In digital electronic circuits the input pin of the microcontroller needs to define the logic level. The logic levels are either HIGH or LOW. If the input pin is not properly assigned the value will makes the program to randomly select the High and LOW signal due to the noise. This could causing an unpredictable behavior in your circuit or devices, to solve this problem by using a Pull up and pull down resistor.
What Is a Floating Input?
When a microcontroller input pin is not connected to the HIGH or LOW voltage, which will causing random input selection because of the faulty signal or noise it is called as floating input.
Without the Input pin resistors the noise can easily change the state of Pin, because the environment can easily affect the pin input states.
What Happen When Floating Input Is Occurs
- False triggering
- Unstable outputs
- Unexpected system behaviour
Pull Up Resistor
Pull Up resistor is the resistor is connected to the positive supply of the circuit and input pin of microcontroller.
How it’s Work
- The resistor is connected between the positive and input pins. when the connected switch in open, The resistor pulls the positive voltage to the input pin of microcontroller. Which means the input pin in initial stage is HIGH.
Logic Behavior
| Switch State | Input Level |
|---|---|
| Open | HIGH (1) |
| Closed | LOW (0) |
Pull Down Resistor
In pull down the resistor is connected between the negative supply and input pin of microcontroller. In its initial state the input level will be in LOW state.
Logic Behavior
| Switch State | Input Level |
|---|---|
| Open | LOW (0) |
| Closed | HIGH (1) |
What is the use of Pull Up and Pull Down Resistors?
- This resistors can prevent the floating inputs. Caused by unconnected input connection and noise.
- A fixed and stable logic levels at initial state.
- These resistors will reduce the noise sensitivity
- Improve working efficiency in digital systems
Common Value of Resistor
The resistor vale of 1 kΩ to 10 kΩ is most commonly used. In most of the circuit 10K is convenient because of it can able to balancing the power consumption and noise reduction.
Too small value resistor > wastes power
Too large value > weak and more noise-sensitive
