4

4543B–AUTO–10/05

U6813B

3.

Fail-safe Functions

A fail-safe IC has to maintain its monitoring function even if there is a fault condition at one of the

pins (e.g., short circuit), ensuring that a microcontroller system does not reach a “critical status”.

A critical status means, for example, if the system is not able to switch off the relay or disable the

power MOSFET, or if the system is not able to provide a signal to the microcontroller via

ENABLE- and RESET-outputs in the case of a fault condition. The U6813B is designed to han-

dle those fault conditions according to Table 3-1 for a maximum of system safety.

Notes:

1. default state at open input

2. P-EN disable: low

3. N-EN disable: high

4.

Watchdog Description

Figure 4-1.

Watchdog Block Diagram

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

ger 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 determined by external R/C com-

ponents. The following description refers to the watchdog timing diagram with tolerances (see

Figure 4-2 on page 5).

Table 3-1.

Truth Table

VCC

3.3V

WDI

RELI

FETI

RELO

FETO

N-RES

P-RES

ok

ok

ok

H

x

on

x

H

L

H

L

ok

ok

ok

x

off

x

H

L

H

L

ok

ok

ok

x

H

x

on

H

L

H

L

ok

ok

ok

x

x

off

HLHL

ok

ok

wrong

x

x

off

off

H

L

L

H

x

wrong

x

x

x

off

off

L

H

L

H

wrong

x

x

x

x

off

off

L

H

L

H

Slope

detector

Up/down

counter

Dual MUX

Binary counter

RS-FF

WDC

WDI

RESET

OSCERR

WD-OK