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CHAPTER 14 A/D CONVERTER
(1) Successive approximation register (SAR)
This register compares the voltage value of analog input with the value of a voltage tap (compare voltage)
from the series resistor string, and holds the result of the comparison starting from the most significant bit
(MSB).
When the result is held down to the least significant bit (LSB) (end of A/D conversion), the contents of SAR
are transferred to the A/D conversion result register (ADCR).
(2) A/D conversion result register (ADCR)
This register holds the result of A/D conversion result. Each time A/D conversion has been completed, the
result of the conversion is loaded to this register from the successive approximation register.
ADCR can be read by an 8-bit memory manipulation instruction.
The contents of this register become undefined when the RESET signal is input.
(3) Sample and hold circuit
The sample and hold circuit samples analog input signals sequentially sent from the input circuit on a one-
by-one basis, and sends the sampled signals to the voltage comparator. This circuit holds the sampled analog
input voltage value during A/D conversion.
(4) Voltage comparator
The voltage comparator compares the analog input with the output voltage of the series resistor string.
(5) Series resistor string
The series resistor string is connected between AV
REF
and AV
SS
and generates a voltage to be compared with
an analog input.
(6) ANI0-ANI7 pins
These are eight channels of analog input pins of the A/D converter. They input analog signals that are
converted to digital values.
Pins other than those pins selected for analog input by A/D converter input select register (ADIS) can be used
as I/O ports.
Cautions 1. Observe the specified input voltage range of ANI0-ANI7. If a voltage of AV
REF
or higher,
or AV
SS
or lower (even within the range of absolute maximum ratings) is applied to a
channel, the converted value of that channel becomes undefined, or the converted value
of the other channels may be affected.
2. The analog input pins (ANI0-ANI7) are also used as I/O port pins (port 1). When A/D
conversion is performed with any of ANI0-ANI7 selected, do not execute the input
instruction for port 1 while conversion is in progress; otherwise, the conversion reso-
lution may be degraded.
If a digital pulse is applied to the pins adjacent to the pins currently used for A/D
conversion, the expected value of the A/D conversion may not be obtained due to
coupling noise. Therefore, do not apply a pulse to the adjacent pins to the pin under A/
D conversion.