EM78458AM datasheet(53/62 Pages) EMC

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EM78458/459

8-Bit Microcontroller with MASK ROM

Product Specification (V1.4) 05.27.2005

• 47

(This specification is subject to change without further notice)

OSCI

EM78458

Vcc

Rext

EM78459

Fig. 23 Circuit for Internal C Oscillator Mode

Table 14 R Oscillator Frequencies

Rext

Average Fosc 5V,25

°C

Average Fosc 3V,25

°C

51k

2.22 MHz

2.17 MHz

100k

1.15 MHz

1.14 MHz

300k

375 KHz

370 KHz

NOTE

1. Measured on DIP packages.

2. Design reference only

4.13 Power-on Considerations

Any microcontroller is not warranted to start proper operation before the power supply

stabilizes in steady state.

EM78458/459 POR voltage range is 1.2V~1.8V. Under customer application, when

power is OFF, Vdd must drop to below 1.2V and remains OFF for 10us before power

can be switched ON again. This way, the EM78458/459 will reset and work 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.

4.13.1

External Power on Reset Circuit

The circuit shown in Fig IV.13.1-1 implements an external RC to produce a reset pulse.

The pulse width (time constant) should be kept long enough to allow Vdd to reach

minimum operation voltage. This circuit is used when the power supply has a slow rise

time. Because the current leakage from the /RESET pin is about

±5µA, it is

recommended that R should not be great than 40 K. In this way, the voltage at Pin

/RESET is held below 0.2V. The diode (D) acts as a short circuit at power-down. The

capacitor, C, is discharged rapidly and fully. Rin, the current-limited resistor, prevents

high current discharge or ESD (electrostatic discharge) from flowing into Pin /RESET.

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