[time-nuts] GPS message jitter (was GPS for Nixie Clock)

[David]

On Tue, 19 Jul 2016 06:16:16 -0700, you wrote:

>On 7/18/16 9:44 PM, David wrote:
>> The aged 16550 has various timeouts so an interrupt is triggered with
>> a partially full buffer even if it is below the interrupt threshold.
>> For implementations which do not do that, I assume they intend for the
>> UART to be polled regularly.
>>
>exactly... you have some sort of blocking read that waits either for an 
>interrupt or for time to expire

Oh, from the application program interface?  Ya, that would be a
problem if it lacks a non-blocking read.  The UART itself has a status
flag which says if there is data available to be read but if you
cannot access that, then you have to wait for the UART's interrupt
timeout assuming it has one.

I seem to recall this issue coming up long ago in connection with
dodgy 16550 implementations where data was getting stuck below the
interrupt threshold but I never encountered it myself.  For the lower
level programming I have done, it was never an issue since I had
direct access to the hardware and could check the flags anytime I
wanted.