EM78P257
OTP ROM
3. The frequency drift about ±30%.
5. Internal RC Oscillator 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.
Table 20 Calibration Selection for Internal RC Mode
RCM 1
RCM 0
Frequency(MHz)
1
1
0
0
1
0
1
0
4
1
455kHz
32.768kHz
<Note>
1. Measured on DIP packages.
2. Design reference only, the frequency value vary with temperature ,VDD and process.
3. The frequency drift about ±35%.
4.10 Power On Considerations
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 andworknormally. 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.
1. Programmable Oscillator Set-Up Time
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.
2. External Power On Reset Circuit
The circuitshown 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 ±5mA, 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
This specification is subject to change without prior notice.
50
07.27.2004 (V1.4)
ELAN [ ELAN MICROELECTRONICS CORP ]
