EM78451AP EMC [ELAN Microelectronics Corp], EM78451AP Datasheet - Page 45

• Manufacturer Part Number: EM78451AP
• Description: 8-Bit Microcontroller
• Manufacturer: EMC [ELAN Microelectronics Corp]

Product Specification (V1.2) 05.27.2004
(This specification is subject to change without further notice)
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 KΩ, the oscillator becomes unstable
because the NMOS cannot properly discharge the current of the capacitor.
Based on the above reasons, it must be kept in mind that all of the supply voltage, the
operation temperature, the components of the RC oscillator, the package types, and
the way the PCB is layout, will affect the system frequency.
Fig. 23 Circuit for External R, Internal C Oscillator Mode
Fig. 22 Circuit for External RC Oscillator Mode
EM 78451
EM 78451
OSCI
OSCI
R e x t
V C C
Cext
VCC
Rext
8-Bit Microcontroller
EM78451
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