Applications for Resistors and Pull-up and Pull-down Resistors

What is A Pull-up Resistor?

In electronic devices, pull-up resistors are resistors that are employed to provide a precise logical level at a pin in all circumstances. Recall that there are three logic states for digital logic circuits: floating (or high impedance), low, and high. When the pin is left “floating” rather than pulled to a high or low logic level, it is said to be in the high-impedance condition. An disconnected microcontroller input pin is a good example of this. The microcontroller may arbitrarily interpret the input value as either a logical high or logical low because it is neither in a high nor low logic state. As illustrated in the following picture, pull-up resistors are employed to pull the value to a logical high state, which helps the microcontroller solve its problem.

The input of the MCU would be floating when the switch is open and would only be pushed down to a logical low when the switch is closed if there was no pull-up resistor.

Pull-up resistors are just fixed-value resistors connected between the voltage supply (usually +5 V, +3.3 V, or +2.5 V) and the corresponding pin. This defines the input or output voltage in the event that there is no driving signal. Pull-up resistors are not a particular type of resistor. As this article will cover later, the value of a pull-up resistor might vary depending on the application. Typically, it has a value of 4.7 kΩ.

What is A Pull-down Resistor?

Apart from pulling the pin to a logical low value, pull-down resistors function similarly to pull-up resistors. They are linked to a device’s proper pin and ground. The following figure shows an example of a pull-down resistor in a digital circuit.

A pushbutton switch is connected in this image to a microcontroller pin and the supply voltage. In this design, the pull-down resistor pulls the input voltage down to ground (logical zero value) when the switch is open, preventing an undefined state at the input. When the switch is closed, the microcontroller input is at a logical high value. In order to prevent the input voltage at the pin from staying at a constant logical low value regardless of the switch position, the pull-down resistor must to have a resistance greater than the logic circuit’s impedance.

Pull Up vs. Pull Down Resistors

In digital electronic circuits, pull-up and pull-down resistors are crucial parts that guarantee the dependability and stability of digital signals in a variety of applications, including aeronautical engineering. Pull-up and pull-down resistors are fixed values that are connected to either GND and a pin or +5V and a pin. They are not separate resistors. Pull-up resistors are resistors that are linked between a pin and +5V, and pull-down resistors are resistors that are connected between a pin and GND.

A pull-down resistor is linked between a signal line and ground, and a pull-up resistor is connected between a signal line and a positive voltage source, usually Vcc. When there is no active input, a pull-up resistor’s principal job is to keep the signal line at a high logic level, which keeps it from floating and guarantees a definite state. On the other hand, when there is no active input driving the signal line, a pull-down resistor is used to maintain the line at a low logic level. Resistors of both kinds are employed to stop digital signals from entering undefinable states, which can cause unpredictable behavior in electronic systems.

Pull-up and Pull-down Resistor Values

Two considerations determine the proper value of the pull-up (or pull-down) resistor. Power dissipation is the first factor. When the switch is closed, a high current will pass through the pull-up resistor if the resistance value is too low, heating the device and consuming extra power. When low power consumption is required, steer clear of this scenario, which is referred to as a strong pull-up. When the switch is open, the pin voltage is the second factor. When the switch is open, the input voltage may not be sufficient if the pull-up resistance value is too high and the input pin leakage current is excessive. This is referred to as having a weak pull-up. The input pin’s impedance, which is directly connected to the leakage current of the pin, determines the pull-up’s resistance in practice.

Generally speaking, you should use a resistor that is at least ten times less than the input pin impedance. Pull-up resistor values in bipolar logic families running at 5 V typically range from 1 to 5 kΩ. The average pull-up resistor value for switch and resistive sensor applications is 1-10 kΩ. If in doubt, 4.7 kΩ is a decent place to start when employing a switch. A tiny input leakage current in some digital circuits, like the CMOS family, allows for significantly greater resistance values, ranging from about 10 kΩ to 1 MΩ. Using a higher resistance value has the drawback of making the input pin respond to voltage changes more slowly. This is the outcome of an RC circuit formed at the switching node by the coupling between the pull-up resistor and the total pin and wire capacitance. The circuit will operate more slowly the larger the product of R and C, as it takes longer for the capacitance to charge and discharge. Large pull-up resistors can occasionally restrict the pace at which a pin can consistently change states in high-speed circuits.

Pull-up and Pull-down Resistors Applications

Pull-up and pull-down resistors are frequently used to interface a microcontroller or other digital gates with a switch or other input. Fewer external components are required because most microcontrollers come with programmable pull-up and/or pull-down resistors built in. Direct interaction between these microcontrollers and switches is achievable. Although some microcontroller families include both pull-up and pull-down resistors, pull-up resistors are typically utilized more frequently than pull-down resistors.

They are frequently used to supply a regulated current flow into a resistive sensor before the output voltage signal from the sensor is converted from analog to digital. Pull-up resistors are also utilized in the I2C protocol bus, which allows a single pin to function as both an input and an output. In its high-impedance condition, the pin floats when it is not linked to a bus. Another method for providing a specified output impedance on outputs is to employ pull-down resistors.

Pull-up resistor applications in aerospace

I/O Interfaces: In order to communicate with external devices or peripheral equipment, aerospace systems commonly use input/output (I/O) interfaces. In these interfaces, pull-up resistors are frequently used to set default logic levels and guarantee correct signal integrity. Pull-up resistors, for example, create a high logic level in digital input interfaces when the input signal is not actively driven, preventing undefined or inaccurate readings.

Systems Critical to Safety: In the aerospace industry, systems critical to safety, including flight control or engine management, must function with extreme dependability and safety. Pull-up resistors are used in circuits for safety monitoring and defect detection to construct default states. The system can identify disconnected or open signals by supplying a pull-up resistor, since a shift from the default high state denotes a fault condition.

Interfaces for Avionics: Avionics systems comprise a variety of electronic parts, such as control panels, displays, and interfaces for data exchange. In these interfaces, pull-up resistors are frequently used to provide default high logic levels in the event that signals are not actively driven. Pull-up resistors, for example, guarantee that input pins stay high when switches are in an open or inactive state in control panels and switch interfaces, providing a recognized default condition.

Data Buses: Aerospace systems often employ data buses like ARINC 429 or MIL-STD-1553 for communication between different avionics subsystems. Pull-up resistors are used in these data buses to maintain a defined voltage level when no active device is driving the bus. By providing a pull-up resistor on the bus lines, the system ensures that the voltage levels stay high and prevents undetermined states or floating conditions, which could lead to data corruption or errors.