ADM825 Analog Devices, ADM825 Datasheet - Page 10

• Manufacturer Part Number: ADM825
• Description: Supervisory Circuit with Watchdog and Manual Reset in 5-Lead SC70 and SOT-23
• Manufacturer: Analog Devices

Specifications of ADM825

• # Of Monitored Voltages: 1
• Backup-battery Switch: No
• Manual Reset Capability: Yes
• Package: SC70,SOT
• Reset Threshold Summary: 7 Options--2.19 to 4.63V
• Reset Output-stage: Active-High/Push-Pull,Active-Low/Push-Pull
• Min Reset Timeout (ms): 140
• Watchdog Timer: No

ADM823/ADM824/ADM825
APPLICATIONS INFORMATION
WATCHDOG INPUT CURRENT
To minimize the 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 ADM823/ADM824/ADM825 are equipped with
glitch rejection circuitry. The typical performance characteristic
in Figure 12 plots V
nitude. The curves show 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 larger in magnitude or duration, a reset is generated. An
optional 0.1 μF bypass capacitor mounted close to V
additional glitch rejection.
ENSURING RESET VALID TO V
Both active low and active high reset outputs are guaranteed to
be valid for V
resistor with push-pull configured reset outputs, valid outputs
for V
a resistor connected between RESET and ground pulls the output
low when it is unable to sink current. For an active high reset
output, a resistor connected between RESET and V
output high when it is unable to source current. A large resist-
ance such as 100 kΩ should be used so that the reset output is
not overloaded when V
ADM823/
ADM824/
ADM825
CC
V
as low as 0 V are possible. For an active low reset output,
CC
CC
Figure 17. Ensuring Reset Valid to V
as low as 1 V. However, by using an external
RESET
CC
transient duration vs. the transient mag-
CC
100kΩ
CC
is above 1 V.
TRANSIENTS
ADM824/
ADM825
CC
V
CC
= 0 V
CC
= 0 V
CC
RESET
CC
pulls the
provides
100kΩ
Rev. C | Page 10 of 12
WATCHDOG SOFTWARE CONSIDERATIONS
In implementing the microprocessor 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 timing out. However, if the subroutine becomes
stuck in an infinite loop, the watchdog cannot detect this cond-
ition 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 (see Figure 18). 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
watchdog times out, and the microprocessor is reset.
SUBROUTINE
PROGRAM
MR
SET WDI
SET WDI
RETURN
START
CODE
HIGH
ADM823
LOW
Figure 19. Typical Application Circuit
Figure 18. Watchdog Flow Diagram
V
CC
RESET
WDI
RESET
I/O
MICROPROCESSOR
INFINITE LOOP:
WATCHDOG
TIMES OUT
RESET