ST72521 datasheet(25/211 Pages) STMICROELECTRONICS

Part #	ST72521

Description	8-BIT MCU WITH NESTED INTERRUPTS, FLASH, 10-BIT ADC, FIVE TIMERS, SPI, SCI, I2C, CAN INTERFACE

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6.2 MULTI-OSCILLATOR (MO)

The main clock of the ST7 can be generated by

three different source types coming from the multi-

oscillator block:

s

an external source

s

4 crystal or ceramic resonator oscillators

s

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.

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 (>16MHz.),

putting the ST7 in an unsafe/undefined state. The

product behaviour must therefore be considered

undefined when the OSC pins are left unconnect-

ed.

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: External clock source is not supported with

the PLL enabled.

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 14.1 on page 199 for more details on

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

oscillator, the resonator and the load capacitors

have to be placed as close as possible to the oscil-

lator 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

This oscillator allows a low cost solution for the

main clock of the ST7 using only an internal resis-

tor and capacitor. Internal RC oscillator mode has

the drawback of a lower frequency accuracy and

should not be used in applications that require ac-

curate timing.

In this mode, 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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