ADM8617 Analog Devices, ADM8617 Datasheet - Page 9

• Manufacturer Part Number: ADM8617
• Description: Low Voltage Supervisory Circuit with Watchdog in 4-Lead SC70 (Active-low with an Open-Drain Output)
• Manufacturer: Analog Devices

Specifications of ADM8617

• Batt-backup-flg: No
• Manual Reset Capability: No
• Package: SC70
• Power Fail Indicator: No
• Reset Threshold Summary: 9 Options--1.58 to 4.63V
• Reset Output-stage: Active-Low/Open-Drain
• Min Reset Timeout (ms): 1,1120,140,20
• Typ Watchdog Timeout (ms): 102,1600,6.3
• Watchdog Timer: Yes

APPLICATION INFORMATION
WATCHDOG INPUT CURRENT
To minimize watchdog input current (and minimize overall
power consumption), leave WDI low for the majority of the
watchdog timeout period. When driven high, WDI can draw as
much as 160 μA. Pulsing WDI low-high-low at a low duty cycle
reduces the effect of the large input current. When WDI is
unconnected, a window comparator disconnects the watchdog
timer from the RESET output circuitry so that RESET
is not asserted when the watchdog timer times out.
NEGATIVE-GOING V
To avoid unnecessary resets caused by fast power supply
transients, the ADM8616/ADM8617 are equipped with glitch
rejection circuitry. The typical performance characteristic in
Figure 8 plots V
The curve shows combinations of transient magnitude and
duration for which a RESET is not generated for 4.63 V and
2.93 V reset threshold parts. For example, with the 2.93 V
threshold, a transient that goes 100 mV below the threshold
and lasts 8 μs typically does not cause a RESET , but if the
transient is any bigger in magnitude or duration, a RESET is
generated. An optional 0.1 μF bypass capacitor mounted close
to V
ENSURING RESET VALID TO V
The active-low RESET output is guaranteed to be valid for
V
valid outputs for V
connected between RESET and ground, pulls the output low
when it is unable to sink current. A large resistance, such as
100 kΩ, should be used so that it does not overload the RESET
output when V
CC
CC
as low as 1 V. However, by using an external resistor,
provides additional glitch rejection.
Figure 13. Ensuring RESET Valid to V
CC
CC
is above 1 V.
transient duration vs. transient magnitude.
CC
ADM8616/
ADM8617
as low as 0 V are possible. The resistor,
V
CC
CC
TRANSIENTS
RESET
CC
100kΩ
= 0 V
CC
= 0 V
Rev. B | Page 9 of 12
WATCHDOG SOFTWARE CONSIDERATIONS
In implementing the microprocessors watchdog strobe
code, quickly switching WDI low to high and then high to
low (minimizing WDI high time) is desirable for current
consumption reasons. However, a more effective way of
using the watchdog function can be considered.
A low-high-low WDI pulse within a given subroutine prevents
the watchdog from timing out. However, if the subroutine
becomes stuck in an infinite loop, the watchdog does not detect
this because the subroutine continues to toggle WDI. A more
effective coding scheme for detecting this error involves using a
slightly longer watchdog timeout. In the program that calls
the subroutine, WDI is set high. The subroutine sets WDI low
when it is called. If the program executes without error, WDI is
toggled high and low with every loop of the program. If the
subroutine enters an infinite loop, WDI is kept low, the watch-
dog times out, and the microprocessor is reset.
SUBROUTINE
PROGRAM
ADM8616/
SET WDI
SET WDI
RETURN
ADM8617
START
CODE
HIGH
LOW
Figure 15. Typical Application Circuit
Figure 14. Watchdog Flow Diagram
V
CC
RESET
WDI
ADM8616/ADM8617
RESET
I/O
INFINITE LOOP:
WATCHDOG
µP
TIMES OUT
RESET