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CHAPTER 11
A/D CONVERTER
User’s Manual U12790EJ2V0UD
(1) Successive approximation register (SAR)
This register compares the analog input voltage value to the voltage tap (compare voltage) value applied from
the series resistor string and holds the result from the most significant bit (MSB).
When up to the least significant bit (LSB) is set (end of A/D conversion), the SAR contents are transferred
to the A/D conversion result register.
(2) A/D conversion result register 3 (ADCR3)
This register holds the A/D conversion result. Each time A/D conversion ends, the conversion result is loaded
from the successive approximation register (SAR).
ADCR is read by an 8-bit memory manipulation instruction.
Reset input makes ADCR undefined.
Caution
When data is written to A/D converter mode register 3 (ADM3) and analog input channel
specification register 3 (ADS3), the contents of ADCR3 are undefined. Read the result of
conversion after conversion has been completed and before writing data to ADM3 and ADS3.
Otherwise, the correct conversion result may not be read.
(3) Power-fail comparison threshold value register 3 (PFT3)
This register sets the threshold value to be compared with the value of A/D conversion result register 3
(ADCR3).
PFT3 is read or written by using an 8-bit memory manipulation instruction.
(4) Sample & hold circuit
The sample & hold circuit samples the input signal of the analog input pin selected by the selector when
A/D conversion starts, and holds the sampled analog input voltage value during A/D conversion.
(5) Voltage comparator
The voltage comparator compares the sampled analog input voltage to the series resistor string output voltage.
(6) Resistor string
The resistor string is connected between AVDD and AVSS, and generates a voltage to be compared to the analog
input.
(7) ANI0 to ANI7 pins
These are 8-channel analog input pins used to input analog signals to undergo A/D conversion to the A/D
converter.
Cautions 1. Use the ANI0 to ANI7 input voltages within the specified range. If a voltage of AVDD or
higher or AVSS or lower is applied (even if within the absolute maximum ratings), the
converted value of the corresponding channel becomes undefined and may adversely
affect the converted values of other channels.
2. The analog input pins (ANI0 to ANI7) function alternately as input port pins (P10 to P17).
When one of ANI0 to ANI7 is selected for A/D conversion, do not execute an input
instruction to port 1; otherwise, the conversion resolution may drop.
If a digital pulse is applied to a pin adjacent to the pin being A/D converted, the expected
A/D conversion value may not be obtained due to coupling noise. Do not apply a pulse
to a pin adjacent to a pin being A/D converted.