Re: [Discuss-gnuradio] USRP2, is that possible to skip the Ethernet and pass data through general purpose (physically accessible) inputs to the FPGA?

[Malihe Ahmadi]

Hi MArcus,
Assuming I want to use Ethernet, let's say I want to send the stream 
'0100001', and I pick DBPSK as the modulation. can you please explain 
what is the relation of the DBPSK modulated data and "GMII_RXD" input to 
the FPGA or "sample" input to the dsp_core_tx? is that FPGA receives 8 
bits per symbol sent over Ethernet?
Also, can I get those occasional programming instructions you mentioned 
to be hard-coded (for example changing the firmware so that it uses some 
default values instead of passing those values through GiGe) if I am 
interested in removing Ethernet and using debug bus to pass stream of 
data to the FPGA?
Thanks,
Malihe
Marcus D. Leech wrote:
> On 09/02/2010 02:48 PM, Malihe Ahmadi wrote:
> > I need 5 mega bit per sec of bandwidth and if I understand correct 
> > the rate of CMII_TX_CLK is 100 mega bit per second which is higher 
> > than what I want. The first reason I don't like GiGe is that it 
> > chunks the data and it can cause delay in its stream (I rather have 
> > my own protocol to transmit data to the FPGA) but I think I can bear 
> > with that! The second reason is that I don't know what is the 
> > relation between the stream of data generated at the source (for 
> > example by sig_source_i()) and the "GMII_TXD" input signal to the 
> > FPGA and eventually "sample_tx" input of dsp_core_tx. Can somebody 
> > explain that relation for me?
> > Also, does anybody have a ready to use Python code for USRP2 device 
> > which generates for example a SIN wave at the transmitter and 
> > captures it at the receives?
> >
> > Thanks,
> > Malihe
> The rate that data is actually sent over the GiGe depends on your 
> decimation/interpolation settings.
>
> The FPGA "sees" a continuous stream of numbers that represent a 
> signal, generally sinusoidal
>   in nature.  That "stream" is a complex-baseband representation of 
> your signal, which the
>   FPGA will interpolate, possibly digitally upconvert, and present to 
> the RF transmitter hardware.
>   Similarly on receive the FPGA gathers the complex baseband data from 
> the A/D, decimates and
>   filters it, and presents it to the GiGe interface for transmission 
> to the host as complex baseband
>   samples.
>
> I think you may be confusing the data rate of whatever modulation 
> scheme you want to use in
>   your application, with the rate that the *waveform data* is 
> presented into/out-of your
>   Gnu Radio flow-graph.
>
> The packetization is generally not an issue, unless you are running at 
> the "edge" where the
>   host computer can't keep up, which produces overruns on receive, and 
> under-runs on transmit.
>
> But for modest-bandwidth signals, with a decent CPU, this doesn't 
> happen very often.
>
> The GiGe interface is simply a convenient and relatively-cheap way of 
> getting complex-baseband
>   data into/out-of the FPGA (along with the occasional programming 
> instructions for the
>   PLL synthesizers, gain controls, etc, on the daughter-cards).