ST72324BJ2 datasheet(28/188 Pages) STMICROELECTRONICS

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Supply, reset and clock management

ST72324B

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6.3.2

Crystal/ceramic oscillators

This family of oscillators has the advantage of producing a very accurate rate on the main

clock of the ST7. The selection within a list of four oscillators with different frequency ranges

has to be done by option byte in order to reduce consumption (refer to Section 14.1 on page

172 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 oscillator

pins in order to minimize output distortion 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.

6.3.3

Internal RC oscillator

This oscillator allows a low cost solution for the main clock of the ST7 using only an internal

resistor and capacitor. Internal RC oscillator mode has the drawback of a lower frequency

accuracy and should not be used in applications that require accurate timing.

In this mode, the two oscillator pins have to be tied to ground.

In order not to exceed the maximum operating frequency, the internal RC oscillator must not

be used with the PLL.

Table 8.

ST7 clock sources

Hardware configuration

OSC1

OSC2

External

ST7

source

OSC1

OSC2

Load

capacitors

ST7

CL2

CL1

OSC1

OSC2

ST7

ST72324BJ2 Datasheet(PDF) 28 Page - STMicroelectronics