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CHAPTER 17 RESET
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17.3.3 Condition Required for Use of the Power-Down Reset Function
The power-down reset function can be enabled or disabled using software. The following condition is required to
use this function:
The power voltage must be within 4.5 to 5.5 V during normal operation, including the standby state.
The frequency of the system clock oscillator must be 400 kHz to 4 MHz.
Caution When the microcontroller is used with a power voltage of 2.7 to 4.5 V, add an external reset circuit
instead of using the internal power-down reset circuit. If the internal power-down reset circuit
is used with a power voltage of 2.7 to 4.5 V, reset operation may not terminate.
17.3.4 Power-Down Reset Function and Operation
This function is enabled by setting the power-down reset enable flag (PDRESEN) using software.
When this function detects a power voltage drop, it issues the reset signal to the microcontroller. It then initializes
the microcontroller. Stopping oscillation during reset prevents the power voltage in the microcontroller from fluctuating
out of control. When the specified power voltage recovers and the power-down reset operation is terminated, the
microcontroller waits the time required for stable oscillation using the timer. The microcontroller then enters normal
operation (starts from address 0).
Figure 17-4 shows an example of the power-down operation. Figure 17-5 shows an example of reset operation
during the period from power-down reset to power recovery.
Functions of the power-down reset
(1) This circuit always monitors the voltage applied to the V
DD
pin.
(2) When this circuit detects a power voltage drop, it issues a reset signal to the other parts of the microcontroller.
It continues to send this reset signal until the power voltage recovers or all the functions in the microcontroller
stop.
(3) This circuit stops oscillation during the reset operation to prevent software crashes.
When the power voltage recovers to the low-voltage detection level (3.5 V TYP., 4.5 V MAX.) before the power-
down reset function stops, the microcontroller waits the time required for stable oscillation using timer 1, then
enters normal operation mode.
(4) When the power voltage recovers from 0 V, the power-on reset function has priority.
(5) After the power-down reset function stops and the power voltage recovers before it reaches 0 V, the
microcontroller waits using timer 1 until oscillation becomes stable and the power voltage (V
DD
) reaches 2.7
V. The microcontroller then enters normal operation mode.