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EM78P809N
8-Bit Microcontroller
Product Specification (V1.0) 07.26.2005
• 57
(This specification is subject to change without further notice)
740
4
4.7
K
10
K
XTAL
740
4
C1
Vdd
10
K
C2
OSCI
EM78P809N
Fig. 33. Crystal/Resonator-Parallel Mode Circuit
4.16.3 External RC Oscillator Mode
For applications that do not need very precise timing calculation, the RC oscillator
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 by the operation temperature. Moreover, the frequency
also varies 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 the value of Rext should not be greater than 1 M
, otherwise, 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
Ω, the oscillator becomes unstable
because the NMOS cannot correctly discharge the current of the capacitance.
Hence, it must be noted that the supply voltage, the operation temperature, the RC
oscillator components, the package types, and the PCB layout, will affect the system
frequency.
OSCI
EM78P809N
Vdd
Rext
Cext
Fig. 34. External RC Oscillator Mode Circuit