The PLL spreadsheet makes the precise register ranges allowed for each mode
quite clear, and shows a few inaccuracies in the b-spec. In particular, the
N register value may range from 1 to 6 instead of 3 to 8. This should close
the gap we've seen in the reachable frequencies.
This shows one of the reasons for the gaps: with the other settings, the VCO
is too low inside the gap. However, it also points out another issue: we
aren't using the high end of the VCO range due to some other limits being hit.
Keith Packard
Eric Anholt