TPU3: alternate PTA and DIO

I am using PTA to measure a signal's period.  No problem.  If there is
not a transition within X ms I need to check if it is high or low.  I am
trying to switch, in that case, to the DIO input function.  All I get
for PIN_LEVEL is 0x0000.  (My DIO input works fine by itself.)

Is there some magic to switching functions?  I set CP=0 and HSR=0 before
setting up the DIO.

Steve