Trying to latch a 200 mS, LOW event to log the event. LOW only occurs when external Watchdog timer sends reset pulse to the Arduino.

Is a D Latch suitable for this application? It has been years since my digital class; help most appreciated.

How would you proceed?

  • Just as a side-note; you know the arduino chips have build in watchdog timers, right?
    – Gerben
    Commented Jan 24, 2016 at 19:32
  • I am using Switchdoc Labs, "Dual Watchdog Timer" with an Arduino Mega running a Data logger/web server.
    – William
    Commented Jan 26, 2016 at 2:44
  • Project web page: tinyurl.com/Data500
    – William
    Commented Jan 26, 2016 at 9:11

1 Answer 1


I understand your question to be : you want to latch (and indicate) whenever an external watchdog timer has reset your Arduino.

D type flip flop (latch) can be used. You would use the clock input as the trigger. However, most D types clock on a rising edge. You could use an inverter on the input to the clock to get the D type to clock on a falling edge. You would also connect the D (Data) input high (logic 1). Then when your input (event) goes low, the Q output will latch high. The 200 mS is irrelevant because the latch ocurrs at the change from high to low (low to high on clock input).

You would also need a means of resetting the latch, which many D types have a reset input or a clear input.

enter image description here

You could also use a more simple SR latch (set reset latch) using 2 Nand gates. SR latch changes state when the “set” input goes low (providing that R input was high). Make the R input go low to reset the latch.

enter image description here

POSSIBLE ALTERNATIVE : If you have some means of operator intervention (or even a switch). You could have your program light an LED at startup. Operator (person) could reset LED. Then afterwards, any reset (restart of program) would light the LED.

  • I tried a S-R Latch (74LS279); so far it is not working. What voltage should I expect on a LOW to the S input? Measured 150mV, is this a failed 74LS279?
    – William
    Commented Jan 25, 2016 at 19:13
  • First, power from 5 volts. Second : Tie all unused inputs HI. On the SR latch that you are using, R must be high (except when resetting). When S goes low, the Q output should be near 5 volts.
    – Marla
    Commented Jan 26, 2016 at 0:43
  • I have the 74LS279 wired and working; however, there is not enough drive from the external Watchdog timer output. Will two inverters in series give me more drive? Measured 2.5 volts at the bottom of the pull-up resistor. Need to go to the Arduino Mega reset and to the S input of the Latch. --Thank you for the help!
    – William
    Commented Jan 26, 2016 at 2:40
  • Documentation on the Dual WatchDog is poor. The output is reported as "pulled to ground", "otherwise high-impedance". You didn't say what value of pull up resistor you are using. Measuring 2.5 volts indicates that your pull up resistor might be too large of a resistance.
    – Marla
    Commented Jan 26, 2016 at 12:54
  • Normal operation of Arduino (program running) Q= HIGH, S" HIGH, and R= LOW. When a WATCHDOG reset occurs I see Q go momentarily LOW (0.565 Volts. --HIGH being 4.77 Volts.) Observation made with voltmeter. Problem is that it happens so fast by the time the Arduino reads the pin for Q status; it is already a HIGH. I never get a Q value Of LOW when I read the pin connect to Q. I am trying to do a conditional based on the status of Q. Suggestions?
    – William
    Commented Jan 27, 2016 at 20:33

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