Re: [Discuss-gnuradio] USRP DDC usage

[Eric Blossom]

On Thu, May 26, 2005 at 04:41:17PM -0700, Matt Ettus wrote:
> 
> > Even if you set the frequency to 0, there is a "random" initial phase
> > that is unknown, and thus you cannot separate the two channels.
> 
> The initial phase is always 0 at power up.  If you never set the
> frequency to something other than 0, it will always stay that way.  Its
> a simple change to make the phase go back to zero.  Patches welcome...
> 
> Matt

A couple of hints:

You'll need a bit in an FPGA register to control the resetting to
zero.  Edit usrp/firmware/include/fpga_regs_standard.h and add

// Reset phase accumulator to zero for those with bits set.
// E.g., writing a 0x000f resets them all.  Writing 0x0001 resets 
// the phase accumulator associated with DDC 0.

#define FR_PHI_RESET           44


Doing a make in usrp/firmware/include will generate
fpga_regs_standard.v from fpga_regs_standard.h

Then look at these files to figure out the rest:
  usrp/fpga/sdr_lib/serial_io.v
  usrp/fpga/toplevel/usrp_std/usrp_std.v

Eric