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Re: [gpsd-dev] Very basic PPS question:

From: Hal Murray
Subject: Re: [gpsd-dev] Very basic PPS question:
Date: Sat, 19 Oct 2013 21:27:07 -0700

address@hidden said:
> If the pulse needs to be "stretched", how is it going to capture the rising
> edge. If the pulse has to be long, then there will be lots of jitter in the
> time measurement.

 I don't understand what you are trying to say.

"Stretched" means that you take a pulse that starts at T0 and lasts a short 
time and turn it into one that starts at T0+ and lasts a long(er) time.  
There are chips that do that.  T0+ is a few ns after T0, not important on 
this scale.

> It is best to have some kind of timer input capture, but failing that a
> high-priority (perhaps even non-maskable) edge interrupt on a GPIO pin that
> can store (and/or "set-to-.000") whatever counter-timer resource is being
> used for timing. 

That's a different issue.  If your hardware supports a counter/timer type 
capture, then you can avoid the jitter in interrupt processing latency.  The 
delay in the interrupt processing isn't a problem, that can be corrected.  
The problem is the jitter in the delay.

> It might be better to miss pulses than to recognize them very late. 

Yup.  The refclock corner of ntpd has a buffer of up to 64 samples.  When 
it's time to pass a batch of  samples up to the next layer (the polling 
interval), ntpd sorts that buffer and discards roughly 1/3 of the samples as 
outliers, then takes the average of what's left.  That works pretty well.

These are my opinions.  I hate spam.

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