EM78P258N
8-Bit Microprocessor with OTP ROM
4.7K
10K
7404
Vdd
OSCI
7404
10K
EM78P258N
XTAL
10K
C1
C2
Fig. 6-16 Parallel Mode Crystal/Resonator Circuit Diagram
6.10.3 External RC Oscillator Mode
For some applications that do not require
precise timing calculation, the RC
Vcc
Rext
oscillator (Fig. 6-17 right) could offer you
with effective 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 changes slightly from one
chip to another due to the manufacturing
process variation.
OSCI
EM78P258N
Cext
Fig. 6-17 External RC Oscillator Mode Circuit
In order to maintain a stable system frequency, the values of the Cext should be no less
than 20pF, and that of Rext should be no greater than 1MΩ. If the frequency cannot be
kept within this range, the frequency can be affected easily by noise, humidity, and
leakage.
The smaller the Rext in the RC oscillator is, the faster its frequency will be. On the
contrary, for very low Rext values, for instance, 1 KΩ, the oscillator will become
unstable because the NMOS cannot discharge the capacitance current correctly.
Based on the above reasons, it must be kept in mind that all supply voltage, the
operation temperature, the components of the RC oscillator, the package types, and
the way the PCB is layout, have certain effect on the system frequency.
58 •
Product Specification (V1.0) 06.16.2005
(This specification is subject to change without further notice)
ELAN [ ELAN MICROELECTRONICS CORP ]
