Re: [Discuss-gnuradio] Strobe signals

[Brian Padalino]

On Sun, Aug 10, 2008 at 7:53 PM, Sebastiaan Heunis <address@hidden> wrote:
> Hi
>
> I read a lot of the USRP's Verilog lately and sort of figured out what
> happens on the FPGA.  master_control.v generates all the control
> signals as far as I understand.  I would just like to know how the
> strobe signals work.
>
> Looking at master_control.v it seems as if rx_sample_strobe is always
> 1.  strobe_decim is a clock signal (if decim=8, strobe_decim is going
> to be an 8MHz clock)?  Is this correct?  hb_strobe is going to be a
> 4MHz clock then and is used to tell the rx buffer how often we have
> samples available for the USB link?

rx_sample_strobe is strobed at the rate of the ADC - 64MHz.  Since
this is also the master clock rate, assigning it to be 1 makes sense.

The strobe generator is here:

    
http://gnuradio.org/trac/browser/gnuradio/trunk/usrp/fpga/sdr_lib/strobe_gen.v

It takes in an enable signal which performs the downcount, and when
that count is reached, a one-shot pulse comes out.

The halfband filter generates it's own output strobe here:

    
http://gnuradio.org/trac/browser/gnuradio/trunk/usrp/fpga/sdr_lib/hb/halfband_decim.v

The rx_buffer just gets fed samples until there is enough data
available for the FX2 to perform a DMA transfer of 512-bytes.

I think there's an old testbench laying around somewhere that I saw if
you wanted to still simulate and watch what the signals do.

As always, I am not an authority - Matt or Johnathan are the real
experts.  I just read the code.

Hope that helps.

Brian