Why Pull UP Resistors In MUX control lines ?

[ranaya]

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.

 

[FvM]

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.

 

[demetal]

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...

 

[Tahmid]

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.
Tahmid.

 

[nikhildascl]

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

 

[Tahmid]

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

 

[nikhildascl]

So the pull up does not make any change in voltage levels right?

 

[Tahmid]

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).