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EM78P221/2N
8-Bit Microcontroller with OTP ROM
Product Specification (V1.0) 10.19.2007 • 45
(This specification is subject to change without further notice)
6.8.4 Internal RC Oscillator Mode
The EM78P221/2N offers a versatile internal RC mode with default frequency value of
4MHz. Internal RC oscillator mode has other frequencies (1MHz, 16MHz, and 455kHz)
that can be set by Code Option (Word 1), RCM1, and RCM0. The Table below
describes the EM78P221/2N internal RC drift with the variations on voltage,
temperature, and process.
Internal RC Drift Rate (Ta=25
°
C, VDD=5.0V ± 5%, VSS=0V)
Drift Rate
Internal
RC Frequency
Temperature
(-40
°
C~+85
°
C)
Voltage
(2.1V~5.5V)
Process Total
4MHz ±5% ±5% ±4% ±14%
16MHz ±5% ±5% ±4% ±14%
1MHz ±5% ±5% ±4% ±14%
455kHz ±5% ±5% ±4% ±14%
Theoretical values are for reference only. Actual values may vary depending on actual process.
6.9 Power-on Considerations
Any microcontroller is not warranted to start operating properly before the power supply
stabilizes to a steady state. EM78P221/2N has a built-in Power-on Voltage Detector
(POVD) with detection level range of 1.7V to 1.9V. The circuitry eliminates the extra
external reset circuit. It will work well if Vdd rises quickly enough (50 ms or less).
However, under critical applications, extra devices are still required to assist in solving
power-on problems.
6.9.1 External Power-on Reset Circuit
The circuit shown in the
following figure
implements an external
RC to produce a reset
pulse. The pulse width
(time constant) should
be kept long enough to
allow Vdd to reach the
minimum operating
voltage. This circuit is
used when the power
/RESET
VDD
D
R
Rin
C
Fig. 6-15 External Power on Reset Circuit
supply has a slow power rise time. Since the current leakage from the /RESET pin is
about ±5μA, it is recommended that R should not be greater than 40K. This way, the
voltage at Pin /Reset is held below 0.2V. The diode (D) functions as a short circuit at
power-down. The “C” capacitor is discharged rapidly and fully. Rin, the current-limited
resistor, prevents high current discharge or ESD (electrostatic discharge) from flowing
into Pin /RESET.