CHAPTER 12 A/D CONVERTER
197
User’s Manual U11302EJ4V0UM
12.4 A/D Converter Operations
12.4.1 Basic operations of A/D converter
[1] Set the number of analog input channels using the A/D converter input select register (ADIS).
[2] From among the analog input channels set by ADIS, select the channel for A/D conversion using the A/
D converter mode register (ADM).
[3] The sample & hold circuit samples the voltage input to the selected analog input channel.
[4] Sampling for the specified period of time sets the sample & hold circuit to the hold state so that the circuit
holds the input analog voltage until the end of A/D conversion.
[5] Bit 7 of the successive approximation register (SAR) is set. The tap selector sets the series resistor string
voltage tap to (1/2) AV
REF
.
[6] The voltage difference between the series resistor string voltage tap and analog input is compared by
the voltage comparator. If the analog input is larger than (1/2) AV
REF
, the MSB of the SAR remains set.
If the input is smaller than (1/2) AV
REF
, the MSB is reset.
[7] Next, bit 6 of the SAR is automatically set and the operation proceeds to the next comparison. In this
case, the series 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
REF
The voltage tap and analog input voltage are compared and bit 6 of the SAR is manipulated using the
result as follows.
•
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 the SAR.
[9] Upon completion of the comparison of 8 bits, an effective digital result value remains in the SAR and the
result value is transferred to and latched in the A/D conversion result register (ADCR).
At the same time, the A/D conversion end interrupt request (INTAD) can also be generated.
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