Pull up resistor on Schmitt trigger IC input?

Question

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.

Answer 1

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.

Answer 2

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.