I've been working on a software keying loop with the GPIO on an X310, and I 
measured the latency for setting a GPIO pin output wrapped back into a GPIO 
input to be around 5 ms on average.  This was done by setting a pin manually, 
and then immediately polling the input pin to see how long before it switched 
states.

I haven't had anyone who could look at the FPGA side of the X310 yet, but I was 
wondering if it would be even possible to implement something in the FPGA like 
the ATR functionality, except it would allow for a keying loop (a Clear to Send 
GPIO output wrapping to a Request to Send GPIO input) and prevent transmission 
until the condition was true.  I'm hoping that would significantly decrease the 
latency (less than 1 ms, hopefully) if I don't have to bother polling the pin.

Right now I'm using:  UHD 4.0, and GNU Radio 3.9.5.

Thanks and regards,
Jeff

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