Pull up resistance:
1. When TTL circuit drives COMS circuit, if the output high level of TTL circuit is lower than the lowest high level of COMS circuit (generally 3.5V), pull-up resistor should be connected at the output end of TTL to improve the output high level.
2. OC gate circuit must be added with a pull resistor to use.
3. In order to increase the driving ability of the output pin, some SCM pins often use pull-up resistance.
4. In COMS chip, in order to prevent static damage, unused pins can not be suspended. Generally, pull-up resistance is used to reduce input impedance and provide load relief path.
5. In order to improve the noise tolerance and anti-interference ability of the input signal of the chip, the pin of the chip is added with a pull resistor to improve the output level.
6. Improve the anti electromagnetic interference ability of the bus. When the pin is suspended, it is easier to accept external electromagnetic interference.
7. In the long line transmission, the resistance mismatch is easy to cause the reflected wave interference, and the pull-down resistance is the resistance matching, which can effectively suppress the reflected wave interference.
The selection principles of pull-up resistance include:
1. It should be large enough to save power consumption and current injection capability; High resistance, low current.
2. It should be small enough to ensure enough driving current; Low resistance, high current. 3. For high-speed circuits, excessive pull-up resistance may flatten the edge. Comprehensive consideration
The above three points are usually selected between 1K and 10K. The same is true for pull-down resistors
The selection of pull-up resistance and pull-down resistance should be combined with the characteristics of switch and the input characteristics of lower circuit
1. Balance of driving ability and power consumption. Take the pull-up resistor as an 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. Driving requirements of lower level circuit. Similarly, take the pull-up resistor as an example. When the output is high, the switch is disconnected, and the pull-up resistor should be properly selected to provide sufficient current to the lower circuit.
3. Setting of high and low level. The threshold level of high and low level of different circuits will be different. The resistance should be set properly to ensure the correct output level. Take the pull-up resistor as an example, when the output is low, the switch tube is on, and the partial voltage value of pull-up resistor and switch tube on resistor should be below the zero level threshold.
4. Frequency characteristics. Take the pull-up resistor 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 circuits will form RC delay. The larger the resistance is, the greater the delay is. The setting of pull-up resistance should consider the needs of the circuit.
The setting principle of pull-down resistance is the same as that of pull-up resistance.
When the output high level of OC gate is a high resistance state, the pull-up current should be provided by the pull-up resistance. Set the input port not more than 100ua, the output port driving current 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 door limit).
When selecting pull-up resistor:
500uA x 8.4k = 4.2, that is to say, when the value is greater than 8.4k, the output terminal can be pulled down to below 0.8V, which is the minimum resistance value, no matter how small it is, it can not be pulled down. If the driving current of the output port is large, the resistance can be reduced to ensure that the pull-down voltage can be lower than 0.8V.
When the output level is high, the leakage current of the tube is ignored, and the two input ports need 200uA
200uA x15k = 3V, that is to say, the voltage drop of pull-up resistance is 3V, and the output port can reach 2V, which is the maximum resistance, no matter how large it is, it can not reach 2V. 10K is available. Refer to 74hc series for COMS door
In the design, the leakage current of the tube can not be ignored, and the actual current of the IO port is also different under different levels. The above is just the principle. In a word, it can be summarized as follows: when the output level is high, the back input port should be fed, and when the output level is low, the output port should not be fed (otherwise, the excess current will be fed to the cascade input port, If it is higher than the limit value of the low level gate, it is not reliable.)
In the digital circuit, the input pins that are not used should be connected to the fixed level, and connected to the high level or ground through 1K resistance.
1. Resistance action:
The purpose of power connection group is to prevent the input end from hanging
Reduce the interference of external current to the chip
The protection diode in the protection CMOS, the general current is not more than 10mA
Pull up and pull down, current limiting
1. Change the level of the potential, commonly used in TTL-CMOS matching
2. There is a definite state when the pin is suspended
3. Increase the driving ability of high level output.
4. Supply 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, it needs to add a pull resistance.
If there is a pull-up resistor, the default value of its port is high level. If you want to control it, you must use low level to control it. For example, the collector of a tristate gate transistor or the positive pole of a diode is used to control the current of the pull-up resistor and pull it down to low level. conversely,
|Especially in the interface circuit, in order to get a certain level, this method is generally used to ensure the correct circuit state, so as to avoid accidents. For example, in the motor control, the upper and lower arms of the inverter bridge can not be directly connected. If they are all driven by the same single chip microcomputer, the initial state must be set to prevent direct connection!
2. Definition:
Pull up is the uncertain signal through a resistor embedded in the high level! At the same time, the resistor has the function of limiting current! Pull down the same way!
Pull up is to inject current into the device, pull down is to output current
The difference between weak and strong is just the resistance value of pull-up resistor
|For non collector (or drain) open circuit output circuit (such as ordinary gate circuit), the ability to increase current and voltage is limited. The function of pull-up resistor is to output current channel for collector open circuit output circuit.
3. Why use pull resistors:
If the IC itself has no internal resistor, another resistor must be connected to the outside of the IC in order to keep the single key in the non triggered state or return to the original state after triggering.
Digital circuit has three states: high level, low level, and high resistance state. In some applications, high resistance state is not desired. You can make it in a stable state by pulling up resistance or pulling down resistance, depending on the design requirements!
The output of I / O port is similar to that of a triode C. when C is connected to the power supply through a resistor, the resistor becomes a pull-up resistor. That is to say, if the port is normal, it is high level, When C is connected to ground through a resistance, the resistance is called pull-down resistance, which makes the port low
