126CHAPTER 10 A/D CONVERTER10.4 A/D Converter Operations10.4.1 Basic operations of A/D converter(1) Set the number of analog input channels with A/D converter input select register (ADIS).(2) From among the analog input channels set with ADIS, select one channel for A/D conversion with A/D convertermode register (ADM).(3) Sample the voltage input to the selected analog input channel with the sample & hold circuit.(4) Sampling for the specified period of time sets the sample & hold circuit to the hold state so that the circuitholds the input analog voltage until termination of A/D conversion.(5) Bit 7 of the successive approximation register (SAR) is set. The series resistor string voltage tap is set at(1/2) AV REF by the tap selector.(6) The voltage difference between the series resistor string voltage tap and the analog input is compared by thevoltage comparater. If the analog input is larger than (1/2) AV REF, the MSB of the SAR remains set. If it issmaller than (1/2) AV REF, the MSB is reset.(7) Next, bit 6 of SAR is automatically set and the operation proceeds to the next comparison. In this case, theseries resistor string voltage tap is selected according to the preset value of bit 7 as described below.• Bit 7 = 1 : (3/4) AV REF• Bit 7 = 0 : (1/4) AV REFThe voltage tap and analog input voltage are compared and bit 6 of SAR is manipulated with the result asfollows.• Analog input voltage ≥ Voltage tap : Bit 6 = 1• Analog input voltage ≤ Voltage tap : Bit 6 = 0(8) Comparison of this sort continues up to bit 0 of SAR.(9) Upon completion of the comparison of 8 bits, any effective digital resultant value remains in SAR and theresultant value is transferred to and latched in the A/D conversion result register (ADCR).At the same time, the A/D conversion termination interrupt request (INTAD) can also be generated.
