sure the UART does not introduce the
> delay, have you checked the datasheet?
>
> --
Gilles.
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?
Thanks,
Philip Ha
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