LTC6995-1/LTC6995-2

18

699512f

For more information www.linear.com/6995

applicaTions inForMaTion

Watchdog Timers

Using the same circuits as shown in Figure 15 with pe-

riodic pulsing of the reset input can create an effective

watchdog timer. A watchdog pulse is required from a

system within each timing interval. The watchdog timeout

interval can be programmed from 500µs to 4.8 hours. If a

pulse is missed the output goes high to indicate that the

system software may be caught in an infinite loop. This

high level can be used to initiate software diagnostic or

restart procedures. The LTC6995 internal clock stops and

the output remains high until the software recovers and

returns to issuing watchdog pulses. Figure 16 shows the

timing for this application.

Watchdog timers are used to detect if a system operating

software is diverted from the designed program sequence

for any reason. It is always a possibility that the software

could get stuck in a way that keeps the watchdog pulse in

the state that holds the timer in the reset so it can never

time out. In this condition the watchdog timer is ineffective

and will never force corrective action. To help to prevent

this a second one shot can be used to reset the watchdog

timer as shown in Figure 17.

Figure 16. Watchdog Timer. Same Circuits as Shown in Figure 15

Figure 17. Extra-Reliable Watchdog Timer. Allows Timeout if System Watchdog Pulse Gets Stuck in the Timer Reset State.

Both Timer Devices Can Share the Same DIVCODE Setting

699512 F16

OUTPUT

RST (LTC6995-1)

RST (LTC6995-2)

SERVICE WATCHDOG

MISSED PULSE

WATCHDOG PULSES

RESUME

TIMEOUT

TIMER RESTARTS

LTC6995-1

OUTPUT

FALLING EDGE TRIGGERED

POL = 0

DIVCODE = 1

RST

GND

SET

OUT

DIV

604k

R2

102k

R1

976k

0.1µF

699512 F17

LTC6993-1

RISING EDGE TRIGGERED

POSITIVE OUTPUT PULSE

DIVCODE = 1

50ms WATCHDOG TIMER

100µs ONE SHOT

SYSTEM POSITIVE

WATCHDOG PULSE

TRG

GND

SET

OUT

DIV

619k