The main function of the pull-up resistor in the circuit is to limit the current. In some designs, pull-up and pull-down resistors are often used, but the designers of the power supply often don’t know much about these two resistors. Because of this, when the circuit has problems caused by pull-up and pull-down resistors, the designer will not find a corresponding solution. In this article, Xiaobian will share some basic knowledge and experience about pull-up resistors, hoping to help everyone.
Pull-up resistor experience summary
1. When the TTL circuit drives the COMS circuit, if the high level of the TTL circuit output is lower than the lowest level of the COMS circuit (generally 3.5V), then the pull-up resistor needs to be connected at the output end of the TTL to improve Output a high value.
2. The OC gate circuit must be connected with a pull-up resistor before it can be used.
3, in order to increase the drive capability of the output pin, some of the microcontroller pins often use pull-up resistors.
4. On the COMS chip, in order to prevent damage caused by static electricity, the unused pins cannot be suspended. Generally, the pull-up resistor is connected to reduce the input impedance and provide a discharge path.
5. The pin of the chip is added with a pull-up resistor to increase the output level, thereby improving the noise margin of the input signal of the chip and enhancing the anti-interference ability.
6. Improve the anti-electromagnetic interference capability of the bus. When the pins are suspended, it is easier to accept external electromagnetic interference.
7. The resistance mismatch in long-line transmission is easy to cause reflected wave interference, and the pull-down resistor is resistance matching, which effectively suppresses reflected wave interference.
Principle of selection of pull-up resistor values
From the perspective of the ability to sink current and save energy, it should have the characteristics of large resistance and small current. It should be large enough to save power and the current sinking capacity of the chip; the resistance is large and the current is small. From the perspective of ensuring sufficient drive current, it should be small enough; the resistance is small and the current is large. For high speed circuits, excessive pull-up resistors may flatten the edges.
Between 10K and 1K, it applies to the above three points. There is a similar reason for the pull-down resistor. The selection of the pull-up resistor and the pull-down resistor should be set in combination with the characteristics of the switching transistor and the input characteristics of the lower-level circuit. The following factors should be considered:
1. Balance between drive capability and power consumption. The above pull-up resistor is an example. Generally speaking, the smaller the pull-up resistor is, the stronger the driving capability is, but the greater the power consumption, the design should pay attention to the balance between the two.
2. The drive requirements of the lower level circuit. Similarly, for the above pull-up resistor, when the output is high, the switch is turned off, and the pull-up resistor should be properly selected to provide sufficient current to the lower-level circuit.
3. High and low level settings. The threshold levels of the high and low levels of different circuits will be different, and the resistors should be properly set to ensure that the correct level can be output. Take the above pull-up resistor as an example. When the output is low, the switch is turned on, and the pull-up resistor and the on-resistance of the switch should be kept below the zero level threshold.
4. Frequency characteristics. For example, the above pull-up resistor, the capacitance between the pull-up resistor and the drain-source of the switch and the input capacitance between the lower-level circuits form an RC delay. The larger the resistance, the greater the delay. The pull-up resistor should be set to account for the needs of the circuit in this regard.
This article introduces the relevant experience and selection principles of pull-up resistors. Among them, the choice of pull-up resistors is the focus of this article. I hope everyone can use the knowledge points in this article to apply to their own. Design and enrich your knowledge base.
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Pull-up resistor experience summary
1. When the TTL circuit drives the COMS circuit, if the high level of the TTL circuit output is lower than the lowest level of the COMS circuit (generally 3.5V), then the pull-up resistor needs to be connected at the output end of the TTL to improve Output a high value.
2. The OC gate circuit must be connected with a pull-up resistor before it can be used.
3, in order to increase the drive capability of the output pin, some of the microcontroller pins often use pull-up resistors.
4. On the COMS chip, in order to prevent damage caused by static electricity, the unused pins cannot be suspended. Generally, the pull-up resistor is connected to reduce the input impedance and provide a discharge path.
5. The pin of the chip is added with a pull-up resistor to increase the output level, thereby improving the noise margin of the input signal of the chip and enhancing the anti-interference ability.
6. Improve the anti-electromagnetic interference capability of the bus. When the pins are suspended, it is easier to accept external electromagnetic interference.
7. The resistance mismatch in long-line transmission is easy to cause reflected wave interference, and the pull-down resistor is resistance matching, which effectively suppresses reflected wave interference.
Principle of selection of pull-up resistor values
From the perspective of the ability to sink current and save energy, it should have the characteristics of large resistance and small current. It should be large enough to save power and the current sinking capacity of the chip; the resistance is large and the current is small. From the perspective of ensuring sufficient drive current, it should be small enough; the resistance is small and the current is large. For high speed circuits, excessive pull-up resistors may flatten the edges.
Between 10K and 1K, it applies to the above three points. There is a similar reason for the pull-down resistor. The selection of the pull-up resistor and the pull-down resistor should be set in combination with the characteristics of the switching transistor and the input characteristics of the lower-level circuit. The following factors should be considered:
1. Balance between drive capability and power consumption. The above pull-up resistor is an example. Generally speaking, the smaller the pull-up resistor is, the stronger the driving capability is, but the greater the power consumption, the design should pay attention to the balance between the two.
2. The drive requirements of the lower level circuit. Similarly, for the above pull-up resistor, when the output is high, the switch is turned off, and the pull-up resistor should be properly selected to provide sufficient current to the lower-level circuit.
3. High and low level settings. The threshold levels of the high and low levels of different circuits will be different, and the resistors should be properly set to ensure that the correct level can be output. Take the above pull-up resistor as an example. When the output is low, the switch is turned on, and the pull-up resistor and the on-resistance of the switch should be kept below the zero level threshold.
4. Frequency characteristics. For example, the above pull-up resistor, the capacitance between the pull-up resistor and the drain-source of the switch and the input capacitance between the lower-level circuits form an RC delay. The larger the resistance, the greater the delay. The pull-up resistor should be set to account for the needs of the circuit in this regard.
This article introduces the relevant experience and selection principles of pull-up resistors. Among them, the choice of pull-up resistors is the focus of this article. I hope everyone can use the knowledge points in this article to apply to their own. Design and enrich your knowledge base.
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