External Rc Oscillator Mode - IBM EM78P809N Specification

Ibm 8-bit microcontroller product specification

Table of Contents

EM78P809N

Fig. 33. Crystal/Resonator-Parallel Mode Circuit

4.16.3

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 Ω , 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.

Fig. 34. External RC Oscillator Mode Circuit

Product Specification (V1.0) 07.26.2005

(This specification is subject to change without further notice)

OSCI

740