Pull-up resistors and pull-down resistors are commonly used components in digital circuits. Their main function is to determine the voltage level of circuit nodes when there is no driver and to improve circuit stability. The following is a brief introduction to pull-up and pull-down resistors: 12
Pull-up resistors connect circuit nodes to the positive power supply (typically Vcc or the supply voltage) to ensure that the node remains high (logic "1") when there is no external signal input. When an external device generates a logic low level, this signal can pull the signal line low. Pull-up resistors also improve the drive capability of output pins and increase voltage levels, ensuring circuit stability and reliability.
Pull-down resistors connect circuit nodes to ground (usually GND) to ensure that the node remains low (logic "0") when there is no external signal input. When an external device generates a logic high signal, this signal can pull the signal line high. Pull-down resistors also help improve the stability and reliability of the circuit.
In addition, pull-up and pull-down resistors are also used to maintain the stable state of the input pins, preventing unpredictable input results or output oscillations. In specific applications, such as level shifting circuits implemented with transistors, pull-up and pull-down resistors are also essential components.
Resistors in circuits limit current, and pull-up and pull-down resistors are frequently mentioned and used. Every system design utilizes numerous pull-up and pull-down resistors; these are collectively referred to as "pull-up resistors." Their most basic function is to clamp an uncertain signal line to a high (pull-up) or low (pull-down) level through a resistor. Regardless of the specific application, this basic function remains the same; only the required resistor value differs in different applications. Let's learn more about them:
1. Pull-up resistor
Principle: On the wire connected to the pull-up resistor, if the external component is not enabled, the pull-up resistor "weakly" pulls the input voltage signal "high". When the external component is not connected, it "appears" to be a high impedance to the input terminal. At this time, the voltage at the input port can be pulled high through the pull-up resistor. If the external component is enabled, it will cancel the high level set by the pull-up resistor. In this way, the pull-up resistor can make the pin maintain a defined logic level even when no external component is connected.
2. Pull-down resistor
Concept: An uncertain signal is connected to GND through a resistor and fixed at a low level.
3. Main functions
The main function of a pull-down resistor, together with an up resistor, is to provide a fixed voltage level to the line (node) when the circuit driver is off.
(1) Improve voltage level:
a) When a TTL circuit drives a CMOS circuit, if the high level output by the TTL circuit is lower than the minimum high level of the CMOS circuit (usually 3.5V), a pull-up resistor needs to be connected to the output of the TTL circuit to increase the value of the output high level.
b) An open-collector (OC) gate circuit must have a pull-up resistor to increase the high-level output value.
(2) Increase the driving capability of the output pins. Pull-up resistors are often used on the pins of some microcontrollers.
(3) Resistance matching to suppress reflected wave interference: Resistance mismatch in long-distance transmission can easily cause reflected wave interference. Adding a pull-down resistor is resistance matching, which effectively suppresses reflected wave interference.
(4) N/A pin anti-static and anti-interference: On CMOS chips, in order to prevent damage caused by static electricity, unused pins cannot be left floating. Generally, pull-up resistors are connected to reduce the input impedance and provide a discharge path. At the same time, floating pins are more susceptible to external electromagnetic interference.
(5) Preset Space State/Default Potential: Pull-up or pull-down resistors are connected to some CMOS inputs to preset the default potential. When these pins are not in use, these inputs are pulled down to 0 or pulled up to 1. On buses such as I2C, the idle state is obtained by the pull-up or pull-down resistors.
(6) Improve the noise margin of the chip's input signal: If the input terminal is in a high-impedance state, or if the high-impedance input terminal is in a floating state, a pull-up or pull-down switch needs to be added to prevent random voltage levels from affecting the circuit operation. Similarly, if the output terminal is in a passive state, a pull-up or pull-down switch needs to be added, such as if the output terminal is only the collector of a transistor. This improves the noise margin of the chip's input signal and enhances its anti-interference capability.
The above explains the functions of pull-up and pull-down resistors. The selection of pull-up and pull-down resistors should be based on the characteristics of the switching transistor and the input characteristics of the downstream circuit; factors to consider include: the balance between driving capability and power consumption, the driving requirements of the downstream circuit, the setting of high and low levels, frequency characteristics, etc.
Resistors in circuits limit current, and pull-up and pull-down resistors are frequently mentioned and used. Every system design utilizes numerous pull-up and pull-down resistors; these are collectively referred to as "pull-up resistors." Their most basic function is to clamp an uncertain signal line to a high (pull-up) or low (pull-down) level through a resistor. Regardless of the specific application, this basic function remains the same; only the required resistor value differs in different applications. Let's learn more about them:
1. Pull-up resistor
(1) Concept: An uncertain signal is connected to the power supply VCC through a resistor and fixed at a high level.
(2) Principle: On the wire connected to the pull-up resistor, if the external component is not enabled, the pull-up resistor will "weakly" pull the input voltage signal "high". When the external component is not connected, the external "appears" to be a high impedance to the input terminal. At this time, the voltage at the input port can be pulled high through the pull-up resistor. If the external component is enabled, it will cancel the high level set by the pull-up resistor. In this way, the pull-up resistor can make the pin maintain a defined logic level even when no external component is connected.
2. Pull-down resistor
Concept: An uncertain signal is connected to GND through a resistor and fixed at a low level.
3. Based on the meanings of pull-up resistors and pull-down resistors, the most common uses are as follows.
(1) Used in OC/OD gates
An OC gate is an Open Collector, meaning the collector is open-circuited, as shown in the diagram below:
An OD gate is an Open Drain gate, as shown in the diagram below.
Therefore, the OC gate applies to transistors, while the OD gate applies to MOSFETs. From the OC and OD circuits, it can be seen that when the input level is H, the output level is L. When the input level is L, the output level is unstable, i.e., in a high-impedance state, and is easily affected by external interference.
Open-collector (OC) and closed-collector (OD) gates do not have the ability to output a high level. In this case, adding a pull-up resistor to the collector or drain, as shown in the diagram below:
When the input is high, the output remains low; when the output is low, the output level is VCC. In this case, the OC and OD gates have the function of outputting high and low levels, and the level is clamped to either VCC or GND.
(2) Used in button circuits
The button circuit works by inverting the signal level when the button is not pressed and when it is pressed. The MCU determines whether the button is pressed by detecting the inverted signal level of the pin. The schematic diagram is as follows:
When the button is not pressed, the MCU's I/O port detects a high level; when the button is pressed, it detects a low level. The pull-up resistor is to ensure that the button is at a fixed high level when it is not pressed.
(3) Used in IIC bus
Pull-up resistors must be added to the IIC bus to ensure that SDA and SCL are both at a high level when idle.
When using the IIC protocol with a voltage level above 3.3V, a 4.7K pull-up resistor is recommended. When the voltage is below 3.3V, a pull-up resistor of approximately 2.2~3K is recommended (empirical value).
(4) Used for floating pins in logic ICs
In digital logic circuits, because internal logic gates are turned on and off simultaneously, SSN noise is relatively large compared to general circuits. Floating pins are more susceptible to electromagnetic interference from both inside the chip and from the outside. In digital circuits, unused input pins should be connected to a fixed level. It is recommended to use a 1k resistor to connect them to a high level or to ground.
(5) Used in terminal matching
Pull-up and pull-down resistors are frequently used in high-speed circuits for termination matching. Impedance discontinuities in transmission lines can cause signal reflections, leading to waveform overshoot, channeling, ringing, and other issues. Therefore, source-end and termination matching is essential in transmission.
Terminal matching generally comes in two types: parallel termination and Thevenin termination.
Parallel termination – A resistor with the same characteristic impedance as the transmission line is connected in parallel to VCC or GND at the termination point. The advantage of parallel termination is that signal energy is eliminated at the load end before being reflected back to the source. The disadvantage is that pull-up and pull-down resistors introduce power consumption.
Thevenin termination—also known as voltage divider termination—uses both pull-up and pull-down resistors connected to the terminal. The advantage is that it reduces the driving capability requirements of the source end; the disadvantage is that both pull-up and pull-down resistors consume power.
Of course, pull-up and pull-down resistors are used in many other situations depending on the requirements of the chip or the design of the circuit. For example, pull-up resistors can be used in level conversion circuits to switch between different levels.
