Pins 11 to 14 are shown on datasheet as multiplexed. Which configuration register is to be used to select DAC or ADC operation?
Modified the part number
Assume you mean ADuC7126 ... as long as the DACs are in tri-state you can apply external signals to the PINs to be converted by the ADC. If you enable the DACs you can convert the actual output from the DAC back with the ADC. No further configuration required.
Basically powering down a DAC "enables" the corresponding ADC to measure an input.
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