PCB News - Pull up and pull down resistors in electronic circuits

Pull up resistors and pull down resistors are two common types of resistors used in electronic circuits, and they are responsible for pulling a signal up or down to a specific level value, respectively. In this case, a pull-up resistor refers to pulling a signal up to a supply voltage value, while a pull-down resistor refers to pulling a signal down to a ground voltage value.

Pull up resistor:

1.When the TTL circuit drives the COMS circuit, if the output high level of the TTL circuit is lower than the lowest high level of the COMS circuit (usually 3.5V), then it is necessary to connect a pull-up resistor at the output of the TTL to increase the value of the output high level.

2.OC gate circuits must be added with pull-up resistors before they can be used.

3.In order to increase the driving ability of the output pins, some microcontroller pins also often use pull-up resistors.

4.In the COMS chip, in order to prevent damage caused by electrostatic discharge, the pins not used can not be suspended, generally connected to the pull-up resistor generated to reduce the input impedance, to provide a leakage path. At the same time, the pin suspension is more susceptible to external electromagnetic interference (MOS devices for high input impedance, very easy to introduce external interference).

5.Chip pin plus pull resistance to improve the output level, thereby increasing the chip input signal noise tolerance to enhance the anti-jamming ability.

6.Improve the anti-electromagnetic interference ability of the bus. Pin suspension is easier to receive external electromagnetic interference.

7.Long line transmission resistance mismatch is easy to cause reflection wave interference, coupled with the pull-down resistor is a resistance match, effective inhibition of reflection wave interference. 

Pull-up resistor resistance selection principles.

1.From saving poer consumption and the chip's ability to fill the current should be large enough to consider: large resistance, small current. 

2.From ensuring sufficient drive current should be small enough to consider: small resistance, high current. 

3.For high-speed circuits, too large a pull-up resistor may flatten the edge.

Considering the above three points, usually between 1k and 10k selection. Similar reasoning for pull-down resistors.

Pull-up resistor and pull-down resistor selection should be combined with the characteristics of the switching tube and the input characteristics of the lower stage circuit to set, there are several factors:

1.The balance between driving capability and power consumption.

2.The driving requirements of the lower circuit.

3.High and low level setting.

4.Frequency characteristics.

Applications in On-Board Circuits

Pull-up resistors and pull down resistors have different applications in on-board circuits. In general digital circuits, it is usually necessary to fix an input signal at a high or low level. In this case, pull-up resistors or pull-down resistors can be used to achieve this. Specifically, the following two cases are performed:

1.When the signal needs to be held high, a pull-up resistor can be used to connect the signal to the power supply. In this case, if the input signal is unstable, the pull-up resistor can counteract the interference generated by the electronics to a certain extent, thus ensuring that the signal is read correctly.

2.When the signal needs to be kept low,a pull-down resistor can be used to connect the signal to ground.In this case, if the input signal is unstable, the pull-down resistor can offset the interference generated by the electronics to a certain extent, thus ensuring that the signal is read correctly. In addition, the pull-down resistor prevents one input from floating up and causing circuit interference or miscommunication.

Applications in real circuits

Pull-up and pull-down resistors have many other applications. For example, in microcontrollers, pull-up resistors or pull-down resistors can be used to implement software interrupts or external interrupts to realize responses to external events. In addition, in digital circuits, pull-up resistors and pull-down resistors can be used to control the state of analog switching devices, enabling interconversion between analog and digital circuits.

Why Use Pull-Up Resistors

Generally, if the IC itself does not have a built-in resistor when used as a single-key trigger, another resistor must be connected outside the IC in order to keep the single key in a non-triggered state or to return to the original state after triggering. 

Digital circuits have three states: high level, low level, and high resistance state, some applications do not want to appear in the high resistance state, you can pull-up resistors or pull-down resistors to make the way in a stable state, depending on the design requirements! 

Generally speaking, the I/O port, some can be set, some can not be set, some are built-in, some need external, the output of the I/O port is similar to a transistor C, when the C connected through a resistor and the power supply is connected together, the resistor becomes the upper C pull-down resistor, that is to say, if the port is normally a high level, the C is connected through a resistor and the ground is called pull-down resistor, so that the port is not a high level. is called a pull-down resistor, making that port normally low.

Pull-up resistors are used to provide current when the bus drive capacity is insufficient. The general term is pull current, and pull-down resistors are used to absorb current, also known as current sinking. These two types of resistors play a key role in electronic circuits to ensure signal stability and enhance circuit performance and reliability. Reasonable selection to help optimize the circuit design.