PULL UP AND PULL DOWN RESISTOR

 Floating condition

If there is nothing connected to your controller pin and that pin read condition of your sensor connected to it or peripherals connected to it. Then sometimes undefined voltage appears at pin and controller take that pin as a state either high or low and perform action according to that. That condition is called as a floating condition.

What is pull up and pull down condition ?

To eliminate this floating condition pin of controller that read state either high or low is pulled to high or low. When pin of controller is pulled to high then it is called pull up and if pin is pulled to low then it is called as a pull down. Based on initial condition of pin pull up and pull down is selected. In pull up pin of controller is connected to VCC through a resistor and in pull down pin is connected to ground through resistor. Below are the pictures for reference.



During this selection of pull down and pull up resistor plays major role. Mostly 4k7 is preferred resistor in schematic of datasheet of a controller and we use same as it is but there may be changes in our power supplies and input current and EMI and EMC of circuits also occur false intrusion. So proper calculation of pull up and pull down resistor is important.

Note : For reference please prefer above images

For Pull up resistor

Rpullup = (Vout-VIH)/Ipullup

where Vout is VCC of a circuit, VIH is minimum voltage that is required to read as a high and Ipullup is a current flowing through pull up resistor. In above quantities Vout is input voltage of controller, VIH is mentioned in datasheet of controller and Ipullup is sum of leakage current and current flowing from controller to the peripheral or sensor.

For Pull down resistor
Rpulldown = VIL/Ipdl
where VIL is maximum low level input voltage which is mentioned in datasheet of a controller and Ipdl is current at pull down resistor which is calculated as sum of leakage current from controller and low level output current these both quantities are mentioned in datasheet of a controller and sensor.

Where to place pull up and pull down resistor in circuit?

We must place pull up resistor as close as controller and also we should place peripherals as close as possible. Because if we placed both far away from each other then all traces has some impedance then value of our pull up or pull down resistor will be equal to the sum of value of resistor and impedance of trace.

What type of resistor is good as a pull up and pull down resistor?

Film type and ceramic resistors are mostly used as pull up and pull down resistor. Ceramic resistors are used for SMC package and film type resistors are used for through hole package. Carbon resistors should not be used  for pull up and pull down purpose as they have high tolerance than ceramic and film type resistor.


Hope you will find this helpful ......

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