If you are not using the ADC, you can ground both reference pins, or leave them
unconnected. If you leave the pins unconnected, be aware that any coupled noise
could exceed the absolute maximum ratings listed on page 18 of the datasheet.
If the voltage between the external REFIN(+) and REFIN(-) pins goes below 0.3 V
or either the REFIN(+) or REFIN(-) inputs is open circuit, the ADuC816 detects
that it no longer has a valid reference. In this case, the NOXREF bit of the
ADCSTAT SFR is set to a 1. If the ADuC816 is performing normal conversions and
the NOXREF bit becomes active, the conversion results revert to all 1s.
Therefore, it is not necessary to continuously monitor the status of the
NOXREF bit when performing conversions. It is only necessary to verify its
status if the conversion result read from the ADC Data Register is all 1s.