[Discuss-gnuradio] Diverting data through external FPGA board

[Bahn William L Civ USAFA/DFCS]

Is it possible (in a relatively easy manner) to make the data from the ADC (or, 
probably better and more doable, from the DDC) available at the external pins 
of the USRP and to get external data from the USRP pins and feed that to the 
PC? Essentially we want to use the FPGA board as a front-end preprocessor.

Here is what we are trying to do:

Instead of passing the data up to the PC and processing it there, we would like 
to take the data (the 12-bits of I data should be sufficient for our needs) and 
pass it off to an external FPGA/DSP board. After that board is finished 
cranking on it, we would like to pass the data back to the USRP for upload to 
the PC over the USB cable for further processing. We don't need to feed the raw 
IQ data to the PC at all.

We realize that this sort of defeats the purpose of SDR but, for what we are 
doing, we have a lot of front-end processing that has to be done and we need to 
show that it can be done efficiently in parallelized hardware.

It would be nice if it can be done without having to delve into the USRP FPGA 
firmware code. We are willing to accept a significant performance penalty and 
run at very low data rates because this is purely proof-of-concept type stuff. 
In other words, if we have to read the IQ data from USRP normally and then send 
the I data back down to the external pins and then read another set of pins to 
get processed data back we are willing to do that as a start.

Thanks.