7.5.2 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 must

have low ESR, low leakage (< 5nA) and low tolerances because 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 tboundary,max and twindow,min. The trigger pulse has to fit into this window frame.

The external capacitor must have a value between a minimum of 155pF and a maximum of 63nF.

Table 7-3. Setting Upper Window Using External Capacitor

SELECTED OPERATION MODE

WINDOW FRAME

WDT = external capacitor C(ext)

WDR = 0V and WDR = VDD

twindow,max = 1.25 × twindow,typ

twindow,min = 0.75 × twindow,typ

t

window,typ +

C

(ext)

15.55 pF )

1

6.25 ms

(2)

7.5.3 Lower Boundary Calculation

The lower boundary can be calculated based on the values given in Section 6.7 . Additionally, facts must be

considered to verify that the lower boundary is where it is expected. Because the internal oscillator of the window

watchdog is running free, any rising edge at the WDI pin is considered at the next internal clock cycle. This

happens regardless of the external source. Because the shift between internal and external clock is not known, it

is best to consider the worst-case condition for calculating this value.

Table 7-4. Setting Lower Boundary Using External Cap

SELECTED OPERATION MODE

LOWER BOUNDARY OF FRAME

WDT = external capacitor C(ext)

WDR = 0V

tboundary,max = twindow,max / 23.5

tboundary,typ = twindow,typ / 25.8

tboundary,min = twindow,min / 28.7

WDR = VDD

tboundary,max = twindow,max / 51.6

tboundary,typ = twindow,typ / 64.5

tboundary,min = twindow,min / 92.7

7.5.4 Watchdog Software Considerations

To benefit from the window watchdog feature and help the watchdog timer monitor the software execution more

closely, TI recommends 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 must be set to different timings inside the window frame to release a defined reset, if the program

must hang in any subroutine. This allows the window watchdog to detect time-outs of the trigger pulse, as well

as pulses that distort the lower boundary.

7.5.5 Power-Up Considerations

Many microcontrollers use general-purpose input and 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 TPS3813xxx, then a pulldown resistor (shown as R2 in Figure 8-1) must be added

to keep the WDI pin from floating during power up.

TPS3813

SLVS331J – DECEMBER 2000 – REVISED AUGUST 2024

www.ti.com

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