em78p257 ELAN Microelectronics Corp, em78p257 Datasheet - Page 50

• Manufacturer Part Number: em78p257
• Description: 8-bit Microcontroller
• Manufacturer: ELAN Microelectronics Corp

4.10 Power On Considerations
This specification is subject to change without prior notice.
5. Internal RC Oscillator Mode
1. Programmable Oscillator Set-Up Time
2. External Power On Reset Circuit
Table 20 Calibration Selection for Internal RC Mode
EM78P257A/B offers a versatile internal RC mode with default frequency of 4MHz. The frequency can be
configured by programming the bit RCM0 and bit RCM1 of the Option code. Table 20 describes a typical instance
of the calibration.
<Note>
Any microcontroller is not guaranteed to start to operate properly before the power supply stabilizes at its steady
state.
EM78P257A/B 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 EM78P257A/B
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.
The Option word (SUT) is used to define the oscillator Set-Up time (18ms or 1ms). Theoretically, the range is from
1 ms to 18 ms. For most of crystal or ceramic resonators, the lower the operation frequency, the longer is the
required Set-up time.
The circuit shown in Fig.22 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 slow rise time. Because the current leakage from the /RESET pin is about 5 A, it is recommended
that R should not be greater than 40 K. In this way, the voltage in pin /RESET will be held below 0.2V. The diode
RCM 1
1. Measured on DIP packages.
2. Design reference only, the frequency value vary with temperature ,VDD and process.
3. The frequency drift about ±30%.
3. The frequency drift about ±35%.
1
1
0
0
RCM 0
1
0
1
0
50
Frequency(MHz)
32.768kHz
455kHz
4
1
EM78P257
07.27.2004 (V1.4)
OTP ROM