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Why Pull UP Resistors In MUX control lines ?

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Advanced Member level 4
Jan 22, 2012
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Hello all......

Can any 1 explain me what is the importance of using pull up resistors in multiplexer control lines ?? In many circuits I have seen this, :


Current flows from vcc to digital pin of MCU.

Can any 1 explain me
Not without knowing what's on the left side. Pull-up makes sense e.g. for 8051 port0, which has open drain outputs. In other cases, it may be waste of components and energy, respectively brainless copy-and-paste.

The pull-up resistor ensures that the wire is at a defined logic level even if no active devices are connected to it... It helps to maintain a logic state even if the input to the mux is disconnected suddenly and avoids error outputs... It actually maintains a state of logic High when input is inactive and an activeness of input will override the logic High state...

The pull-up resistors shouldn't be necessary if the microcontroller outputs are always either driven high or low.

However the pull-up resistors may be used to ensure that there is a logic level present at all times as demetal has said. This could be, as FvM has said, if the outputs are open-drain, or if they have mistakenly been configured as inputs or if the outputs are high-impedance tri-state, eg when the microcontroller is reset, etc.

Hope this helps.

But how the data's are differentiated? What will be the voltage at the pin if the data is logic high or logic low?

It's a pull-up. So, if the pin is driven low by the microcontroller, the voltage at that pin will be low.

Not when the pin is low. But it pulls up the voltage (as the name suggests) when the pin is neither high nor low (floating, tristate, etc).

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