CHAPTER 12 A/D CONVERTER
Preliminary User’s Manual U16315EJ1V0UD
232
12.4 A/D Converter Operations
12.4.1 Basic operations of A/D converter
<1>
Select one channel for A/D conversion with analog input channel specification register (ADS).
<2>
The voltage input to the selected analog input channel is sampled by the sample & hold circuit.
<3>
When sampling has been done for a certain time, the sample & hold circuit is placed in the hold state and the
input analog voltage is held until the A/D conversion operation is ended.
<4>
Bit 9 of the successive approximation register (SAR) is set. The series resistor string voltage tap is set to
(1/2) AV
REF
by the tap selector.
<5>
The voltage difference between the series resistor string voltage tap and analog input is compared by the
voltage comparator. If the analog input is greater than (1/2) AV
REF
, the MSB of SAR remains set to 1. If the
analog input is smaller than (1/2) AV
REF
, the MSB is reset to 0.
<6>
Next, bit 8 of SAR is automatically set to 1, and the operation proceeds to the next comparison. The series
resistor string voltage tap is selected according to the preset value of bit 9, as described below.
•
Bit 9 = 1: (3/4) V
DD
•
Bit 9 = 0: (1/4) V
DD
The voltage tap and analog input voltage are compared and bit 8 of SAR is manipulated as follows.
•
Analog input voltage
≥
Voltage tap: Bit 8 = 1
•
Analog input voltage < Voltage tap: Bit 8 = 0
<7>
Comparison is continued in this way up to bit 0 of SAR.
<8>
Upon completion of the comparison of 10 bits, an effective digital result value remains in SAR, and the result
value is transferred to the A/D conversion result register (ADCR) and then latched.
At the same time, the A/D conversion end interrupt request (INTAD) can also be generated.
Caution
The first A/D conversion value immediately after A/D conversion operations start may not fall
within the rating.