ST72321 datasheet(30/185 Pages) STMICROELECTRONICS

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ST72321

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SYSTEM INTEGRITY MANAGEMENT (Cont’d)

6.4.3 Clock Security System (CSS)

The Clock Security System (CSS) protects the

ST7 against breakdowns, spikes and overfrequen-

cies occurring on the main clock source (fOSC). It

is based on a clock filter and a clock detection con-

trol with an internal safe oscillator (fSFOSC).

Caution: The CSS function is not guaranteed. Re-

fer to Section 15

6.4.3.1 Clock Filter Control

The PLL has an integrated glitch filtering capability

making it possible to protect the internal clock from

overfrequencies created by individual spikes. This

feature is available only when the PLL is enabled.

If glitches occur on fOSC (for example, due to loose

connection or noise), the CSS filters these auto-

matically, so the internal CPU frequency (fCPU)

continues deliver a glitch-free signal (see Figure

17).

6.4.3.2 Clock detection Control

If the clock signal disappears (due to a broken or

disconnected resonator...), the safe oscillator de-

livers a low frequency clock signal (fSFOSC) which

allows the ST7 to perform some rescue opera-

tions.

Automatically, the ST7 clock source switches back

from the safe oscillator (fSFOSC) if the main clock

source (fOSC) recovers.

When the internal clock (fCPU) is driven by the safe

oscillator (fSFOSC), the application software is noti-

fied by hardware setting the CSSD bit in the SIC-

SR register. An interrupt can be generated if the

CSSIE bit has been previously set.

These two bits are described in the SICSR register

description.

6.4.4 Low Power Modes

6.4.4.1 Interrupts

The CSS or AVD interrupt events generate an in-

terrupt if the corresponding Enable Control Bit

(CSSIE or AVDIE) is set and the interrupt mask in

the CC register is reset (RIM instruction).

Figure 17. Clock Filter Function

Mode

Description

WAIT

No effect on SI. CSS and AVD interrupts

cause the device to exit from Wait mode.

HALT

The CRSR register is frozen.

The CSS (including the safe oscillator) is

disabled until HALT mode is exited. The

previous CSS configuration resumes when

the MCU is woken up by an interrupt with

“exit from HALT mode” capability or from

the counter reset value when the MCU is

woken up by a RESET.

Interrupt Event

Event

Flag

Enable

Control

Bit

Exit

from

Wait

Exit

from

Halt

CSS event detection

(safe oscillator acti-

vated as main clock)

CSSD

CSSIE

Yes

AVD event

AVDF

AVDIE

Yes

fOSC2

fCPU

fOSC2

fCPU

fSFOSC

Clock Filter Function

Clock Detection Function

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