- Pull-up resistor
resistorsare used in electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected. Pull-up resistors may also be used at the interface between two different types of logic devices, possibly operating at different power supply voltages.
The idea of a pull-up resistor is that it weakly "pulls" the voltage of the wire it's connected to towards 5V (or whatever voltage represents a logic "high"). However, the resistor is intentionally weak (high-resistance) enough that, if something else strongly pulls the wire toward 0V, the wire will go to 0V. An example of something that would strongly pull a wire to 0V would be the transistor in an open-collector output.
Similarly, pull-down resistors are used to hold the input to a zero (low) value when no other component is driving the input. They are used less often than pull-up resistors. Pull-down resistors can safely be used with
CMOSlogic gates because the inputs are voltage-controlled. TTLlogic inputs that are left un-connected inherently float high, thus they require a much lower valued pull-down resistor to force the input low. This also 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
CMOSand 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.
Pull-up resistors may be used at logic outputs where the logic device cannot source current, such as open-collector TTL logic devices. Such outputs are used for driving external devices, for a wire-OR function in
combinatorial logic, or for a simple way of driving a logic bus with multiple devices connected to it. For example, the circuit shown on the right uses 5 V logic level inputs to actuate a relay. If the input is left unconnected, pull-down resistor R1 ensures that the input is pulled down to a logic low. The 7407 TTL device, an open collector buffer, simply outputs whatever it receives as input, but as an open collector device, the output is left effectively unconnected when outputting a "1". Pull-up resistor R2 thus pulls the output all the way up to 12 V when the buffer outputs a "1", providing enough voltage to turn the power MOSFETall the way on and actuate the relay.
Pull-up resistors may be discrete devices mounted on the same circuit board as the logic devices. Many
microcontrollersintended for embedded control applications have internal, programmable pull-up resistors for logic inputs so that minimal external components are needed.
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 supplytransients introduced into logic inputs through pull-up resistors, which may force the use of a separate filtered power source for the pull-ups.
I²C requires pull-up resistors on its clock (SCL) and data line (SDA) because the pins on the chips are of open-collector design.This means that a chip can only pull the lines low, otherwise they float up to VDD.In I²C, pulling the line to ground indicates a logical zero while letting it float to VDD is a logical one.As a
channel access method, this allows one node to determine if another is transmitting by sensing that when asserting a logical 1 (letting it float) and sensing if the line is still at a logical 1 (no other node is pulling the line to ground) then it's possible no other node is simultaneously transmitting.However, if a second node pulls the line to zero then the first node can detect that the other is transmitting.
Paul Horowitzand Winfield Hill, " The Art of Electronics2nd Edition", Cambridge University Press, Cambridge, England, 1989 ISBN 0-521-37095-7
* [http://www.seattlerobotics.org/encoder/mar97/basics.html Really basic introduction at Encoder]
Wikimedia Foundation. 2010.
См. также в других словарях:
Pull up — can mean:* Pull up (exercise), an upper body compound pull exercise * Pull up resistor, a technique in digital electronics * Pull up transistor, a transistor used in analog electronics * Pull Up refactoring, a technique used in object oriented… … Wikipedia
Pull up — Résistance de tirage Circuit avec résistance de tirage Une résistance de tirage (ou pull up resistor) est une résistance dans un circuit électronique, située entre la source d alimentation et une ligne, et qui amène délibérément à l état positif… … Wikipédia en Français
Push–pull converter — A push–pull converter is a type of DC to DC converter that uses a transformer to change the voltage of a DC power supply. The transformer s ratio is arbitrary but fixed; however, in many circuit implementations the duty cycle of the switching… … Wikipedia
Open drain — is one of the many different electrical input/output standards in digital designs today. Definition The word drain in the term Open drain refers to the drain terminal of a MOSFET transistor. (The analogous term for BJT devices is open collector.) … Wikipedia
Diode logic — (DL) or diode resistor logic constructs Boolean logic gates from diodes acting as electrically operated switches. While diode logic has the advantage of simplicity, the lack of an amplifying stage in each gate limits its application. Not all… … Wikipedia
Open collector — A simple schematic of an open collector of an integrated circuit (IC). An open collector is a common type of output found on many integrated circuits (IC). Instead of outputting a signal of a specific voltage or current, the output signal is… … Wikipedia
Wired logic connection — A wired logic connection is a logic gate that implements boolean algebra (logic) using only diodes and resistors. It differs from a digital logic integrated circuit which utilizes transistor transistor logic (TTL). A wired logic connection can… … Wikipedia
Virtual ground — In the theory of electrical networks, a virtual ground ( or virtual earth) is a node of the circuit that is maintained at a steady reference potential, without being connected directly to the reference potential. In some cases the reference… … Wikipedia
Comparator — For other uses, see Comparator (disambiguation). In electronics, a comparator is a device that compares two voltages or currents and switches its output to indicate which is larger. They are commonly used in devices such as Analog to digital… … Wikipedia
Interrupt — This article is about computer interrupts. For the study of the effect of disruptions on job performance, see Interruption science. In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in… … Wikipedia