osmo-trx: Generating frame sync output on GPIO?

[Tom Tsou]

Hi Harald,

On Fri, May 26, 2017 at 1:59 PM, Harald Welte <laforge at gnumonks.org> wrote:
> I was wondering if it is possible to generate a TDMA frame sync output
> on a GPIO line of a USRP device (and/or any other SDR supported by
> osmo-trx).  This way it would be much easier to do e.g. BER testing
> by using a pure waveform/pattern generator for the "MS side".

I recently tested GSM frame triggering with a B210. The UHD code
itself for GPIO triggering is fairly straightforward.

        /* Configure GPIO-0 pin*/
        usrp->set_gpio_attr("FP0", "CTRL", 0x00);
        usrp->set_gpio_attr("FP0", "DDR", 0x01);

        /* Set rise and fall times */
        usrp->set_command_time(TIME_A);
        usrp->set_gpio_attr("FP0", "OUT", 0x01);
        usrp->set_command_time(TIME_B);
        usrp->set_gpio_attr("FP0", "OUT", 0x00);

I tested the frame trigger using the B210 with osmo-trx in a new
signal generator application (to be announced shortly), which works
fine.

But, when I attempt to trigger with full-duplex osmo-trx, the trigger
becomes unstable for reasons that I do not fully understand. I'm still
looking into the cause.

So right now we can generate a frame trigger for the BTS downlink,
but, unfortunately, not while the uplink receiver is enabled.

  -TT