7470/7471/7477/7478 GROUP USER’S MANUAL
1-142
HARDWARE
1.14 A-D converter
2
2 Conversion time
q After a start of A-D conversion, this A-D conversion terminates after 50 cycles (12.5
s at f(XIN)
= 8 MHz).
q Main clock input oscillation frequency f(XIN)/2 is used as an operating clock for the A-D converter,
so the A-D conversion time can be basically obtained by the following expression.
A-D conversion time =
f(XIN)
× conversion cycle (50: cycles)
Note: Because the comparator is configurated by capacity coupling, use the A-D converter in the
following condition.
f(XIN) > 1 MHz
Accordingly, use the A-D converter in the condition that bit 7 of the CPU mode register
(address 00FB16) is “0” (ordinary mode).
[Setting method]
In the 7470/7471 group
1 Clear the bit of the Port P2 direction register corresponding to the used analog input pin to “0”
(input mode).
2 Clear the port pull-up control bit corresponding to the used analog input pin to “0” (no pull-up).
3 Clear the A-D conversion interrupt request bit of the Interrupt request register 1 to “0.”
Note: After A-D conversion is started, the A-D conversion interrupt request bit is not cleared to “0”
automatically.
4 When using an A-D conversion interrupt, set the A-D conversion interrupt enable bit to “1” to
provide an interrupt enable state.
5 Set the A-D control register as follows.
q Select an analog input pin by the analog input pin selection bit.
q Set the VREF connection selection bit to “1” and connect VREF to a ladder resistor.
6 Wait for 1.0
s or more as VREF stabilizing time.
7 Clear the A-D conversion end bit of the A-D control register to “0.” (With this setting, A-D conversion
is started.)
In the 7477/7478 group
1 Clear the A-D conversion interrupt request bit of the Interrupt request register 1 to “0.”
Note: After A-D conversion is started, the A-D conversion interrupt request bit is not cleared to “0”
automatically.
2 When using an A-D conversion interrupt, set the A-D conversion interrupt enable bit to “1” to
provide an interrupt enable state.
3 Set the A-D control register as follows.
q Select an analog input pin by the analog input pin selection bit.
q Clear the A-D conversion end bit to “0.” (With this setting, A-D conversion is started.)
Don't read the contents of the A-D conversion register during A-D conversion.
For register setting, refer to “Table 1.14.1 Setting at A-D Conversion.”