ST72334J datasheet(32/125 Pages) STMICROELECTRONICS

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ST72334J/N, ST72314J/N, ST72124J

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4.3 CLOCK SECURITY SYSTEM (CSS)

The Clock Security System (CSS) protects the

ST7 against main clock problems. To allow the in-

tegration of the security features in the applica-

tions, it is based on a clock filter control and an In-

ternal Safe Oscillator. The CSS can be disabled by

OPTION BYTE.

4.3.1 Clock Filter Control

The Clock Filter is based on a clock frequency lim-

itation function.

This filter function is able to detect and filter high

frequency spikes on the ST7 main clock.

If the oscillator is not working properly (e.g. work-

ing at a harmonic frequency of the resonator), the

current active oscillator clock can be totally fil-

tered, and then no clock signal is available for the

ST7 from this oscillator anymore. If the original

clock source recovers, the filtering is stopped au-

tomatically and the oscillator supplies the ST7

clock.

4.3.2 Safe Oscillator Control

The Safe Oscillator of the CSS block is a low fre-

quency back-up clock source (see Figure 24).

If the clock signal disappears (due to a broken or

disconnected resonator...) during a Safe Oscillator

period, the Safe oscillator delivers a low frequency

clock signal which allows the ST7 to perform some

rescue operations.

Automatically, the ST7 clock source switches back

from the Safe Oscillator if the original clock source

recovers.

Limitation detection

The automatic Safe Oscillator selection is notified

by hardware setting the CSSD bit of the CRSR

SIE bit has been previously set.

These two bits are described in the CRSR register

description.

Figure 23. Clock Filter Function

Figure 24. Safe Oscillator Function

MAIN

OSCILLATOR

CLOCK

INTERNAL

ST7

CLOCK

MAIN

OSCILLATOR

CLOCK

INTERNAL

ST7

CLOCK

SAFE

OSCILLATOR

CLOCK

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