
7542 Group
Rev.3.02 Oct 31, 2006 Page 55 of 134
REJ03B0006-0302
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 3 is the A/D conversion clock selection bit. When “0” is set to this
bit, the A/D conversion clock is f(X
IN
)/2 and the A/D conversion time
is 122 cycles of f(X
IN
). When “1” is set to this bit, the A/D conversion
clock is f(X
IN
) and the A/D conversion time is 61 cycles of f(X
IN
).
Bit 4 is the A/D conversion completion bit. The value of this bit re-
mains 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
AV
SS
and V
REF
by 1024, and outputs the divided voltages.
[Channel selector]
The channel selector selects one of ports P2
7
/AN
7
to P2
0
/AN
0
,
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 A/D conversion completion bit and the A/D
interrupt request bit to “1”. Because the comparator is constructed
linked to a capacitor, set f(X
IN
) in order that the A/D conversion
clock is 250 kHz or over during A/D conversion.
■
Notes on A/D converter
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 V
REF
voltage is set up lower than Vcc
voltage, accuracy may become low rather than the case
where V
REF
voltage and Vcc voltage are set up to the same
value..
(2) When V
REF
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 V
REF
=3.0 V or more is recom-
mended.
Fig. 65 Structure of A/D control register
Fig. 66 Structure of A/D conversion register
Read 8-bit (Read only address 0035
16
)
b7
b0
b9
b8
b7
b6
b5
b4
b3
b2
(Address 0035
16
)
Read 10-bit (read in order address 0036
16
, 0035
16
)
b7
b0
b9
b8
(Address 0036
16
)
b7
b0
b7
b6
b5
b4
b3
b2
b1
b0
(Address 0035
16
)
Note: High-order 6-bit of address 0036
16
returns “0” when read.
Not used (returns “0” when read)
A/D conversion completion bit
0 : Conversion in progress
1 : Conversion completed
b7 b0
Analog input pin selection bits
000 : P2
0
/AN
0
001 : P2
1
/AN
1
010 : P2
2
/AN
2
011 : P2
3
/AN
3
100 : P2
4
/AN
4
101 : P2
5
/AN
5
110 : P2
6
/AN
6
(Note 1)
111 : P2
7
/AN
7
(Note 1)
Notes 1: These can be used only for 36 pin version.
2: A/D conversion clock=f(X
IN
) can be used
only when ceramic oscillation or on-chip oscillator is used.
Select f(X
IN
)/2 when RC oscillation is used.
A/D control register
(ADCON : address 0034
16
, initial value: 10
16
)
A/D conversion clock selection bit
(Note 2)
0 : f(X
IN
)/2
1 : f(X
IN
)