[time-nuts] USB to Serial Connectors for GPS Modules

Jim Lux James.P.Lux at jpl.nasa.gov
Thu Jun 5 19:39:26 EDT 2008

At 04:10 PM 6/5/2008, Matthew Smith wrote:
>Quoth Ulrich Bangert at 2008-06-05 13:58...
> > consider the latency times of ANYTHING on a USB provided virtual serial
> > connection to be in the order of 1 ms. If no serial data is send but
> > only status signals change their state the latency can even be higher.
>Thanks (and to the others who responded to this).  Just wanted to know
>where I was.
>I will make sure that I've got "real" serial ports with "real" kernel
>drivers for timing applications and use USB only for non time-critical
>stuff, like control messages, etc. (given a module with with 2 serial
>ports and only 1 port available on the house computer, for instance.)

I wouldn't assume that the latency on a hardware serial port, 
depending on the OS, is any better than 1 ms, either.

Particularly if you are expecting the process to be something like:

value change on serial port line (Carrier Det)
interrupt from hardware
fielded by serial port ISR
blocked process waiting on CD interrupt marked as ready to run
<indeterminate time waiting for next scheduler pass>
process starts running.

While it might only take microseconds to fire up the ISR, it could be 
milliseconds before your process starts to run (unless you're using 
something at the device driver level?)

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