EM78468AQ EMC [ELAN Microelectronics Corp], EM78468AQ Datasheet - Page 42

• Manufacturer Part Number: EM78468AQ
• Description: 8-BIT Microcontroller
• Manufacturer: EMC [ELAN Microelectronics Corp]

EM78468
8-BIT Microcontroller
38 •
4.7 Power-on Considerations
4.6.4 RC Oscillator Mode with Internal Capacitor
4.7.1 External Power-on Reset Circuit
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.
Table 6 RC Oscillator Frequencies
Note: Measured from QFP packages with frequency drift of about ± 30%.
Values are provided for design reference only.
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.
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 ± 5 µ A, 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.
R-OSCI
Pin
Xin
Rext
100k
300k
2.2M
51k
Fig. 11 Circuit for Internal C Oscillator Mode
Average Fosc 5V, 25°C Average Fosc 3V, 25°C
R-OSCI or Xin
2.2221 MHz
1.1345 MHz
381.36KHz
32.768KHz
(This specification is subject to change without further notice)
Product Specification (V1.1) 04.11.2005
VDD
Rext
2.1972 MHz
1.1203 MHz
374.77 KHz
32.768KHz