ST72361 datasheet(21/224 Pages) STMICROELECTRONICS

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ST72361

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6.2 MULTI-OSCILLATOR (MO)

The main clock of the ST7 can be generated by

two different source types coming from the multi-

oscillator block:

■ an external source

■ a crystal or ceramic resonator 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 6. Refer to the

electrical characteristics section for more details.

Caution: The OSC1 and/or OSC2 pins must not

be left unconnected. For the purposes of Failure

Mode and Effect Analysis, it should be noted that if

the OSC1 and/or OSC2 pins are left unconnected,

the ST7 main oscillator may start and, in this con-

figuration, could generate an fOSC clock frequency

in excess of the allowed maximum (> 16 MHz),

putting the ST7 in an unsafe/undefined state. The

product behavior must therefore be considered

undefined when the OSC pins are left unconnect-

ed.

External Clock Source

In external clock mode, a clock signal (square, si-

nus 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 five oscilla-

tors with different frequency ranges must be done

by option byte in order to reduce consumption (re-

fer to Section 14.1 on page 210 for more details on

the frequency ranges). The resonator and the load

capacitors must be placed as close as possible to

the oscillator pins in order to minimize output dis-

tortion and start-up stabilization time. The loading

capacitance values must be adjusted according to

the selected oscillator.

These oscillators are not stopped during the

RESET phase to avoid losing time in the oscillator

start-up phase.

Table 6. ST7 Clock Sources

Hardware Configuration

OSC1

OSC2

EXTERNAL

ST7

SOURCE

OSC1

OSC2

LOAD

CAPACITORS

ST7

CL2

CL1

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