deveopmen
Rev.B2 for proof reading
Usage precaution
Mitsubishi Microcomputers
M32C/83 group
SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
337
A-D Converter
(1) Write to each bit (except bit 6) of A-D i (i=0,1) control register 0, to each bit of A-D i control register 1,
and to each bit of A-D i control register 2 when A-D conversion is stopped (before a trigger occurs).
In particular, when the Vref connection bit is changed from
“
0
”
to
“
1
”
, start A-D conversion after an
elapse of 1
μ
s or longer.
(2) When changing A-D operation mode, select analog input pin again.
(3) Using one-shot mode or single sweep mode
Read the correspondence A-D register after confirming A-D conversion is finished. (It is known by A-
D conversion interrupt request bit.)
(4) Using repeat mode, repeat sweep mode 0 or repeat sweep mode 1
Use the undivided main clock as the internal CPU clock.
(5) When f(X
IN
) is faster than 10 MHz, make the frequency 10 MHz or less by dividing.
(6) Output impedance of sensor at A-D conversion (Reference value)
To carry out A-D conversion properly, charging the internal capacitor C shown in Figure 1.31.1 has to
be completed within a specified period of time T. Let output impedance of sensor equivalent circuit be
R0, microcomputer
’
s internal resistance be R, precision (error) of the A-D converter be X, and the A-
D converter
’
s resolution be Y (Y is 1024 in the 10-bit mode, and 256 in the 8-bit mode).
Vc is generally V
C
= V
IN
{1
–
e
}
And when t = T,
V
C
=V
IN
–
X
IN
=V
IN
(1
–
X
Y
e
–
T
C (R0 +R)
T
Hence, R0 =
–
–
R
With the model shown in Figure 1.31.1 as an example, when the difference between V
IN
and V
C
becomes
0.1LSB, we find impedance R0 when voltage between pins V
C
changes from 0 to V
IN
-(0.1/1024) V
IN
in
time T. (0.1/1024) means that A-D precision drop due to insufficient capacitor charge is held to 0.1LSB at
time of A-D conversion in the 10-bit mode. Actual error however is the value of absolute precision added
to 0.1LSB. When f(X
IN
) = 10 MHz, T = 0.3
μ
s in the A-D conversion mode with sample & hold. Output
impedance R0 for sufficiently charging capacitor C within time T is determined as follows.
T = 0.3
μ
s, R = 7.8 k
, C = 3 pF, X = 0.1, and Y = 1024 . Hence,
R0 =
–
–
7.8 X10
3
3.0 X 10
3
C (R0 + R)
T
–
C (R0 + R)
t
–
Y
Y
X
Y
X
C
ln
Y
X
3.0 X 10
–
12
ln
1024
0.1
0.3 X 10
-6