Electronic – Why do some people use resistors to create a HIGH connection on a chip


This is a follow-up question to my previous one, What does Low/High mean on the connections of a chip?.

To create a HIGH connection, you often just connect the pin to VDD, which is often above VIH (and therefore sets it to HIGH). However, some people choose to add resistors to those pins, as shown below:

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Credit: Wolfson

In this case, DVDD is 3.3V and the VIH is 0.7Ă—DVDD. Looking at that, it seems that the resistors aren't necessary. What are these resistors for, and do I even need them?

Best Answer

These are called pull up resistors. These are used to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected or high-impedance is introduced. Usually used with open-collector or open drain outputs.

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