APPLICATION
2.6 A-D converter
2–143
3825 GROUP USER’S MANUAL
Fig. 2.6.1 Changes in A-D conversion register and comparison voltage during A-D conversion
2
f(XIN)
(3) Conversion time
In the high-speed operation mode, A-D conversion terminates in a maximum 50 cycles (12.5
s at
f(XIN) = 8 MHz) after a start of A-D conversion.
In the middle-speed operation mode, A-D conversion terminates in a maximum 56 cycles (14
s at
f(XIN) = 8 MHz) after a start of A-D conversion.
For the A-D converter, the main clock input oscillation frequency f(XIN) divided by 2 is used (Note 1),
so A-D conversion time is obtained basically by the following expression.
Conversion clock period =
A-D conversion time = Conversion clock period ! Conversion cycle
However, the number of conversion cycles varies depending on internal clock
φ and trigger.
Notes 1: Use the A-D converter in the state where bits 5 and 7 of the CPU mode register (address
003B16) are “0” (high-speed mode or middle-speed mode).
As the comparator is composed of a capacitance coupling, use the A-D converter in a state
of f(XIN)
500 kHz.
2: When an external trigger is selected, the A-D conversion being executed is stopped by
inputting a falling signal to the ADT pin during A-D conversion, and A-D conversion is
resumed.
The A-D conversion register holds the previous conversion result until A-D conversion is
completed.
3: When an external trigger is selected, an ADT/A-D conversion interrupt may occur by switch-
ing the interrupt source selection bit from “1” to “0” or “0” to “1.”
1 00 00 0 0 0
1 2 1 0 00 00
10 00
0 0
0
1
12 3 4 56 7 1
0 0 0 0 00 00
Contents of A-D conversion register
Reference voltage
[V]
0
A-D conversion start
1st comparison start
3rd comparison start
8th comparison start
2nd comparison start
m
12 3 4 5
6 7 8
Disital value corresponding to
analog input voltage
A-D conversion completion
(8th comparison
completion)
: Value determined by
m th (m = 1 to 8) result
m
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
±
.......
.....
≥