[time-nuts] PPS over USB (Was SiLabs)

Hal Murray hmurray at suespammers.org
Fri Aug 11 20:05:11 UTC 2006


> Is the USB bus really a good way to get 1 PPS into a machine? I would
> think it would have quite a bit of jitter in it. 

I'm not a USB wizard, but I work with people who are.

There is no interrupt mechanism on a USB cable.  The host/master polls the 
devices/slaves.  Typical numbers are 10 ms or 1 ms.

At setup time (I forget what USB calls it), the device can tell the host how 
often it wants to be polled.  (I assume the host can ignore that or round up 
to what it can handle.)


For a homebrew project, it might be possible to turn things inside out.  Have 
the host flap a signal, say from the printer port, and the GPS device records 
the time.  Then the device reports that time via USB.

One of the popular GPS over serial port devices works that way.  (I forget 
which one.)

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