M37544 Datasheet, PDF (31/68 Pages) Renesas Technology Corp – SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

7544 Group
A/D Converter
The functional blocks of the A/D converter are described below.
[A/D conversion register] AD
The A/D conversion register is a read-only register that stores the
result of A/D conversion. Do not read out this register during an A/
D conversion.
[A/D control register] ADCON
The A/D control register controls the A/D converter. Bit 2 to 0 are
analog input pin selection bits. Bit 4 is the AD conversion comple-
tion bit. The value of this bit remains at “0” during A/D conversion,
and changes to “1” at completion of A/D conversion.
A/D conversion is started by setting this bit to “0”.
[Comparison voltage generator]
The comparison voltage generator divides the voltage between
AVSS and VREF by 256, and outputs the divided voltages.
[Channel selector]
The channel selector selects one of ports P25/AN5 to P20/AN0,
and inputs the voltage to the comparator.
[Comparator and control circuit]
The comparator and control circuit compares an analog input volt-
age with the comparison voltage and stores its result into the A/D
conversion register. When A/D conversion is completed, the con-
trol circuit sets the AD conversion completion bit and the AD
interrupt request bit to “1”. Because the comparator is constructed
linked to a capacitor, set f(XIN) to 500 kHz or more during A/D con-
version.
b7
b0
A/D control register
(ADCON : address 003416, initial value: 1016)
Analog input pin selection bits
000 : P20/AN0
001 : P21/AN1
010 : P22/AN2
011 : P23/AN3
100 : P24/AN4
101 : P25/AN5
110 : Disable
111 : Disable
Disable (returns “0” when read)
AD conversion completion bit
0 : Conversion in progress
1 : Conversion completed
Disable (returns “0” when read)
Fig. 33 Structure of A/D control register
■ Notes on A/D converter
The comparator uses internal capacitors whose charge will be lost
if the clock frequency is too low.
Make sure that f(XIN) is 500 kHz or more during A/D conversion.
As for AD translation accuracy, on the following operating condi-
tions, accuracy may become low.
(1) Since the analog circuit inside a microcomputer becomes sen-
sitive to noise when VREF voltage is set up lower than Vcc
voltage, accuracy may become low rather than the case
where VREF voltage and Vcc voltage are set up to the same
value..
(2) When VREF voltage is lower than [ 3.0 V ], the accuracy at the
low temperature may become extremely low compared with
that at room temperature. When the system would be used at
low temperature, the use at VREF=3.0 V or more is recom-
mended.
Data bus
b7
b0
A/D control register
(Address 003416)
P20/AN0
P21/AN1
P22/AN2
P23/AN3
P24/AN4
P25/AN5
3
A/D control circuit
A/D interrupt request
Comparator
A/D conversion register (low-order) _{(Address 003516)}
10
Resistor ladder
Fig. 34 Block diagram of A/D converter
Rev.1.04 2004.06.08 page 31 of 66
REJ03B0012-0104Z
VREF
VSS