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TPS3813J25 Datasheet(PDF) 5 Page - Texas Instruments |
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TPS3813J25 Datasheet(HTML) 5 Page - Texas Instruments |
5 / 16 page www.ti.com TIMING RULES OF WINDOW-WATCHDOG PROGRAMMABLE WINDOW-WATCHDOG BY USING AN EXTERNAL CAPACITOR t window,typ + C (ext) 15.55 pF ) 1 6.25 ms (1) LOWER BOUNDARY CALCULATION WATCHDOG SOFTWARE CONSIDERATIONS APPLICATION EXAMPLE TPS3813J25, TPS3813L30 TPS3813K33, TPS3813I50 SLVS331B – DECEMBER 2000 – REVISED JUNE 2005 After the reset of the supervisor is released, the lower boundary of the first WDI window is disabled. However, after the first WDI pulse low-to-high transition is detected, the lower boundary function of the window is enabled. All further WDI pulses will need to fit into the configured window frame. 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 tboundary,max and twindow,min. The trigger pulse has to fit into this window frame. The external capacitor should have a value between a minimum of 47 pF and a maximum of 63 nF. SELECTED OPERATION MODE WINDOW FRAME twindow,max = 1.25 × twindow,typ WDT = external capacitor C(ext) WDR = 0 V and WDR = VDD twindow,min = 0.75 × twindow,typ 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 BOUNDRY OF FRAME tboundary,max = twindow,max / 23.5 WDR = 0 V tboundary,typ = twindow,typ / 25.8 tboundary,min = twindow,min / 28.7 WDT = external capacitor C(ext) tboundary,max = twindow,max / 51.6 WDR = VDD tboundary,typ = twindow,typ / 64.5 tboundary,min = twindow,min / 92.7 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. A typical application example (see Figure 2) is used to describe the function of the watchdog in more detail. To configure the window watchdog function, two pins are provided by the TPS3813. These pins set the window timeout and ratio. The window watchdog ratio is a fixed ratio, which determines the lower boundary of the window frame. It can be configured in two different frame sizes. 5 |
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