ST72C171 datasheet(19/152 Pages) STMICROELECTRONICS

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ST72C171

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4.2.4.1 Multi-Oscillator (MO)

The Multi-Oscillator (MO) block is the main clock

supplier of the ST7. To insure an optimum integra-

tion in the application, it is based on an external

clock source and six different selectable oscilla-

tors.

The main clock of the ST7 can be generated by 8

different sources comming from the MO block:

an External source

4 Crystal or Ceramic resonator oscillators

1 External RC oscillators

1 Internal High Frequency RC oscillator

Each oscillator is optimized for a given frequency

range in term of consumption and is selectable

through the Option Byte.

External Clock Source

The default Option Byte value selects the External

Clock in the MO block. In this mode, a clock signal

(square, sinus or triangle) with ~50% duty cycle

has to drive the OSCin pin while the OSCout pin is

tied to ground (see Figure 13).

Figure 13. MO External Clock

Crystal/Ceramic Oscillators

This family of oscillators allows a high accuracy on

the main clock of the ST7. The selection within the

list of 4 oscillators has to be done by Option Byte

according to the resonator frequency in order to

reduce the consumption. In this mode of the MO

block, the resonator and the load capacitors have

to be connected as shown in Figure 14 and have

to be mounted as close as possible to the oscilla-

tor pins in order to minimize output distortion and

start-up stabilization time.

These oscillators, when selected via the Option

Byte, are not stopped during the RESET phase to

avoid losing time in the oscillator starting phase.

Figure 14. MO Crystal/Ceramic Resonator

OSCin

OSCout

EXTERNAL

ST7

SOURCE

OSCin

OSCout

LOAD

CAPACITORS

ST7

CL1

CL0

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