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232
CHAPTER 13 A/D CONVERTER
User
’
s Manual U14260EJ3V1UD
13.4 A/D Converter Operation
13.4.1 Basic operations of A/D converter
<1>
<2>
<3>
Select one channel for A/D conversion using analog input channel specification register 0 (ADS0).
Set bit 0 (ADCE0) of A/D converter mode register 0 (ADM0) to 1 and wait for 14
μ
s or longer.
Set bit 7 (ADCS0) of the ADM0 register to 1 to start the A/D conversion operation.
(<4> to <10> are operations performed by hardware)
The voltage input to the selected analog input channel is sampled by the sample & hold circuit.
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 finished.
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.
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. If the analog
input is smaller than (1/2) AV
REF
, the MSB is reset.
Next, bit 8 of SAR is automatically set, 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) AV
REF
Bit 9 = 0: (1/4) AV
REF
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
Comparison is continued in this way up to bit 0 of SAR.
<10> Upon completion of the comparison of 10 bits, an effective digital result value remains in SAR, and the result
value is transferred to and latched in A/D conversion result register 0 (ADCR0).
At the same time, the A/D conversion end interrupt request (INTAD0) can also be generated.
<11> Repeat steps <4> to <10>, until ADCS0 is cleared to 0.
To stop the A/D converter, clear ADCS0 to 0.
To restart A/D conversion from the status of ADCE0 = 1, start from <3>. To restart A/D conversion from
the status of ADCE0 = 0, however, start from <2>.
<4>
<5>
<6>
<7>
<8>
<9>
Cautions 1. If bit 7 (ADCS0) of A/D converter mode register 0 (ADM0) is set to 1 without setting bit 0
(ADCE0) to 1, the first A/D conversion value immediately after A/D conversion has been
started may not satisfy the rated value. Take measures such as polling the A/D conversion
end interrupt request (INTAD0) and removing the first conversion results.
The same may apply if ADCS0 is set to 1 without the lapse of a wait time of 14
μ
s (MIN.) after
ADCE0 has been set to 1. Make sure that the specified wait time elapses.
2. The A/D converter stops operation in standby mode.