Hardware Watchdog

the PCB (> v1.3) is equipped with an external power supply monitor and hardware watchdog IC STM6822S. The watchdog is periodically reset by the firmware.

Note

The external watchdog is only activated in Release mode. When building the application in Debug mode, it is deactivated because setting breakpoints or other events that distrb the program flow would trigger a the external watchdog and force a software reset.

During startup of the power supply, it keeps the controller in reset until the power supply is stable. The following picture shows the 3.3 Volt supply rail on channel 2 and the low active reset signal of the STM32F407 on channel 1.

The total startup delay is larger than 200 ms.

The follwoing figure shows the startup of the voltage rgulator and the reset signal.

The power supply is gradually ramped up over approximately 500 us. The external watchdog does not work fpr voltages below 0.9 Volt. Therefore the reset signal rises simultaneously. Once the supply voltage exceeds 0.9 Volts, the reset signal is pulled low. This ensures that the controller is not able to start until the > 200 ms startup time have passed.