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CHAPTER 16 RESET
173
16.3 POWER-ON/POWER-DOWN RESET FUNCTION
The
μ
PD17120 subseries is provided with two reset functions to prevent malfunctions from occurring in the
microcontroller. They are the power-on reset function and power-down reset function. The power-on reset function
resets the microcontroller when it detects that power was turned on. The power-down reset function resets the
microcontroller when it detects drops in the power voltage.
These functions are implemented by the power-voltage monitoring circuit whose operating voltage has a different
range from the logic circuits in the microcontroller and the oscillation circuit (which stops oscillation at reset to put
the microcontroller in a temporary stop state). Conditions required to enable these functions and their operations
will be described next.
Caution When designing an applied circuit requiring a high level of reliability, make sure that its resetting
relies on the built-in power-on/power-down reset function only. Also, design the circuit in such
a way that the RESET signal is input externally.
16.3.1 Conditions Required to Enable the Power-On Reset Function
This function is effective when used together with the power-down reset function.
The following conditions are required to validate the power-on reset function:
<1> The power voltage must be 4.5 to 5.5 V during normal operation, including the standby state.
<2> The frequency of the system clock oscillator must be 400 kHz to 4 MHz.
Note
<3> The power-down reset function must be enabled during normal operation, including the standby state.
<4> The power voltage must rise from 0 V to the specified voltage.
<5> The time it takes for the power voltage to rise from 0 to 2.7 V must be shorter than the oscillation stabilization
wait time counted in timer of the
μ
PD17120 subseries. (System clock 256
×
256 counts: approx. 16 ms,
at f
X
=4 MHz/approx. 32 ms, at f
CC
=2 MHz)
Note
μ
PD17121/17133/17P133 only
Cautions 1. If the above conditions are not satisfied, the power-on reset function will not operate
effectively. In this case, an external reset circuit needs to be added.
2. In the standby state, even if the power-down reset function operates normally, general-
purpose data memory (except for DBF) retains data up to V
DD
=2.7 V. If, however, data is
changed due to an external error, the data in memory is not guaranteed.