ST72C254G2 datasheet(21/135 Pages) STMICROELECTRONICS

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6.3 MULTI-OSCILLATOR (MO)

The main clock of the ST7 can be generated by

four different source types coming from the multi-

oscillator block:

an external source

4 crystal or ceramic resonator oscillators

an external RC oscillator

an internal high frequency RC oscillator

Each oscillator is optimized for a given frequency

range in terms of consumption and is selectable

through the option byte. The associated hardware

configuration are shown in Table 3. Refer to the

electrical characteristics section for more details.

External Clock Source

In this external clock mode, a clock signal (square,

sinus or triangle) with ~50% duty cycle has to drive

the OSC1 pin while the OSC2 pin is tied to ground.

Crystal/Ceramic Oscillators

This family of oscillators has the advantage of pro-

ducing a very accurate rate on the main clock of

the ST7. The selection within a list of 4 oscillators

with different frequency ranges has to be done by

option byte in order to reduce consumption. In this

mode of the multi-oscillator, the resonator and the

load capacitors have to be placed as close as pos-

sible to the oscillator pins in order to minimize out-

put distortion and start-up stabilization time. The

loading capacitance values must be adjusted ac-

cording to the selected oscillator.

These oscillators are not stopped during the

RESET phase to avoid losing time in the oscillator

start-up phase.

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. The frequency of the

external RC oscillator (in the range of some MHz.)

is fixed by the resistor and the capacitor values.

Consequently in this MO mode, the accuracy of

the clock is directly linked to the accuracy of the

discrete components.

Internal RC Oscillator

The internal RC oscillator mode is based on the

same principle as the external RC oscillator includ-

ing the resistance and the capacitance of the de-

vice. 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.

Table 3. ST7 Clock Sources

Hardware Configur ation

OSC1

OSC2

EXTERNAL

ST7

SOURCE

OSC1

OSC2

LOAD

CAPACITORS

ST7

CL2

CL1

OSC1

OSC2

ST7

CEX

REX

OSC1

OSC2

ST7

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