ST72334J datasheet(31/125 Pages) STMICROELECTRONICS

31 / 125 page

ST72334J/N, ST72314J/N, ST72124J

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MULTI-OSCILLATOR (Cont’d)

External RC Oscillator

This oscillator allows a low cost solution for the

main clock of the ST7 using only an external resis-

tor and an external capacitor (see Figure 21). The

selection of the external RC oscillator has to be

done by OPTION BYTE.

The frequency of the external RC oscillator (in the

range of some MHz.) is fixed by the resistor and

the capacitor values:

The previous formula shows that in this MO mode,

the accuracy of the clock is directly linked to the

accuracy of the discrete components.

Figure 21. MO External RC

Internal RC Oscillator

The Internal RC oscillator mode is based on the

same principle as the External RC oscillator in-

cluding the resistance and the capacitance of the

device. This mode is the most cost effective one

with the drawback of a lower frequency accuracy.

Its frequency is in the range of several MHz.

In this mode, the two oscillator pins have to be tied

to ground as shown in Figure 22.

The selection of the internal RC oscillator has to

be done by OPTION BYTE.

Figure 22. MO Internal RC

Note:

1) This formula provides an approximation of the frequency with typical REX and CEX values at VDD=5V.

It is given only as design guidelines.

fOSC ~

REX .CEX

OSC1

OSC2

ST7

CEX

REX

OSC1

OSC2

ST7

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