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Pluto RX timing indication

I have a radar related application that requires timing correlation between RX data from the Pluto (controlled by a host PC) and another signal (controlled by a microcontroller).

The TX signal is generated at precise intervals. However, I don't have knowledge of precisely when the Pluto starts to fill the RX buffer, so I have not found a good way to correlate the two. 

My first thought was to use TDD mode and use the GPO to generate a high signal when in RX mode. The microcontroller would monitor that pin, and report to the host PC the offset between it's last pulse and the GPO being driven high. I've found that this is also inconsistent because the GPO signal is asserted with the ENSM "RX"  state is entered, NOT when the buffer is filled. 

I'm curious to know if anyone has ways I could get some indication of when the RX buffer is filled. I'm not necessarily above modifying the HDL or firmware (i've already downloaded Vivado), but I'd prefer not to go down that rabbit hole. 

I should also probably mention: I'm receiving the data with a python script and the libiio python bindings.