In electronic

logic circuit A logic gate is a device that performs a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, one that has, for ...

s, a pull-up resistor (PU) or pull-down resistor (PD) is a

resistor A resistor is a passive two-terminal electronic component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active e ...

used to ensure a known state for a signal. More specifically, a pull-up resistor or pull-down resistor ensures that a wire will have a high logic level or low logic level, respectively, in the absence of a driving signal. It is typically used in combination with components such as

switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...

es and

transistor A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...

s, which physically interrupt the connection of subsequent components to

ground Ground may refer to: Geology * Land, the solid terrestrial surface of the Earth * Soil, a mixture of clay, sand and organic matter present on the surface of the Earth Electricity * Ground (electricity), the reference point in an electrical circ ...

or to V_CC. A closed switch creates a direct connection to ground or V_CC, but without a PU or PD, when the switch is open, the rest of the circuit would be left floating (i.e. it would have an indeterminate voltage), which is generally undesirable. For a switch that is used to connect a circuit to ground, a pull-up resistor (connected between the circuit and V_CC) ensures a well-defined

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

(i.e. V_CC) when the switch is open. For a switch that is used to connect a circuit to V_CC (e.g. if the switch is used to transmit a "high" signal when closed), a pull-down resistor connected between the circuit and ground ensures a well-defined ground voltage (i.e. logical low) across the remainder of the circuit when the switch is open.

Principle

An open switch is not equivalent to a component with infinite impedance. The stationary voltage in any loop with an open switch cannot be determined by Kirchhoff's laws, while that with a component with infinite impedance can be determined by such laws. Consequently, the voltages across those ''critical'' components (such as the digital inputs in the example on the right), which are only in loops involving the open switch, are undefined, too. A pull-up resistor effectively establishes an additional loop over the critical components, ensuring that the voltage is well-defined even when the switch is open.

Optimal resistance

For a pull-up resistor to serve only this one purpose and not interfere with the circuit otherwise, a resistor with an appropriate amount of resistance must be used. For this, it is assumed that the critical components have infinite or sufficiently high impedance, which is guaranteed, for example, for logic gates made from

FET The field-effect transistor (FET) is a type of transistor that uses an electric field to control the current through a semiconductor. It comes in two types: junction FET (JFET) and metal-oxide-semiconductor FET (MOSFET). FETs have three termi ...

s. In this case, when the switch is open, the voltage drop across a pull-up resistor (with ''sufficiently low'' impedance) practically vanishes, and the circuit looks like a wire directly connected to V_CC. On the other hand, when the switch is closed, the pull-up resistor must have ''sufficiently high'' impedance in comparison to the closed switch to not affect the connection to ground. Together, these two conditions can be used to derive an appropriate value for the impedance of the pull-up resistor. However, usually, only a lower bound is derived, assuming that the critical components do indeed have infinite impedance. A resistor with relatively low resistance (relative to the circuit it is in) is often called a "strong" pull-up or pull-down; when the circuit is open, it will pull the output high or low very quickly (just as the voltage changes in an

RC circuit A resistor–capacitor circuit (RC circuit), or RC filter or RC network, is an electric circuit composed of resistors and capacitors. It may be driven by a voltage source, voltage or current source and these will produce different responses. A fi ...

), but will draw more current. A resistor with relatively high resistance is called a "weak" pull-up or pull-down; when the circuit is open, it will pull the output high or low more slowly, but will draw less current. This current, which is essentially wasted energy, only flows when the switch is closed, and technically for a brief period after it is opened until the charge built up in the circuit has been discharged to ground.

Applications

A pull-up resistor may be used when interfacing logic gates to inputs. For example, an input signal may be pulled by a resistor, then a switch or jumper strap can be used to connect that input to ground. This can be used for configuration information, to select options or for troubleshooting of a device. Pull-up resistors may be used at logic outputs where the logic device cannot source current such as

open-collector Open collector, open drain, open emitter, and open source refer to integrated circuit (IC) output pin configurations that process the IC's internal function through a transistor with an exposed terminal that is internally unconnected (i.e. "open ...

TTL logic devices. Such outputs are used for driving external devices, for a wired-OR function in

combinational logic In automata theory, combinational logic (also referred to as time-independent logic) is a type of digital logic that is implemented by Boolean circuits, where the output is a pure function of the present input only. This is in contrast to sequ ...

, or for a simple way of driving a logic bus with multiple devices connected to it. Pull-up resistors may be discrete devices mounted on the same circuit board as the logic devices. Many

microcontrollers A microcontroller (MC, uC, or μC) or microcontroller unit (MCU) is a small computer on a single integrated circuit. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable input/output peripherals. Pro ...

intended for embedded control applications have internal, programmable pull-up resistors for logic inputs so that not many external components are needed. Pull-down resistors can be safely used with

CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss ", , ) is a type of MOSFET, metal–oxide–semiconductor field-effect transistor (MOSFET) semiconductor device fabrication, fabrication process that uses complementary an ...

logic gates because the inputs are voltage-controlled. TTL logic inputs that are left unconnected inherently float high, and require a much lower valued pull-down resistor to force the input low. A standard TTL input at logic "1" is normally operated assuming a source current of 40 μA, and a voltage level above 2.4 V, allowing a pull-up resistor of no more than 50 kohms; whereas the TTL input at logic "0" will be expected to sink 1.6 mA at a voltage below 0.8 V, requiring a pull-down resistor less than 500 ohms. Holding unused TTL inputs low consumes more current. For that reason, pull-up resistors are preferred in TTL circuits. In bipolar logic families operating at 5 VDC, a typical pull-up resistor value will be 1000–5000 Ω, based on the requirement to provide the required logic level current over the full operating range of temperature and supply voltage. For

and MOS logic, much higher values of resistor can be used, several thousand to a million ohms, since the required leakage current at a logic input is small.

Drawbacks

Some disadvantages of pull-up resistors are the extra power consumed when current is drawn through the resistor and the reduced speed of a pull-up compared to an active current source. Certain logic families are susceptible to

power supply A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, electric current, current, and frequency to power ...

transients introduced into logic inputs through pull-up resistors, which may force the use of a separate filtered power source for the pull-ups.

References

Paul Horowitz Paul Horowitz (born 1942) is an United States of America, American physicist and electrical engineering, electrical engineer, known primarily for his work in electronics design, as well as for his role in the search for extraterrestrial intellige ...

and

Winfield Hill Winfield Hill is the Director of the Electronics Engineering Laboratory at the Rowland Institute at Harvard University. A self-proclaimed "electronics circuit-design guru" and trained physicist and electronic engineer, he co-authored the popular ...

, ''

The Art of Electronics ''The Art of Electronics'', by Paul Horowitz and Winfield Hill, is a popular electronics design reference textbook dealing with analog electronics, analog and digital electronics. The third edition was published in 2015. The author accepts repo ...

'', 2nd edition,

Cambridge University Press Cambridge University Press was the university press of the University of Cambridge. Granted a letters patent by King Henry VIII in 1534, it was the oldest university press in the world. Cambridge University Press merged with Cambridge Assessme ...

, Cambridge, England, 1989, {{reflist Electronic circuits Resistive components de:Open circuit#Pull-up

Principle

Optimal resistance

Applications

Drawbacks

See also

References