POWER SAVING FUNCTIONS
7902 Group User’s Manual
17-9
17.4 Stop of oscillation circuit, 17.5 Pin VREF disconnection
17.4 Stop of oscillation circuit
When a stable clock externally generated is input to pin XIN, power consumption can be saved by setting
the external clock input select bit to “1” to stop the drive circuit for oscillation between pins X IN and XOUT.
(See Figure 17.1.1.) At this time, the output level at pin XOUT is fixed to “H.” Also, if the system clock select
bit (bit 5 at address BC16) = “0,” the watchdog timer is not used when the stop mode is terminated owing
to an interrupt request occurrence; therefore, the microcomputer can start instruction execution just after
termination of the stop mode. When the system clock select bit = “1,” in this case, the watchdog timer is
used.
17.5 Pin VREF disconnection
When the A-D converter and D-A converter are not used, power consumption can be saved by setting the
VREF connection select bit to “1.” It is because the reference voltage input pin (VREF) is disconnected from
the ladder resistors of the A-D converter and D-A converter (See Figure 17.5.1.), and there is no current
flow between them.
When the VREF connection select bit has been cleared from “1” (VREF disconnected) to “0” (VREF connected),
be sure to start the A-D conversion or D-A conversion after an interval of 1 s or more has elapsed.
Fig. 17.5.1 Structure of A-D control register 1
Notes 1: These bits are invalid in the one-shot and repeat modes. (They may be either “0” or “1.”)
2: When using pin AN4, be sure that the pin INT3 select bit (bit 5 at address 9416) = “0.”
3: When using pin AN5, be sure that the pin INT4 select bit (bit 6 at address 9416) = “0.”
4: When using pin AN6, be sure that the D-A0 output enable bit (bit 0 at address 9616) = “0” (output disabled).
5: When using pin AN7, be sure that the pin INT2 select bit (bit 4 at address 9416) = “0” and the D-A1 output enable bit (bit 1
at address 9616) = “0.” When an external trigger is selected, pin AN7 cannot be used as an analog input pin.
6: When this bit is cleared from “1” to “0,” be sure to start the A-D conversion or D-A conversion after an interval of 1 s or
more has elapsed.
7: Writing to each bit of the A-D control register 1 must be performed while the A-D conversion halts.
1
0
RW
–
A-D control register 1 (Address 1F16
)
A-D sweep pin select bits
(Valid in the single sweep and repeat
sweep modes.)
(Note 1)
Fix this bit to “0.”
Resolution select bit
A-D conversion frequency (
φAD) select
bit 1
External trigger polarity select bit
(Valid when external trigger selected.)
VREF connection select bit (Note 6)
The value is “0” at reading.
b7 b6 b5 b4 b3 b2 b1 b0
0 0 : Pins AN0 and AN1 (2 pins)
0 1 : Pins AN0 to AN3 (4 pins)
1 0 : Pins AN0 to AN5 (6 pins) (Notes 2, 3)
1 1 : Pins AN0 to AN7 (8 pins) (Notes 2 to 5)
b1 b0
0 : 8-bit resolution mode
1 : 10-bit resolution mode
See Table 13.2.1.
0
0 : Falling edge of the pin ADTRG’s input signal
1 : Rising edge of the pin ADTRG’s input signal
0 : Pin VREF is connected.
1 : Pin VREF is disconnected.
Bit name
Bit
Function
At reset
R/W
0
1
2
3
4
5
6
7