L9803 datasheet(21/126 Pages) STMICROELECTRONICS

21 / 126 page

L9803

Clocks, Reset, Interrupts & Power saving modes

21/126

This flag is useful for distinguishing Safeguard Reset, Power On / Low Voltage Reset and

Watchdog Reset.

b7 = SGFL: Safeguard low flag. Set by an Oscillator Safeguard Reset generated for

frequency too low, cleared by software (writing zero) or Power On / Low Voltage Reset. This

flag is useful for distinguishing Safeguard Reset, Power On / Low Voltage Reset and

Watchdog Reset.

b5 = SFGEN: Safeguard enable when set. It’s cleared only by hardware after a reset.

b4 = CANDS: CAN Transceiver disable. When this bit is set the CAN transceiver goes in

Power Down Mode and does not work until this bit is reset. CANDS is 0 after reset so the

standard condition is with the transceiver enabled. This bit can be used by application

requiring low power consumption (see Section 5.8 for details).

b3,b2,b1 = not used

b0 = PIEN: PWMI input enable. When set, the PWMI input line is connected to Input

Capture 2 of Timer 2. Otherwise, ICAP2_2 is the alternate function of PA7. See Figure 34 for

the explanation of this function.

3.3

Watchdog system (WDG)

3.3.1

Introduction

The Watchdog is used to detect the occurrence of a software fault, usually generated by

external interference or by unforeseen logical conditions, which causes the application

program to give up its normal sequence. The Watchdog circuit generates an MCU reset on

expiry of a programmed time period, unless the program refreshes the counter’s contents

before it is decremented to zero.

3.3.2

Main Features

–

Programmable Timer (64 increments of 12,288 CPU clock)

–

Programmable Reset

–

reset (if watchdog activated) after an HALT instruction or when bit timer MSB

reaches zero

–

Watchdog Reset indicated by status flag.

3.3.3

Functional Description

The counter value stored in the CR register (bits T6:T0), is decremented every 12,288

machine cycles, and the length of the timeout period can be programmed by the user in 64

increments.

If the watchdog is activated (the WDGA bit is set) and when the 7-bit timer (bits T6:T0) rolls

over from 40h to 3Fh (T6 becomes cleared), it initiates a reset cycle pulling low the reset pin

for typically 500ns.

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