ST7LITE2 datasheet(26/131 Pages) STMICROELECTRONICS

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ST7LITE2

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7.4 MULTI-OSCILLATOR (MO)

The main clock of the ST7 can be generated by

four different source types coming from the multi-

oscillator block (1 to 16MHz or 32kHz):

an external source

5 crystal or ceramic resonator oscillators

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

configurations are shown in Table 4. 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.

Note: when the Multi-Oscillator is not used, PB4 is

selected by default as external clock.

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 (refer

to section 15.1 on page 123 for more details on the

frequency ranges). In this mode of the multi-oscil-

lator, the resonator and the load capacitors have

to be placed as close as possible to the oscillator

pins in order to minimize output distortion and

start-up stabilization time. The loading capaci-

tance 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.

Internal RC Oscillator

In this mode, the tunable 1%RC oscillator is used

as main clock source. The two oscillator pins have

to be tied to ground.

Table 4. ST7 Clock Sources

Hardware Configuration

OSC1

OSC2

EXTERNAL

ST7

SOURCE

OSC1

OSC2

LOAD

CAPACITORS

ST7

CL2

CL1

OSC1

OSC2

ST7

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