TRF7970A: TRF7970A

Part Number:TRF7970A

I've got a board with the TRF7970A on it.  I've ported TI code to my platform to run this and (as far as I can tell) is largely the same as that running on TMS430 (with exceptions of timers).

This code ID's Mifare Classic tags no problem.  It doesn't see either a Mifare ultralight C or Ultralight Nano tag at all.  There is nothing coming back from those tags and no ISR from the chip being generated so the search times out eventually.

Code appears to send WUPA to start dealing with the tags (rather than REQA) but according to data sheets, that should be good and I've attempted to force to REQA with no change in behaviour.

Any ideas on what to look for?