Re: [Discuss-gnuradio] Daughterboard clocking on USRP2

[Jason Abele]

On Tue, Mar 16, 2010 at 04:34:44PM -0500, John Orlando wrote:
> Hi all,
> Just looking through the WBX daughterboard code in the latest git repo
> for adding support for the USRP2 (which can be found at
> usrp2/firmware/lib/).  I noticed that in adf4350.c, there is a #define
> for INPUT_REF_FREQ that is defined as 50e6 (50 MHz), which is then
> used in all of the follow-on calculations for determining register
> settings.  I had thought that the USRP2 only provided a 100 MHz ref
> clock for use by the daughterboards on the clock_rx_p line, but I must
> be missing a divide by 2 some place.  Can anyone confirm A) if the WBX
> runs off of 50 or 100 MHz, and B) if it is 50 MHz, how the clk div 2
> occurs on the USRP2?

The adf4350 has a ref clock doubler and ref clock divider bit, in
addition to the R divider setting.  In the case of WBX on USRP2, we use
the ref clock divider bit (adf4350_regs.c, line 34, adf4350_regs_rdiv2),
hence the 100M clock is divided to 50M on the adf4350 before the R
divider.

I could have written the INPUT_REF_FREQ more clearly, my apologies for
the confusion.

Jason