Re: [Discuss-gnuradio] A couple of questions related to UHD

[Josh Blum]

On 02/17/2011 01:57 PM, Feng Andrew Ge wrote:
> I posted one question in an early email, but it might be better to put
> it together with other questions here. I'd greatly appreciate your answers.
> 
> (1)  When transmitting UHD packets (with USRP2 + WBX), after each
> transmission, there is either one or a few "U" printed out. What does
> "U" mean?  Is it used in the update packet from USRP2 to the host?
> 

Underflow on transmit = host not producing samples fast enough.

If you are concerned about performance, I encourage you to checkout the
"next" branch on uhd.git and see if you get less underflows.

> (2) In uhd:usrp:single_usrp, there is a function "read_rssi", is this
> function enabled? If so, does it mean that we can read RSSI value
> directly from USRP2? I suppose that this function is implemented on the
> FPGA since there exits a rssi.v function in the uhd library.
> 

This function is daughterboard dependent and only works with the XCVR2450.

> (3) In uhd:usrp:single_usrp, it seems that "get_rx_rate" is for setting
> sampling rate, but then what is "get_rx_bandwidth" for?
> 

It tells you the IF bandwidth of the daughterboard.

> (4) In flow control,  how to specify "ups_per_fifo", just like
> device_address["ups_per_fifo"] = "xx"? Also, what are the FIFOs on USRP2
> and what are their default sizes?
> 

Yes but I recommend using the default.

And the FIFO is the 1MB SRAM on the USRP2.

> (5) For "concurrency_hint", it says "the number of threads to run the IO
> service", does it mean the total number of threads associated with the
> UDP socket for each USRP_SINK or USRP_SOURCE, or something else?
> 

These are background threads that run the recv() and send() calls on the
udp socket. If you switch to the next branch, this has been removed.

> (6) Do control messages generated by functions like get/set_tx_gain and
> get/set_tx-freq have to wait on the same UDP buffers which are also used
> for exchanging data samples between GNU Radio and USRP?
> 

No, control is independent from data.

-josh