[USRP-users] using GPIO output to trigger external RF switches

Thomas Harder Thomas.Harder at oca.eu
Tue Jan 21 09:54:10 EST 2020


Thank you Jeff and Sam,
I found now also the gpio.cpp example. Really helpful to get familiar with GPIO.

>> To accomplish what you're talking about, I think you'd just need to use timed commands >>on both set_tx_rate() and set_gpio_attr(). If these are set to execute simultaneously, the >>GPIO line you set will go high on the same clock cycle as the LO retune.

In fact in my code which is based on the “tx_samples_from_file” example I start transmitting my data in the file in the future with tx_metadata_t  time specification and with timed commands I set set_gpio_attr() to the same point in future.
But this gives me just 1 output pulse in GPIO what is not sufficient for my application.

I am transmitting  in continuous mode (a fixed modulation in a while loop) and my center frequency stays fix during the whole transmission whereas with my data in the file I am modulating the center frequency to discrete frequencies in the whole bandwidth of 160MHz. I measured a transition time of the usrp  of around 20ns between different frequencies in my modulation.
To be able to postprocess my RF signal I would like to have a pulse each time the tx-frequency is changing.

As the ATR is just showing if the usrp is transmitting, receiving or idle it is not really helpful
And putting set_gpio_attr() in the while loop for my continuous transmission is slowing down the streaming and causes underruns.

Do you think of any other solution to get a pulse each time the transmission frequency of the USRP is changing?

Thomas

From: Sam Reiter
Sent: Tuesday, January 14, 2020 6:44 PM
To: Jeff S
Cc: usrp-users at lists.ettus.com; Thomas Harder
Subject: Re: [USRP-users] using GPIO output to trigger external RF switches

Thomas,

To accomplish what you're talking about, I think you'd just need to use timed commands on both set_tx_rate() and set_gpio_attr(). If these are set to execute simultaneously, the GPIO line you set will go high on the same clock cycle as the LO retune.

Sam


On Tue, Jan 14, 2020 at 5:59 AM Jeff S via USRP-users <usrp-users at lists.ettus.com> wrote:
Thomas,

I am looking at GPIO right now as well.  As to your question #2 on examples, I compiled and am able to run the example I found in the UHD software:

uhd/host/examples/gpio.cpp

I am able to set individual data lines to experiment with (and eventually use as a roadmap for my development) using commands like:

./gpio --dwell 0 --bitbang --ddr 0x00ff --out 0x0055

which I verified works using a logic analyzer.  I eventually want to try to create a keying loop, but that may take some FPGA control (?).

Jeff



From: USRP-users <usrp-users-bounces at lists.ettus.com> on behalf of Thomas Harder via USRP-users <usrp-users at lists.ettus.com>
Sent: Tuesday, January 14, 2020 5:07 AM
To: usrp-users at lists.ettus.com <usrp-users at lists.ettus.com>
Subject: [USRP-users] using GPIO output to trigger external RF switches 
 
Hi,
I am continuously transmitting a waveform with several RF with 200MSPS with my USRP X310 (modified code on the basis of “tx_samples_from_file.cpp” with a waveform in a .dat file). I would like to get a trigger signal from the USRP when it is changing the tx- frequency which I set in my .dat file. 
Could the GPIO deliver this trigger without changing the image? 
I am using the SRAM image with UHD 3.14.1
 
A second question is concerning the GPIO example on https://files.ettus.com/manual/page_gpio_api.html.
I copied and paced it in the “tx_waveforms” example just changing the usrp device handle to “usrp” but when I compiled it and started tx, I couldn’t measure any tension on PIN 4 and 6 as defined in the example. Are there more examples how to program the GPIO?
Thank you,
Thomas
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