1) Only available for TLE4263-2, TLE4268, TLE4271-2, TLE4291, TLE4278, TLE4678, TLE4471, TLE4473
2) Only available for TLE7273-2, TLE7278-2, TLE7469
3) Only available for TLE4278, TLE4678
Watchdog application circuit
|Devices with standard watchdog|
|Devices with window watchdog|
The watchdog monitors the microcontroller to ensure it is operating normally. The function of the watchdog timer is to monitor the timing of the microcontroller and reset it to a known state of operation in case of an obvious timing error. For example, a microcontroller could get stuck in a software loop and stop responding to other inputs. If too much time elapses between triggers, the watchdog senses that something is wrong and sends a reset signal to the microcontroller.
Watchdog timing (analog implementation)
Positive edges at the watchdog input pin “WI” are expected within the watchdog trigger timeframe t WI,tr, otherwise a low signal at pin “WO” is generated and it remains low for t WD,low. All watchdog timings are defined by charging and discharging capacitor C D at pin “D”. Thus, the watchdog timing can be programmed by selecting C D.
In the datasheet, reset timing is given for a certain capacitor, e.g. 100nF.
Example: TLE4678 watchdog timing
|Watchdog trigger time||t WI,tr,100nF||25||36||47||ms||Calculated value;
C D = 100nF
|Watchdog output lowtime||t WD,low,100nF||13||18||23||ms||Calculated value;
C D = 100nF
V Q > V RT,low
|Watchdog period||t WD,p,100nF||38||54||70||ms||
In case a watchdog trigger timeperiod t WI,tr different from the value specified at C D= 100nF is required, the corresponding value of the delay capacitor value can be derived as follows:
Watchdog output low time t WD,low and watchdog period t WD,p can be derived using:
Example: TLE7273-2 watchdog timing
|Ignore window time||t OW||25.6||32||38.4||ms||Fast watchdog timing|
|51.2||64||76.8||ms||Slow watchdog timing|
|Watchdog period||t WD,p||25.6||32||38.4||ms||Fast watchdog timing|
|51.2||64||76.8||ms||Slow watchdog timing|
For safety-critical applications a more advanced watchdog called window watchdog is provided for higher security of the system. The window watchdog operates in a similar manner to the standard watchdog except a trigger must occur within a certain window or time slot. If a trigger occurs outside of the window or does not occur at all within the designated window, the window watchdog will reset the microcontroller. When an unintentional trigger occurs, the standard watchdog is not able to decipher if this trigger is valid. The window requirement enables the window watchdog to detect unintentional triggers.
If a microcontroller is set to sleep mode or to low power mode, its current consumption is very low and it might not be able to send any watchdog pulses to the voltage regulator’s watchdog input “WI”. In order to avoid unwanted wake-up signals due to missing edges at pin “WI”, the watchdog function of some linear voltage regulators can be activated dependent on the regulator’s output current.
The load-dependent watchdog activation feature is available on TLE4268, TLE4278, TLE4678, TLE7273-2 and TLE7278-2.
On voltage regulators TLE4268, TLE7273-2 and TLE7278-2, watchdog activation and deactivation thresholds are fixed.
On voltage regulators TLE4278 and TLE4678, the watchdog can be permanently activated or deactivated, or enabled/ disabled by defining a current threshold through the external resistor at the WADJ pin:
- An external resistor at WADJ to GND determines the watchdog activation threshold.
- Connect WADJ directly to GND to permanently deactivate the watchdog.
- Connect WADJ to the output Q via a 270kΩ resistor to permanently activate the watchdog.
Watchdog timing (window watchdog)
It is possible that the microcontroller could become trapped in a routine of only emitting the pulses. The standard watchdog is not capable of detecting this potential program error and would interpret this signal as valid. The solution in this case would be to use the window watchdog.
Disadvantage of standard watchdog
To further reduce the potential risk of program errors, a more advanced watchdog called window watchdog has been implemented. It offers higher system security. A window watchdog monitors not only the minimum pulse period, but also the maximum pulse period. A watchdog pulse must occur within a certain window or time slot. If a pulse occurs outside of the window or does not occur at all within the designated window, the window watchdog will reset the microcontroller.
Advantage of window watchdog
In some applications, the microcontroller software is stored in an external non-volatile memory and needs to be downloaded to the microcontroller after every start-up. During this download, the microcontroller is not able to send any watchdog pulses. To skip unwanted watchdog alerts due to missing WI-input edges, the watchdog function should be deactivated.
The watchdog function can be easily deactivated by connecting WADJ directly to GND for those regulators with an adjustable watchdog activation threshold (TLE4278 and TLE4678).
For other linear regulators, the watchdog function could be deactivated by connecting the D pin to the output Q via a pull-up resistor to compensate the discharge current of the watchdog. The pull-up resistor can be determined by referring to the delay capacitor discharge current specified in the datasheet.
Example: watchdog deactivation for TLE4263-2
Formula to apply:
R PU,D ≤ (V Q–V D)/I D,wd,max = (5.0V–1.0V)/9.40µA = 425kΩ
Taking some head room for tolerances, a 390kΩ pull-up resistor could be recommended for deactivating the watchdog function on the TLE4263-2.
1) Applicable to TLE4263-2, TLE4268, TLE4271-2, TLE4291, TLE4278, TLE4678,TLE4471, TLE4473
2) Applicable to TLE7273-2, TLE7278-2, TLE7469
3) The window watchdog is available for voltage regulators TLE7273-2 and TLE7469