![](http://datasheet.mmic.net.cn/30000/M37225M6-XXXSP_datasheet_2359636/M37225M6-XXXSP_51.png)
51
SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER for VOLTAGE SYNTHESIZER
with ON-SCREEN DISPLAY CONTROLLER
M37225M6/M8/MA/MC–XXXSP
M37225ECSP
MITSUBISHI MICROCOMPUTERS
Rev. 1.0
Note: VREF indicates the reference voltage (= Vcc).
Fig. 8.8.3 Changes in A-D Conversion Register and Comparison Voltage during A-D Conversion
8.8.6 Conversion Method
Set the A-D conversion interrupt request bit to “0” (even when A-
D conversion is started, the A-D conversion interrupt reguest bit
is not set to “0” automatically).
When using A-D conversion interrupt, enable interrupts by setting
A-D conversion interrupt enable bit to “1” and setting the interrupt
disable flag to “0.”
Set the VCC connection selection bit to “1” to connect VCC to the
resistor ladder.
Select analog input pins by the analog input selection bit of the
A-D control register.
Set the A-D conversion completion bit to “0.” This write operation
starts the A-D conversion. Do not read the A-D conversion regis-
ter during the A-D conversion.
Verify the completion of the conversion by the state (“1”) of the
A-D conversion completion bit, the state (“1”) of A-D conversion
interrupt reguest bit, or the occurrence of an A-D conversion in-
terrupt.
Read the A-D conversion register to obtain the conversion re-
sults.
Note : When the ladder resistor is disconnect from VCC, set the VCC connec-
tion selection bit to “0” between steps and .
8.8.7 Internal Operation
When the A-D conversion starts, the following operations are auto-
matically performed.
The A-D conversion register is set to “0016.”
The most significant bit of the A-D conversion register becomes
“1, ” and the comparison voltage “Vref” is input to the comparator.
At this point, Vref is compared with the analog input voltage “VIN .”
Bit 7 is determined by the comparison results as follows.
When Vref < VIN : bit 7 holds “1”
When Vref > VIN : bit 7 becomes “0”
With the above operations, the analog value is converted into a digi-
tal value. The A-D conversion terminates in a maximum of 50 ma-
chine cycles (8.5
s at f(XIN) = 8 MHz) after it starts, and the conver-
sion result is stored in the A-D conversion register.
An A-D conversion interrupt request occurs at the same time as A-D
conversion completion, the A-D conversion interrupt request bit be-
comes “1.” The A-D conversion completion bit also becomes “1.”
Table 8.8.1 Expression for Vref and VREF
A-D conversion register contents “n”
(decimal notation)
0
1 to 255
Vref (V)
0
VREF
2
VREF
512
VREF
2
VREF
4
VREF
512
VREF
2
VREF
4
VREF
8
VREF
512
VREF
2
VREF
4
VREF
8
VREF
512
VREF
256
12 3 456 7 8
1 0000 0 0 0
12
100
0 0
0
1000
0 0
0
1
12 3 4 5 6
7 1
–
±
–
±±
0 000 0
0 0 0
Contents of A-D conversion register
Reference voltage (Vref)
[V]
0
A-D conversion start
1st comparison start
3rd comparison start
8th comparison start
2nd comparison start
Digital value corresponding to
analog input voltage.
A-D conversion completion
(8th comparison completion)
±±
±
–
±
.......
: Value determined by mth (m = 1 to 8) result
m
.....
VREF
256
(n – 0.5)