6

U6809B

4764A–AUTO–10/03

Description of the Watchdog

Abstract

The microcontroller is monitored by a digital window watchdog which accepts an incom-

ming trigger signal of a constant frequency for correct operation. The frequency of the

trigger signal can be varied in a broad range as the watchdog's time window is deter-

mined by external RC components.

The following description refers to Figure 1 on page 2.

Figure 4. Watchdog Block Diagram

Table 3. Table of Fault Condition

Pin

Function

Short to V

S

Short to V

Batt

Short to GND

Open Circuit

LA2O

Short-circuit proof

driver for warning lamp

LA2O partly on

LA2O off

LA2O on

LA2O off

LA2I

Digital input to activate

warning lamp

LA2O on

LA2O on

LA2O off

LA2O on

FBLA2

Digital feedback of

warning lamp

Faulty feedback

Faulty feedback

Faulty feedback

Faulty feedback

LA1O

Short-circuit proof

driver for warning lamp

LA1O partly on

LA1O off

LA1O on

LA1O off

LA1I

Digital input to activate

warning lamp

LA1O on

LA1O on

LA1O off

LA1O on

FBLA1

Digital feedback of

warning lamp

Faulty feedback

Faulty feedback

Faulty feedback

Faulty feedback

RELI

Digital input to activate

the fail safe relay

Relay off

Relay off

Relay on

Relay off

WDI

Watchdog trigger input Watchdog reset

Watchdog reset

Watchdog reset

Watchdog reset

OSC

Capacitor and resistor

of watchdog

Watchdog reset

Watchdog reset

Watchdog reset

Watchdog reset

Slope

detector

Up/down

counter

Dual MUX

Binary counter

RS-FF

RCOSC

WDI

RESET

OSCERR

WD-OK