Sanguino AREF

I know you can change the internal reference in some units from 1v1 and 2v56 I tried this on my 644 running Sanguino with analogReference(INTERNAL2V56); and got a compiling error.

Are the only options internal (1v1??) and external?

From the 644 datasheet:

REFS1 REFS0 Voltage Reference Selection 0 0 AREF, Internal Vref turned off 0 1 AVCC with external capacitor at AREF pin 1 0 Internal 1.1V Voltage Reference with external capacitor at AREF pin 1 1 Internal 2.56V Voltage Reference with external capacitor at AREF pin

However the standard arduino IDE doesn't support the 644 processor, so you are dependent on modified 'core' library files to correctly handle the external reference commands required.

Lefty