ST72C254G2 datasheet(23/140 Pages) STMICROELECTRONICS

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7.5 CLOCK RESET AND SUPPLY REGISTER DESCRIPTION (CRSR)

Read /Write

Reset Value: 000x 000x (XXh)

Bit 7:5 = Reserved, always read as 0.

Bit 4 = LVDRF

LVD reset flag

This bit indicates that the last RESET was gener-

ated by the LVD block. It is set by hardware (LVD

reset) and cleared by software (writing zero). See

WDGRF flag description for more details. When

the LVD is disabled by option byte, the LVDRF bit

value is undefined.

Bit 3 = Reserved, always read as 0.

Bit 2 = CSSIE

Clock security syst

interrupt enable

This bit enables the interrupt when a disturbance

is detected by the clock security system (CSSD bit

set). It is set and cleared by software.

0: Clock security system interrupt disabled

1: Clock security system interrupt enabled

Refer to Table 5, “Interrupt Mapping,” on page 26

for more details on the CSS interrupt vector. When

the CSS is disabled by option byte, the CSSIE bit

has no effect.

Bit 1 = CSSD

Clock security system detection

This bit indicates that the safe oscillator of the

clock security system block has been selected by

hardware due to a disturbance on the main clock

signal (fOSC). It is set by hardware and cleared by

reading the CRSR register when the original oscil-

lator recovers.

0: Safe oscillator is not active

1: Safe oscillator has been activated

When the CSS is disabled by option byte, the

CSSD bit value is forced to 0.

Bit 0 = WDGRF

Watchdog reset flag

This bit indicates that the last RESET was gener-

ated by the watchdog peripheral. It is set by hard-

ware (Watchdog RESET) and cleared by software

(writing zero) or an LVD RESET (to ensure a sta-

ble cleared state of the WDGRF flag when the

CPU starts).

Combined with the LVDRF flag information, the

flag description is given by the following table.

Application notes

The LVDRF flag is not cleared when another RE-

SET type occurs (external or watchdog), the

LVDRF flag remains set to keep trace of the origi-

nal failure.

In this case, a watchdog reset can be detected by

software while an external reset can not.

Table 4. Clock, Reset and Supply Register Map and Reset Values

LVD

CSS

WDG

RESET Sources

LVDRF

WDGRF

External RESET pin

Watchdog

LVD

Address

(Hex.)

Label

7654

3210

0025h

CRSR

Reset Value

LVDRF

CSSIE

CSSD

WDGRF

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