STWD100NY Connection

tensaisakuragi06 · ‎2025-09-01

I use STWD100NY Watchdog IC and for now, i can not achieve what i want. The problem is about to get output from the WDO pin. I did not connect the WDO to the MCU but i connect it to drive a relay. I set up my timer to 4 seconds. Even if it exceeds the twd time, i can not get the signal from WDO pin. I attach my drawings below and needs your help. My Vcc is 5V( most setups use 3.3v but i think that i can choose it 5V according to datasheet ). R25 and R26 resistors are optional.

tensaisakuragi06 · ‎2025-09-01

My bad, you re right i did not put any resistor but when writing here, i write as "planning to put 0R". By the way when changing the relay resistor, i am so close to my plan, when everything works well, IC reset itself by the help of WDI. By external pull-ups, i can drive relay at normal conditon. The remaining problem is this: when i adjust my timer to 2 sec, every transition, WDO toggles. I dont want the toggling process (i set up timer to high value to test if IC works). I attach a video related with that. The logic to latch the WDO confuses me.

Andrew Neil · ‎2025-09-01

@tensaisakuragi06 wrote:
The remaining problem is this: when i adjust my timer to 2 sec, every transition, WDO toggles. I dont want the toggling process

That seems consistent with what the datasheet says:

So the WDO gives a pulse output with a width between 10us and 280ms.

Are you wanting it to stay asserted permanently once the watchdog has "fired" ?

A complex system that works is invariably found to have evolved from a simple system that worked.
A complex system designed from scratch never works and cannot be patched up to make it work.

tensaisakuragi06 · ‎2025-09-01

If i am not wrong, this situation happens because of my testing way. I think that relay can not toggle if MCU really crash.

Peter BENSCH · ‎2025-09-01

Well, the question is, why are you going to all this trouble to control a relay with a watchdog?

A watchdog is there to reset a device (typically an MCU or MPU) when there are no signs of life, thereby bringing the device out of the ‘death zone’. A relay is rather unsuitable as an intermediary. What is being controlled by the relay?

In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.

tensaisakuragi06 · ‎2025-09-01

Its depending on the need. I do not want to recover the IC but somehow want to know if is fine or not. I use it for different purpose rather than the actual aim.

tensaisakuragi06 · ‎2025-09-01

Relay is used for just give a warning with its led indication.

Andrew Neil · ‎2025-09-01

If it's just an LED, why do you need a relay?

The STWD100NY can easily drive an LED directly!

A complex system that works is invariably found to have evolved from a simple system that worked.
A complex system designed from scratch never works and cannot be patched up to make it work.

tensaisakuragi06 · ‎2025-09-01

Its my prediction that they will use it for the button with LED. I dont have any extra space to embed the watchdog relay. The only want to know that if there is a problem and this problem is caused by malfunctioning of the MCU. This IC comes into my mind for the solution. I don't want to recover IC, i just want to know if the malfunctioning is due to improper working of MCU. Maybe it is not suitable or the best way for that but for now, it satisfy my needs. If you suggest any other method, i will kepp them in my mind for future projects.