Max watchdog delay using MAX6749

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I am designing a circuit using Multi gas sensing platform using SAMD21(programmed in Arduino language).The data will be uploaded to a MQTT server using ESP8266.The circuit should run without any human intervention.As an extra layer of security,i want a supervisory circuit so that if any hang/stall occurs,the circuit should power reset itself.

For that purpose,i have chosen MAX6749 & MIC2042 for the power reset circuit.The SAMD21,Sensors & the ESP8266 will be powered via the MIC2042.MIC2042 & MAX2749 will be powered directly.The MIC2042 enable pin will be connected to MAX6749 reset pin.The MAX6749 will receive pulses from the MCU in some regular interval.If any hang/stall occurs,the SAMD21 will not give output pulse to the MAX6749 & it will reset the MIC2042 & thus the whole  circuit.

My query is,i want a pulse interval of 30s.Is it possible with MAX6749 because as far as i can understand in the datasheet:

In extended mode (Figure 4b), the watchdog timeout period is multiplied by 128. For example, in extended mode, a 0.1μF capacitor gives a watchdog timeout period of 65s

 Has anyone worked with these before?

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Not sure what is unclear about the data sheet.

 

Basic Principles: You need to pulse the watchdog at a shorter interval than the hardware timeout. Some colloquially refer to this as "petting the dog". The criterion is that the watchdog timer has to be reset BEFORE it times out. There is typically some tolerance associated with the watchdog. In the case of the MAX6749, because the timeout clock frequency is set by an external capacitor, the tolerance will be a combination of the capacitor tolerance and tolerances associated with the internal clock oscillator circuitry.

 

So, if you use a 0.047uF capacitor (a standard value), the nominal timeout will be 65sec (0.047/0.1) = 30.55 seconds. But, if your capacitor has a 10% tolerance, then this interval COULD be as short as 27.5 seconds or it could be as long as 33.6 seconds. Internal circuit tolerances will add to this. Thus, it might be safe to pulse the watchdog timer at an interval that is no longer than, lets say, 25 seconds, or maybe a bit shorter.

 

The next larger standard capacitor value, 0.056uF, would give a nominal timeout of 65sec (0.056/0.1) = 36.4 seconds. A 10% tolerance would give a possible range that extends, at the very least, from 32.76 sec to 40.4 sec. Now, you could safely pulse it at a 30 second interval and have it safely reset before timeout.

 

Jim

 

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Last Edited: Mon. Aug 26, 2019 - 07:41 PM