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L4979D Datasheet(PDF) 11 Page - STMicroelectronics |
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L4979D Datasheet(HTML) 11 Page - STMicroelectronics |
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11 / 22 page  L4979D, L4979MD Application information Doc ID 10262 Rev 9 11/22 Table 10. Reset time diagram 3.3 Watchdog The watchdog input Wi monitors a connected microcontroller. If pulses are missing, the reset output Res is set to low. The pulse sequence time can be set within a wide range through the external capacitor Ctw. The watchdog circuit discharges the capacitor Ctw with the constant current Icwd. If the lower threshold Vwlth is reached, a watchdog reset is generated. To prevent this reset, the microcontroller must generate a positive edge during the discharge of the capacitor before the voltage has reached the threshold Vwlth. In order to calculate the minimum time Tdis during which the microcontroller must generate the positive edge, the following equation can be used: Each Wi positive edge switches the current source from discharging to charging; the same happens when the lower Vwlth threshold is reached. When the voltage reaches the upper threshold Vwhth the current switches from charging to discharging. The result is a saw tooth voltage at the watchdog timer capacitor Ctw. Figure 4. Watchdog time diagram V whth V wlth – C tw I cwd T dis = |
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