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I have two different circuits on two different boards. The output of board A, let's say, is optocoupler based and hence, it has a pull up on it. This signal is connected to board B through an interconnect board. In the board B, I have used a Schmitt trigger inverter gate on the signal (to prevent signal chattering) before connecting it to the MCU's GPIO pin for detection.

My question is, should I place a pull up resistor on the input on the Schmitt trigger IC as well or not? If I keep the pin floating (don't add a pull up), will it cause the Schmitt trigger output to be unstable? I am thinking of a case when for some reason where there is a connection issue between two boards and the input of Schmitt trigger is left open in unknown state.

Any advice would be appreciable.

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2 Answers 2

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I'd put a lightweight pull-up (like a 1Mohm) for the occasions when board B is powered but disconnected from board A. The resistor attached to the opto is likely to be only a few kohms and 1Mohm in parallel isn't going to affect its operation hardly at all.

Taking a step back though, I'd be thinking of putting the schmitt trigger on board A and routing a purely digital signal between board A and board B. Depending on the connection between A and B, the opto's output may "suffer" by routing it some distance especially if the signal it transmits is high speed.

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  • \$\begingroup\$ Thanks for the suggestion. Yes, the optocoupler pull up is 22KOhm. I think putting the Schmitt trigger on board A won't be possible, because the PCB layout has already started. But since the output of optocoupler isn't going to switch at high-speed (it will just give a pulse indication), I think I can add a weak pull on the board B schmitt trigger input. However, just to mention, I have an RC filter of (1.5Kohm & 1uF) on this Schmitt trigger input as well. Since we are adding a high value pull up, it won't be a problem. Am I right? \$\endgroup\$
    – LoveEnigma
    Nov 27, 2014 at 8:23
  • \$\begingroup\$ I don't believe it will be a problem. \$\endgroup\$
    – Andy aka
    Nov 27, 2014 at 8:27
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You must not leave CMOS inputs floating since this will cause spurious input changes and increased power consumption, and may result in damaging the device. If it is a TTL device then you can get away without a pull-up, but it would still be recommended.

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