WDI not triggering NRST properly for SMT818

[hoatsosa]

Hello,

I purchased several of the SMT818 chips and was testing its watchdog timer capabilities for my TI MCU. I started with an input pulse that the MCU sent every 500 ms from a 3.3V GPIO pin into the WDI input. Looking at both this input pulse and the NRST output, I observed that the NRST output stayed high but when I removed this input pulse, the NRST output still remained high and did not attempt to reset like it should have, shown below: (blue signal is the NRST signal and yellow is the WDI signal)

I then lowered the input pulse to 31.25 ns which is the fastest my MCU can trigger. This is where the behavior got strange. Instead of remaining high when the WDI signal triggered, the NRST signal started periodically going low for roughly 200ms shown below. When I removed the 31.25ns WDI input, the NRST signal stopped triggering and remained high.

As a watchdog timer is supposed to reset a device that could be deadlocked, the behavior described above is clearly the opposite of what we need. The SMT818 is being powered by a 5V source and the chip-enable gating circuit functionality has been disabled per the datasheet instructions.

Why would this behavior be exhibited and how can we set up this device to exhibit the correct functionality (i.e. if there is a WDI pulse, the NRST stays high, if there is no WDI pulse, the NRST goes low for 200 ms then goes back to high)?

Any suggestions on this issue would be greatly appreciated. Thank you!

[Tesla DeLorean]

For our non-dyslexic members, the STM818

https://www.st.com/resource/en/datasheet/stm690a.pdf

https://estore.st.com/en/stm818mm6f-cpn.html

[TDK]

Pulses need to be at least 50 ns, explains the second result.

[TDK · Accepted solution]

> when I removed this input pulse

How exactly did you remove it?

If the pin is floating, it will not trigger the watchdog. If you tie it to GND or VDD, it should trigger.

[hoatsosa]

Thank you so much! That was it.