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CHAPTER 10 A/D CONVERTER
User’s Manual U15104EJ2V0UD
10.4 Operations of A/D Converter
10.4.1 Basic operations of A/D converter
(1) Select one channel for A/D conversion with A/D converter analog input channel specification register 3 (ADS3).
(2) Sample the voltage input to the selected analog input channel with the sample & hold circuit.
(3) 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 termination of A/D conversion.
(4) Bit 7 of the successive approximation register (SAR) is set and the tap selector sets the series resistor string
voltage tap to (1/2) V
DD
.
(5) The voltage difference between the series resistor string voltage tap and analog input is compared with a
voltage comparator. If the analog input is greater than (1/2) V
DD
, the MSB of SAR remains set. If the input
is smaller than (1/2) V
DD
, the MSB is reset.
(6) Next, bit 6 of 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) V
DD
• Bit 7 = 0: (1/4) V
DD
The voltage tap and analog input voltage are compared and bit 6 of SAR is manipulated with the result as
follows.
• Analog input voltage
≥
Voltage tap: Bit 6 = 1
• Analog input voltage < Voltage tap: Bit 6 = 0
(7) Comparison of this sort continues up to bit 0 of SAR.
(8) Upon completion of the comparison of 8 bits, any valid digital resultant value remains in SAR and the resultant
value is transferred to and latched in A/D conversion result register 3 (ADCR3).
At the same time, the A/D conversion termination interrupt request (INTAD3) can also be generated.
Caution
The value immediately after A/D conversion has been started may not satisfy the ratings.