R

EM6152

Copyright © 2006, EM Microelectronic-Marin SA

8

www.emmicroelectronic.com

rev. B / 06.06

Timer Clearing and RES Action

The watchdog circuit monitors the activity of the processor.

If the user’s software does not send a pulse to the TCL

input within the programmed open window timeout period a

short watchdog RES pulse is generated which is equal to

With the open window constraint, new security is added to

conventional watchdogs by monitoring both software cycle

time and execution. Should software clear the watchdog too

quickly (incorrect cycle time) or too slowly (incorrect

execution) it will cause the system to be reset. If software is

stuck in a loop which includes the routine to clear the

watchdog then a conventional watchdog would not make a

system reset even though the software is malfunctioning;

the circuit would make a system reset because the

watchdog would be cleared too quickly.

If no TCL signal is applied before the closed and open

windows expire, RES will start to generate square waves of

in this state until the next TCL falling edge appears during

an open window, or until a fresh power-up sequence. The

system enable output, EN , can be used to prevent critical

control functions being activated in the event of the system

going into this failure mode (see section “Enable- EN

Output”).

output is not used by the system (see Fig 8).

Combined Voltage and Timer Action

The combination of voltage and timer actions is illustrated

by the sequence of events shown in Fig. 6. On power-up,

POR, delay is initialized and holds RES active for the time

of the POR delay. A TCL pulse will have no effect until this

power-on-reset delay is completed. When the risk exists that

TCL

elapsed, RES goes inactive and the watchdog timer starts

acting. If no TCL pulse occurs, RES goes active low for a

A TCL pulse coming during the open window clears the

watchdog timer. When the TCL pulse occurs too early

(during the closed window), RES goes active and a new

level for longer than typically 3

μs overrides the timer and

immediately forces RES active and EN inactive. Any

further TCL pulse has no effect until the next power-up

sequence has completed.

Enable -

EN Output

The system enable output, EN , is inactive always when

RES

is active and remains inactive after a RES pulse until

the watchdog is serviced correctly 3 consecutive times (i.e.

the TCL pulse must come in the open window). After three

consecutive services of the watchdog with TCL during the

open window, the EN goes active low.

A malfunctioning system would be repeatedly reset by the

watchdog. In a conventional system critical motor controls

could be energized each time reset goes inactive (time

allowed for the system to restart) and in this way the

electrical motors driven by the system could function out of

control. The circuit prevents the above failure mode by using

the EN output to disable the motor controls until software

has successfully cleared the watchdog three times (i.e. the

system has correctly re-started after a reset condition).

Typical Application

Fig. 8

The important parameters of the 22

μF input capacitor are an effective series resistance lower than 3 Ω and a resonant

frequency above 500 kHz.

Regulated Voltage (5V)

GND

Unregulated

Voltage

INPUT

OUTPUT

EM6152

RES

EN

TCL

RES

Microprocessor

Motor

controls

EN

Address decoder

22uF

100nF

100k

Ω

22uF

+

+