Pull-up resistor
Encyclopedia
Pull-up resistor
s are used in electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected or high-impedance. They may also be used at the interface between two different types of logic devices, possibly operating at different power supply voltages.
A pull-up resistor weakly "pulls" the voltage of the wire it is connected to towards its voltage source level when the other components on the line are inactive. When all other connections on the line are inactive, they are high-impedance and act like they are disconnected. Since the other components act as though they are disconnected, the circuit acts as though it is disconnected, and the pull-up resistor brings the wire up to the high logic level
. When another component on the line goes active, it will override the high logic level set by the pull-up resistor. The pull-up resistor assures that the wire is at a defined logic level even if no active devices are connected to it.
A pull-down resistor works in the same way but is connected to ground. It holds the logic signal near zero volts when no other active device is connected.
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
TTL
logic devices. Such outputs are used for driving external devices, for a wired-OR function in combinational 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 MOSFET
all 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 microcontrollers intended 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 supply
transients introduced into logic inputs through pull-up resistors, which may force the use of a separate filtered power source for the pull-ups.
Pull-down resistors can be safely used with CMOS
logic gates because the inputs are voltage-controlled. TTL logic 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 CMOS
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.
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 asserting an output, letting the external pull-up resistor pull the line to logic 1 level, and monitoring the line state. If a second node pulls the line to zero, then the first node can detect that the other is transmitting.
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
s are used in electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected or high-impedance. They may also be used at the interface between two different types of logic devices, possibly operating at different power supply voltages.
A pull-up resistor weakly "pulls" the voltage of the wire it is connected to towards its voltage source level when the other components on the line are inactive. When all other connections on the line are inactive, they are high-impedance and act like they are disconnected. Since the other components act as though they are disconnected, the circuit acts as though it is disconnected, and the pull-up resistor brings the wire up to the high logic level
Logic level
In digital circuits, a logic level is one of a finite number of states that a signal can have. Logic levels are usually represented by the voltage difference between the signal and ground , although other standards exist...
. When another component on the line goes active, it will override the high logic level set by the pull-up resistor. The pull-up resistor assures that the wire is at a defined logic level even if no active devices are connected to it.
A pull-down resistor works in the same way but is connected to ground. It holds the logic signal near zero volts when no other active device is connected.
Applications
Pull-up resistors may be used at logic outputs where the logic device cannot source current, such as open-collector
Open collector
An open collector is a common type of output found on many integrated circuits . Instead of outputting a signal of a specific voltage or current, the output signal is applied to the base of an internal NPN transistor whose collector is externalized on a pin of the IC. The emitter of the...
TTL
Transistor-transistor logic
Transistor–transistor logic is a class of digital circuits built from bipolar junction transistors and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors .TTL is notable for being a widespread...
logic devices. Such outputs are used for driving external devices, for a wired-OR function in combinational logic
Combinational logic
In digital circuit theory, combinational logic is a type of digital logic which is implemented by boolean circuits, where the output is a pure function of the present input only. This is in contrast to sequential logic, in which the output depends not only on the present input but also on the...
, 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 MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
all the way on and actuate the relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
.
Pull-up resistors may be discrete devices mounted on the same circuit board as the logic devices. Many microcontrollers intended 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 supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
transients introduced into logic inputs through pull-up resistors, which may force the use of a separate filtered power source for the pull-ups.
Pull-down resistors can be safely used with CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
logic gates because the inputs are voltage-controlled. TTL logic 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
Bipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...
logic families operating at 5 VDC, a typical pull-up resistor value will be 1000–5000 Ω
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
, based on the requirement to provide the required logic level current over the full operating range of temperature and supply voltage. For CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
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.
I²C
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
Channel access method
In telecommunications and computer networks, a channel access method or multiple access method allows several terminals connected to the same multi-point transmission medium to transmit over it and to share its capacity...
, this allows one node to determine if another is transmitting by asserting an output, letting the external pull-up resistor pull the line to logic 1 level, and monitoring the line state. If a second node pulls the line to zero, then the first node can detect that the other is transmitting.