In electronic logic circuits, the pull-up resistor is the resistor used to ensure the known state of the signal. These are typically used in combination with components such as switches and transistors, which physically disrupt the connection of the next component to ground. The pull-up voltage then ensures a well-defined voltage (ie V CC ) crosses the last during the interrupt.
The open switch is not equivalent to a component with infinite impedance, because in the previous case, the stationary voltage in any loop involved can no longer be determined by Kirchhoff's law. As a result, the voltage on the critical components is like the logic gate in the example to the right) which only in turns involving the open switch is also undefined.
A pull-up resistor effectively forms an additional loop over an important component, ensuring that the voltage is well defined even when the switch is open.
For a pull-up resistor only serves the purpose of this one and does not interfere with the reverse circuits, it is assumed that the critical component has infinite or high impedance, which is guaranteed for example for logic gates made of FET. In this case, when the switch is open, the voltage across the pull-up resistor with low enough impedance disappears into the effect that looks like a wire to VCC. On the other hand, when the switch is closed, the pull-up resistor must have high enough impedance compared to the closed switch to not affect the connection to ground. Together, these two conditions can be used to obtain the appropriate value for the impedance of the pull-up resistor but usually, only the lower limit is obtained assuming that the critical component does have an infinite impedance.
A pull-down resistor works the same way but is connected to ground. It holds logic signals at a low logic level when no other connected devices are connected.
Video Pull-up resistor
Apps
A pull-up resistor can be used when connecting logic gates with inputs. For example, the input signal can be pulled by a resistor, then a jumper switch or rope can be used to connect the input to ground. This can be used for configuration information, to select options or to solve device problems.
Pull-up resistors can be used on logic output where logic devices can not source current such as TTL open-collector logic devices. The output is used to drive an external device, for an OR-cable 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 the input logic level 0 V to run the relay. If the input is left unconnected, the pull-down resistor R1 ensures that the input is pulled down to a low logic. The TTL 7407 device, open collector buffer, only outputs whatever is received as input, but as an open collector device, the output is left unconnected effectively when issuing "1". Pull-up resistor R2 thus pulls output all the way up to 12 V when buffer takes out "1", provides enough voltage to convert MOSFET power all the way and run relays.
Pull-up resistors can be discrete devices mounted on the same circuit board as logic devices. Many microcontrollers intended for embedded control applications have internal pull-up resistors that can be programmed for logic inputs so that minimal external components are required.
Some disadvantages of pull-up resistors are the extra power consumed when the current is pulled through the resistor and the pull-up speed decreases as compared to the current source. Certain logic families are susceptible to transient power supplies being introduced to logic inputs via pull-up resistors, which can force the use of separate filtered resources for pull-ups.
Pull-down resistors can be safely used with CMOS logic gates because their inputs are controlled by voltage. TTL logic inputs left unattached inherently high floating, and require lower pull-down resistors to force low inputs. The standard TTL input on logic "1" is usually operated assuming a 40 ÂμA source current, and a voltage level above 2.4 V, allowing the pull-up resistors to be no more than 50 kohm; whereas TTL input on logic "0" would be expected to sink 1.6 mA at voltages below 0.8 V, requiring pull-down resistors less than 500 ohm. Holding low unused TTL inputs consumes more current. For that reason, pull-up resistors are preferred in TTL circuits.
In a family of bipolar logic operating at 5 VDC, a typical pull-up resistor value would be 1000-5000?, Based on the requirement to provide the required logic levels current over the full operating range of the temperature and the supply voltage. For CMOS and MOS logic, a much higher resistor value can be used, several thousand to one million ohms, because the leakage current is required on small logic inputs.
Maps Pull-up resistor
References
- Paul Horowitz and Winfield Hill, The Art of Electronics , second edition, Cambridge University Press, Cambridge, England, 1989, ISBN: 0-521-37095-7
See also
- Three-state logic
Source of the article : Wikipedia
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