Common sense of electronic components: pull-up resistor and pull-down resistance
Time:2023-04-20
Views:1075
Pull-up resistor:
1. When the TTL circuit drives the COMS circuit, if the high level output by the TTL circuit is lower than the minimum high level of the COMS circuit (generally 3.5V), then it is necessary to connect a pull-up resistor at the output terminal of the TTL to increase the value of the high level output.
2. The OC gate circuit must be equipped with a pull-up resistor before it can be used.
3. In order to increase the driving ability of the output pin, pull-up resistor is often used on the pin of some SCM.
4. On COMS chip, in order to prevent damage caused by static electricity, unused pins cannot be suspended. Generally, pull-up resistor is connected to reduce input impedance and provide a discharge path.
5. The pins of the chip are added with a pull-up resistor to improve the output level, thereby improving the noise tolerance of the chip input signal and enhancing the anti-interference ability.
6. Improve the anti electromagnetic interference capability of the bus. Suspended pins are more susceptible to external electromagnetic interference.
7. Mismatching resistors in long-distance transmission can easily cause reflected wave interference, and coupled with pull-down resistors, resistance matching effectively suppresses reflected wave interference.
The selection principles of pull-up resistor value include:
1. Considering the power consumption savings and the current injection capacity of the chip, it should be sufficiently large; High resistance, low current.
2. Considering ensuring sufficient driving current, it should be small enough; Low resistance, high current.
3. For high-speed circuits, excessive pull-up resistor may flatten the edges. Comprehensive consideration
The above three points are usually selected between 1k and 10k. The same applies to pull-down resistors
The selection of pull-up resistor and pull-down resistance should be set in combination with the switch characteristics and the input characteristics of the lower circuit, and the following factors should be considered:
1. Balance between driving capability and power consumption. For example, generally speaking, the smaller the pull-up resistor is, the stronger the driving ability is, but the greater the power consumption is. The design should pay attention to the balance between the two.
2. Driver requirements for lower level circuits. Similarly, for example, when the switch tube is disconnected when the output voltage is high, the pull-up resistor should be properly selected to provide sufficient current to the lower circuit.
3. Setting of high and low levels. The threshold levels of high and low levels in different circuits may vary, and the resistance should be appropriately set to ensure that the correct level can be output. For example, when the output voltage of the pull up resistance is low, the switch is on, and the voltage division value of the pull-up resistor and the switch on resistance should be below the zero level threshold.
4. Frequency characteristics. Taking the pull-up resistance as an example, the capacitance between the pull-up resistor and the drain source stage of the switch tube and the input capacitance between the lower level circuit will form RC delay. The greater the resistance, the greater the delay. The setting of pull-up resistor shall consider the requirements of the circuit in this respect.
The setting principle of pull-down resistance is the same as that of pull-up resistor.
The output high level of the OC gate is a high resistance state, and its pull-up current should be provided by the pull-up resistor. It is set that each input port is not more than 100uA, the output port drive current is about 500uA, the standard working voltage is 5V, and the high and low level threshold of the input port is 0.8V (lower than this value is low level); 2V (high level gate limit).
When selecting pull-up resistor:
When 500uAx8.4K=4.2, the output terminal can be pulled down to below 0.8V when it is greater than 8.4K. This is the minimum resistance value, and it cannot be pulled down even if it is too small.
If the driving current at the output port is large, the resistance value can be reduced to ensure that it can be pulled down below 0.8V.
When outputting high voltage, ignoring the leakage current of the tube, the two input ports need to be 200uA
200uAx15K=3V, that is, the voltage drop of the pull-up resistor is 3V, and the output port can reach 2V. This resistance is the maximum resistance, and it will not reach 2V if it is larger. Choosing 10K is available. The COMS door can refer to the 74HC series
When designing, the leakage current of the tube cannot be ignored, and the actual current of the IO port varies at different levels. The above is just the principle. In one sentence, it can be summarized as follows: when outputting high voltage, the input port should be fully fed, and when outputting low voltage, the output port should not be fully fed (otherwise, excess current is fed to the cascaded input port, which is unreliable if it exceeds the low voltage threshold)
In digital circuits, unused input pins must be connected to a fixed level, and connected to a high level or ground through a 1k resistor.
1. Resistance effect:
The power connection group is designed to prevent the input terminal from being suspended
Reduce the interference caused by external current on the chip
Protect the protective diode within the CMOS, with a general current not exceeding 10mA
Pull up and pull down, current limiting
1. Change the potential of the level, commonly used in TTL-CMOS matching
2. There is a certain state when the pin is suspended
3. Increase the driving ability during high-level output.
4. Provide current for OC gate
It depends on what device is driven by the output port. If the device needs high voltage and the output voltage of the output port is not enough, pull-up resistor needs to be added.
If there is a pull-up resistor, the default value of its port is high. If you want to control it, you must use a low level to control the collector of the triode in the tristate gate circuit, or the diode positive pole to control the current of the pull-up resistor to be pulled down to a low level. conversely,
Especially used in interface circuits, in order to obtain a certain level, this method is generally used to ensure the correct circuit state and avoid accidents. For example, in motor control, the upper and lower bridge arms of the inverter bridge cannot be directly connected. If they are all driven by the same microcontroller, the initial state must be set to prevent direct connection!
2. Definition:
Pulling up means embedding an uncertain signal at a high level through a resistor! The resistor also has a current limiting effect! Dropdown is the same!
Pulling up injects current into the device, while pulling down outputs current
The difference between weak resistance and strong resistance is only the difference of pull-up resistor. There is no strict distinction between them
For non collector (or drain) open circuit output circuit (such as ordinary gate circuit), the ability to raise current and voltage is limited. The function of pull-up resistor is to output current channel for open collector output circuit.
3. Why use a pull resistor:
When used as a single key trigger, if the IC itself does not have an internal resistor, in order to maintain the single key in a state that is not triggered or to return to its original state after triggering, an additional resistor must be connected externally to the IC.
There are three states of digital circuit: high level, low level, and high resistance state. Some applications do not want high resistance state, but can be stabilized by pull-up resistor or pulling down resistance, depending on the design requirements!
Generally speaking, there are I/O ports that can be set, some cannot be set, some are built-in, and some require external connection. The output of the I/O port is similar to that of a transistor C. When C is connected to the power supply through a resistor, the resistor becomes the pull-up resistor. In other words, if the port is at a high level when it is normal, and C is connected to the ground through a resistor, the resistor is called the pull-down resistor, What is the function of keeping this port at a low level
For example, when a port connected with a pull-up resistor is set to the input as state, its normal state is high level, which is used to detect low level inputs.
The pull-up resistor is used to provide current when the bus drive capability is insufficient. The general saying is to pull current, while a pull-down resistor is used to absorb current.
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