Datasheet

LTC6995-1/LTC6995-2
18
699512fa
For more information www.linear.com/LTC6995-1
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
RESUMETIMEOUTTIMER RESTARTS
V
+
LTC6995-1
OUTPUT
FALLING EDGE TRIGGERED
POL = 0
DIVCODE = 1
RST
GND
SET
OUT
V
+
DIV
R
SET
604k
R2
102k
R1
976k
0.1µF
V
+
699512 F17
LTC6993-1
RISING EDGE TRIGGERED
POSITIVE OUTPUT PULSE
DIVCODE = 1
50ms WATCHDOG TIMER100µs ONE SHOT
SYSTEM POSITIVE
WATCHDOG PULSE
TRG
GND
SET
OUT
V
+
DIV
R
SET
619k
V
+