8-Bit A/D Converter Operations; Basic Operations Of 8-Bit A/D Converter - NEC mPD780344 Series User Manual

CHAPTER 12 8-BIT A/D CONVERTER ( µ PD780344, 780344Y SUBSERIES)

12.4 8-Bit A/D Converter Operations

12.4.1 Basic operations of 8-bit A/D converter

<1> Select one channel for A/D conversion using analog input channel specification register 0 (ADS0).
<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 7 of the successive approximation register (SAR) is set. The series resistor string voltage tap is set to
(1/2) AV by the tap selector.
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<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
input is smaller than (1/2) AV
<6> Next, bit 6 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 7, as described below.
• Bit 7 = 1: (3/4) AV
• Bit 7 = 0: (1/4) AV
The voltage tap and analog input voltage are compared and bit 6 of SAR is manipulated as follows.
• Analog input voltage ≥ Voltage tap: Bit 6 = 1
• Analog input voltage < Voltage tap: Bit 6 = 0
<7> Comparison is continued in this way up to bit 0 of SAR.
<8> Upon completion of the comparison of 8 bits, an effective digital result value remains in SAR, and the result
value is transferred to and latched in A/D conversion result register 1 (ADCR1).
At the same time, the A/D conversion end interrupt request (INTAD0) can also be generated.
Cautions 1. Start conversion (ADCS0 = 1) after 14 µ s have elapsed following the setting of ADCE0. If
ADCE0 is not used, the first conversion result immediately after the setting of ADCS0 is
undefined.
2. In standby mode, the A/D converter operation is stopped.
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, the MSB is reset.
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, the MSB of SAR remains set. If the analog
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