EM78159NAS EMC [ELAN Microelectronics Corp], EM78159NAS Datasheet - Page 32

• Manufacturer Part Number: EM78159NAS
• Description: 8-Bit Microcontroller with OTP ROM
• Manufacturer: EMC [ELAN Microelectronics Corp]

EM78P159N
8-Bit Microcontrollerwith OTP ROM
28 •
4.7.3 External RC Oscillator Mode
For some applications that do not need a very precise timing calculation, the RC
oscillator (Figure 4-9 below) 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 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 MΩ. If they
cannot be kept in this range, the frequency can be easily affected 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 becomes unstable
because the NMOS cannot discharge the current of the capacitance correctly.
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, the
way the PCB is layout, will affect the system frequency.
The values of capacitors C1 & C2 are for reference only.
Figure 4-9 External RC Oscillator Mode Circuit
EM78P159N
OSCI
(This specification is subject to change without further notice)
NOTE
Product Specification (V1.0) 03.10.2006
Cext
Vcc
Rext