What is pull-down and pull-up resistor?

What is pull-down and pull-up resistor?

A pull-up resistor connects unused input pins (AND and NAND gates) to the dc supply voltage, (Vcc) to keep the given input HIGH. A pull-down resistor connects unused input pins (OR and NOR gates) to ground, (0V) to keep the given input LOW.

Why are pull up and pull-down resistors necessary?

To control the current flow, we need those pull-down or pull up resistors. A pull-up resistor allow controlled current flow from supply voltage source to the digital input pins, where the pull-down resistors could effectively control current flow from digital pins to the ground.

How do you determine the value of a pull-up resistor?

A rule of thumb is to use a resistor that is at least 10 times smaller than the value of the input pin impedance. In bipolar logic families which operate at operating at 5V, the typical pull-up resistor value is 1-5 kΩ. For switch and resistive sensor applications, the typical pull-up resistor value is 1-10 kΩ.

What does a 10k resistor look like?

What does a 10K ohm resistor look like? A 10k ohm resistor has 4 color bands: brown, black, orange, and gold for 5% tolerance, respectively. A 1k ohm resistor has 4 color bands: brown, black, red, and gold for 5% tolerance, respectively.

Can you use a pull down resistor on an analog input?

A 10k ohm resistor is recommended. See related links for more information on pull-down resistors. Adding a resistor may have an impact on your analog input readings since they will change the load going to the ADC. If you add a pull-down resistor, check your readings after to make sure that it has not affected them.

Why do microcontrollers use pull up and pull down resistors?

As you stated, using pullup/pulldown resistors on pins expected to be output pins can be there to guarantee an input state. This is done to mitigate firmware/MCU failure, but will only work to protect against Hi-impedance state (basically the pin is configured as input by accident).

What is the maximum pull up resistor for a logic gate?

By knowing the information above, we can calculate the maximum pull-up resistor value required for a single TTL 74LS series logic gate as: Then using Ohms Law, the maximum pull-up resistance required to drop 3 volts for a single TTL 74LS series logic gate would be 150kΩ.

What should the resistive value of a pull up resistor be?

Reducing the resistive value further, will produce a smaller voltage drop but increase the current. Then we can see that while there may be a maximum allowable resistive value, the resistance value for pull-up resistors is not usually that critical with resistance values ranging from between 10k to 100k ohms acceptable.