Datasheet

t
window,typ
+
ǒ
C
(ext)
15.55 pF
) 1
Ǔ
6.25 ms
TPS3813J25, TPS3813L30
TPS3813K33, TPS3813I50
www.ti.com SLVS331G –DECEMBER 2000–REVISED OCTOBER 2013
PROGRAMMABLE WINDOW-WATCHDOG BY USING AN EXTERNAL CAPACITOR
The upper boundary of the watchdog timer can be set by an external capacitor connected between the WDT pin
and GND. Common consumer electronic capacitors can be used to implement this feature. They should have low
ESR and low tolerances since the tolerances have to be considered if the calculations are performed. The first
formula is used to calculate the upper window frame. After calculating the upper window frame, the lower
boundary can be calculated. As in the last example, the most important values are the t
boundary,max
and t
window,min
.
The trigger pulse has to fit into this window frame.
The external capacitor should have a value between a minimum of 155 pF and a maximum of 63 nF.
SELECTED OPERATION MODE WINDOW FRAME
t
window,max
= 1.25 × t
window,typ
WDT = external capacitor C
(ext)
WDR = 0 V and WDR = V
DD
t
window,min
= 0.75 × t
window,typ
(1)
LOWER BOUNDARY CALCULATION
The lower boundary can be calculated based on the values given in the switching characteristics. Additionally,
facts have to be taken into account to verify that the lower boundary is where it is expected. Since the internal
oscillator of the window watchdog is running free, any rising edge at the WDI pin will be taken into account at the
next internal clock cycle. This happens regardless of the external source. Since the shift between internal and
external clock is not known, it is best to consider the worst case condition for calculating this value.
SELECTED OPERATION MODE LOWER BOUNDARY OF FRAME
t
boundary,max
= t
window,max
/ 23.5
WDR = 0 V t
boundary,typ
= t
window,typ
/ 25.8
t
boundary,min
= t
window,min
/ 28.7
WDT = external capacitor C
(ext)
t
boundary,max
= t
window,max
/ 51.6
WDR = V
DD
t
boundary,typ
= t
window,typ
/ 64.5
t
boundary,min
= t
window,min
/ 92.7
WATCHDOG SOFTWARE CONSIDERATIONS
To benefit from the window watchdog feature and help the watchdog timer monitor the software execution more
closely, it is recommended that the watchdog be set and reset at different points in the program rather than
pulsing the watchdog input periodically by using the prescaler of a microcontroller or DSP. Furthermore, the
watchdog trigger pulses should be set to different timings inside the window frame to release a defined reset, if
the program should hang in any subroutine. This allows the window watchdog to detect timeouts of the trigger
pulse as well as pulses that distort the lower boundary.
POWER-UP CONSIDERATIONS
Many microcontrollers use general-purpose input/output (GPIO) pins that can be programmed to be either inputs
or outputs. During power-up, these I/O pins are typically configured as inputs. If a GPIO pin is used to drive the
WDI input pin of the TPS3813, then a pull-down resistor (shown as R2 in Figure 8) should be added to keep the
WDI pin from floating during power-up.
In applications where the WDI input may experience a negative voltage while V
DD
is ramping between 0 V and
0.8 V, then the V
DD
slew rate in this range should be greater than 10 V/s. A negative voltage on the WDI input
along with a slew rate less than 10 V/s could result in a greatly reduced watchdog window time and reset output
delay time.
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