EM78468 Datasheet, PDF (42/71 Pages) ELAN Microelectronics Corp – 8-BIT Microcontroller

EM78468
8-BIT Microcontroller
4.6.4 RC Oscillator Mode with Internal Capacitor
If both precision and cost are taken into consideration, This LSI also offers a special
oscillation mode, which is equipped with an internal capacitor and an external resistor
connected to VDD. The internal capacitor functions as temperature compensator. In
order to obtain more accurate frequency, a precise resistor is recommended.
VDD
Rext
R-OSCI or Xin
Fig. 11 Circuit for Internal C Oscillator Mode
Table 6 RC Oscillator Frequencies
Pin
Rext
Average Fosc 5V, 25°C Average Fosc 3V, 25°C
51k
2.2221 MHz
2.1972 MHz
R-OSCI
100k
1.1345 MHz
1.1203 MHz
300k
381.36KHz
374.77 KHz
Xin
2.2M
32.768KHz
32.768KHz
Note:Measured from QFP packages with frequency drift of about ±30%.
Values are provided for design reference only.
4.7 Power-on Considerations
Any microcontroller (as with this LSI) is not warranted to start operating properly before
the power supply stabilizes in steady state. This LSI is equipped with Power On Reset
(POR) with detection level range of 1.9V to 2.1V. The circuitry eliminates the extra
external reset circuit but it will work well only if the VDD rises quickly enough (50 ms or
less). However, under critical applications, extra devices are still required to assist in
solving power-on problems.
4.7.1 External Power-on Reset Circuit
This circuit implements an external RC to produce a reset pulse (see Fig.12). 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 rise time is slow. Because
the current leakage from the /RESET pin is about ±5µA, it is recommended that R should
not be great than 40K. 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.
38 •
Product Specification (V1.1) 04.11.2005
(This specification is subject to change without further notice)