How to check external watched dog IC when toggling MCU oscillator?

[Md Mubdiul Hasan]

Hi There,

I am working with a MCU STM32F405VET for any particular purpose. Lets a at this time firmware code has burn on MCU. Lets talk about HW issue.

Take a look the datasheet here https://www.qdatasheet.com/datasheet-download/190671/1/ST-Microelectronics/STM32F405VET6

In my design portion a 25MHz oscillator supports MCU for clocking. See the attached file.

In this time a watched dog ic STWD100NXWY3F working externally,https://www.st.com/resource/en/datasheet/stwd100.pdf 

What it does is watchdog timer is as a system monitor to detect and reset an "out of control" microprocessor. When program execution goes awry it will not properly execute the code that restarts the watchdog. In such a case the watchdog timer will timeout and cause a microcontroller reset. In a properly designed system, the reset will correct the error. Is the case of independent watched dog from main MCU timer.

I want to connect a toggle swish in 2 and 3 point of crystal oscillator XT1 and wish to see the input out put of 1 and 4 of U29 watched dog.

I did see in oscillator when I push the switch meaning that 3 and 2 pin of XT1 has been shorted then input of U29 distorted but wont disappeared.

It wont clear instantly , what do you think?

Datasheet of STWD100NXWY3F says, While the system is operating correctly, it periodically toggles the watchdog input, WDI. If the system fails, the watchdog timer is not reset, a system alert is generated and the watchdog output, WDO , is asserted. BUT MY MCU NEVER GOES TO RESET.

What do you think?

[KnarfB · Accepted solution]

Not sure if I got everything right from your problem description, but: The STM32 has an on-chip clock-security-system (CSS) which, when active, will detect failure of your external clock (HSE), switch to internal oscillator (HSI) and generate an interrupt for further software treatment. Moreover, the on-chip independent watchdog (IWDG) is clocked by an independent internal oscillator and can safely reset the MCU. So there is IMHO no need for such a complex external circuitry. Even if you use STWD100 you may connect /WDO to /NRST via a simple 4k7 resistor (as shown in the data sheet).

[Md Mubdiul Hasan]

An expert says ' it's kinda hard to say something quantitative, about code reliability and WatchDog tripped frequency. Zero resets being the goal, during development and testing.

Previously who handled this project has gone. This is a charger module controller, at display control( display has external pad input) voltage values comes correctly but current does not.

Someone explains just very few information but he doesn't know what to do.

He said, f/w cant make watchdog action in this case. But I do believe WD command has inactivated.

Now looking at left bottom side of circuit you can see 2 signal WADCH_DOG_EVENT and WADCH_DOG_EV_CLEAR is appeared.

What I did is connect a IN4148 at the base of Q15 that goes to pin 1 of U29 WD IC.

Then I stared push and pull the switch again in XT1 and saw the WDO input pulse diminishes and MCU start to reset.

After "time out" system restart again!