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EM78P152/3S

8-Bit Microcontroller with OTP ROM

5.9 Power-on Considerations

Any microcontroller is not guaranteed to start to operate properly before the power

supply stabilizes at its steady state. Under customer application, when power is OFF,

Vdd must drop to below 1.8V and remains OFF for 10µs before power can be switched

ON again. This way, the EM78P152/3S will reset and operate normally. The extra

external reset circuit will work well if Vdd can rise at very fast speed (50 ms or less).

However, under most cases where critical applications are involved, extra devices are

required to assist in solving the power-up problems.

5.10 Programmable Oscillator Set-up Time

The Option word contains SUT0 and SUT1 which can be used to define the oscillator

set-up time. Theoretically, the range is from 4.5 ms to 72 ms. For most of crystal or

ceramic resonators, the lower the operation frequency, the longer the Set-up time may

be required. Table 12 describes the values of the Oscillator Set-up Time.

5.11 External Power-on Reset Circuits

The circuitry in the figure

implements an external RC

to produce the reset pulse.

The pulse width (time

constant) should be kept

long enough for Vdd to

reach minimum operation

voltage. This circuit is

used when the power

supply has a slow rise

time.

EM78P153S

/RESET

Vdd

Rin

Figure 5-14 External Power-up Reset Circuit

Since the current leakage from the /RESET pin is

± 5 μA, it is recommended that R

should not be greater than 40K. In this way, the /RESET pin voltage is held below 0.2V.

The diode (D) acts as a short circuit at the moment of power down. The capacitor C will

discharge rapidly and fully. Rin, the current-limited resistor, will prevent high current or

ESD (electrostatic discharge) from flowing to pin /RESET.

•

Product Specification (V1.8) 09.08.2009

(This specification is subject to change without further notice)

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