IBM EM78P447N manual External RC Oscillator Mode Circuit

EM78P447N

8-Bit Microcontroller with OTP ROM

Table 12 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonator

Oscillator Type	Frequency Mode	Frequency	C1(pF)	C2(pF)
		455 kHz	100~150	100~150
Ceramic Resonators	HXT	2.0 MHz	20~40	20~40
		4.0 MHz	10~30	10~30
		32.768kHz	25	15
	LXT	100KHz	25	25
		200KHz	25	25
Crystal Oscillator		455KHz	20~40	20~150
	HXT	1.0MHz	15~30	15~30
		2.0MHz	15	15
		4.0MHz	15	15

4.7.3 External RC Oscillator Mode

For some applications that do not need a very precise timing calculation, the RC oscillator (Fig. 15) 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 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 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.

OSCI

EM 78P447S

VCC

Rext

Cext

Fig. 12 External RC Oscillator Mode Circuit

Product Specification (V1.1) 03.30.2005 • 25

(This specification is subject to change without further notice)