EM78P153EN EMC [ELAN Microelectronics Corp], EM78P153EN Datasheet - Page 26

• Manufacturer Part Number: EM78P153EN
• Description: EM78P153E is an 8-bit microprocessor with low-power and high-speed CMOS technology
• Manufacturer: EMC [ELAN Microelectronics Corp]

EM78P153E
OTP ROM
3. External RC Oscillator Mode
For some applications that do not need to have its timing to be calculated precisely, the RC oscillator
(Fig. 16) offers a lot of cost savings. Nevertheless, it should be noted that the frequency of the RC
oscillator is influenced by the supply voltage, the values of the resistor (Rext), the capacitor (Cext),
and even the operation temperature. Moreover, the frequency also changes slightly from one chip to
another due to the manufacturing process variation.
In order to maintain a stable system frequency, the values of the Cext should not be less than 20pF,
and that the value of Rext should not be greater than 1 M ohm. If they cannot be kept in this range, the
frequency is easily affected by noise, humidity, and leakage.
The smaller the Rext in the RC oscillator, the faster its frequency will be. On the contrary, for very low
Rext values, for instance, 1 K , the oscillator becomes unstable because the NMOS cannot
discharge the current of the capacitance correctly.
Based on the reasons above, it must be kept in mind that all of the supply voltage, the operation
temperature, the components of the RC oscillator, the package types, the way the PCB is layout, will
affect the system frequency.
Vcc
Rext
OSCI
Cext
EM78P153E
Fig. 16 Circuit for External RC Oscillator Mode
This specification is subject to change without prior notice.
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2002/03/01