EM78P372KSS20 datasheet(86/118 Pages) EMC

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EM78P372K

8-bit Microcontroller



Product Specification (V1.4) 05.11.2016

(This specification is subject to change without prior notice)

7.11.1

External RC Oscillator Mode

For some applications that do not require

precise timing calculation, the RC

oscillator (Figure 7-35) could offer an

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.

ERCin

Vcc

Rext

Cext

Figure 7-35 External RC Oscillator Mode

Circuit

In order to maintain a stable system frequency, the values of the Cext should be no less

than 20 pF, and the value of Rext should not be greater than 1 M

. 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 K

, the oscillator will become

unstable because the NMOS cannot correctly discharge the capacitance current.

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 PCB layout have certain effects on the system frequency.

The RC Oscillator frequencies:

Cext

Rext

Average Fosc 5V, 25

C

Average Fosc 3V, 25

C

20 pF

3.3k

2.064 MHz

1.901 MHz

5.1k

1.403 MHz

1.316 MHz

10k

750.0 kHz

719.0 kHz

100k

81.45 kHz

81.33 kHz

100 pF

3.3k

647.0 kHz

615.0 kHz

5.1k

430.8 kHz

414.3 kHz

10k

225.8 kHz

219.8 kHz

100k

23.88 kHz

23.96 kHz

300 pF

3.3k

256.6 kHz

245.3 kHz

5.1k

169.5 kHz

163.0 kHz

10k

88.53 kHz

86.14 kHz

100k

9.283 kHz

9.255 kHz

Note:

1: Measured based on DIP packages.

2: The values are for design reference only.

3: The frequency drift is  30%

EM78P372KSS20 Datasheet(PDF) 86 Page - ELAN Microelectronics Corp

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