UPD784938 Datasheet, PDF (411/733 Pages) NEC – 16-Bit Single-Chip Microcontrollers

CHAPTER 16 A/D CONVERTER
16.4 Operation
16.4.1 Basic A/D converter operation
(1) A/D conversion operation procedure
A/D conversion is performed by means of the following procedure:
(a) Analog pin selection and operation mode specification are set with the A/D converter mode register (ADM).
(b) Bit 7 (CS) of the ADM is set (to 1), and A/D conversion is started.
(c) When conversion starts, the MSB (bit 7) of the successive approximation register (SAR) is set (to 1) automatically.
(d) When bit 7 of the SAR is set (to 1), the tap selector sets the series resistor string voltage tap to
225 _{AVREF1 (=.. 1/2 AVREF1).}
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(e) The voltage difference between the series resistor string voltage tap and the analog input is determined by the
voltage comparator. If the analog input is greater than (1/2) AVREF1, the MSB of the SAR remains set (to 1), and
if it is less than (1/2) AVREF1, the MSB is cleared (to 0).
(f) Next, bit 6 of the SAR is set (to 1) automatically, and the next comparison is performed. Here, the series resistor
string voltage tap is selected according to the value of bit 7 for which the result has already been set, as shown
below.
• Bit 7 = 1 ........ ³⁸³ AVREF1 ^{.=. 3} AVREF1
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4
• Bit 7 = 0 ........ ¹²⁷ AVREF1 ^{.=. 1} AVREF1
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4
This voltage tap is compared with the analog input voltage, and bit 6 of the SAR is manipulated as follows according
to the result:
• Analog input voltage ≥ voltage tap: Bit 6 = 1
• Analog input voltage < voltage tap: Bit 6 = 0
(g) The same kind of comparison is continued up to the LSB (bit 0) of the SAR (binary search method).
Preliminary User’s Manual U13987EJ1V0UM00
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