Pull-up resistor:
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.
The selection criteria for the value of the pull-up resistor include:
1. 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.
2. It should be small enough to ensure sufficient driving current; the resistance is small and the current is large.
3. For high-speed circuits, excessive pull-up resistors may flatten the edges. Consider the above three points, usually between 1k and 10k. There is a similar reason for the pull-down resistor. The selection of the pull-up resistor and the pull-down resistor should be combined with the characteristics of the switch tube and the input characteristics of the lower-level circuit. The following factors should be considered:
1. The 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 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.
The principle of setting the pull-down resistor is the same as the pull-up resistor.
When the OC gate outputs a high level, it is a high-impedance state. The pull-up current is provided by a pull-up resistor. The input terminal is not more than 100uA per port, and the output current of the output port is about 500uA. The standard operating voltage is 5V. The high and low thresholds are 0.8V (below this value is low); 2V (high threshold).
When the pull-up resistor is selected:
500uA x 8.4K= 4.2 When the output is greater than 8.4K, the output can be pulled down to 0.8V or less. This is the minimum resistance value, and it will not be pulled down. If the output current of the output port is large, the resistance value can be reduced, and it can be lower than 0.8V when the pull-down is performed.
When the output is high, the leakage current of the tube is ignored. The two input ports need 200uA.
200uA x15K=3V, that is, the pull-up resistor has a voltage drop of 3V, and the output port can reach 2V. This resistance value is the maximum resistance value, and then it will not reach 2V. Choose 10K available. For the COMS door, the leakage current of the tube can not be ignored when designing the 74HC series. The actual current of the IO port is different at different levels. The above is only the principle. In one sentence, it is summarized as follows: when the output level is high, the input port must be fed. , output low level do not feed the output port (otherwise the excess current is fed to the cascade input port, higher than the low level threshold is not reliable)
Input pins that are not used in digital circuits must be connected to a fixed level and connected to a high level or ground through a 1k resistor.
1. Resistance effect:
l The power pack is used to prevent the input from floating.
l Reduce the interference of external current on the chip
l Protect the protection diode in cmos, the current is generally less than 10mA
l Pull-up and pull-down, current limit
l 1. Change the level potential, commonly used in TTL-CMOS matching
2. There is a certain state when the pin is floating
3. Increase the drive capability when outputting at high level.
4. Supply current to the OC gate
l It depends on what device is driven by the output port. If the device requires a high voltage and the output voltage of the output port is not enough, a pull-up resistor is required.
l If there is a pull-up resistor, its port is at the default value. You must control it to use a low level to control the collector of a tri-state gate transistor, or the diode anode to control the current drawn by the pull-up resistor. Go down to low level. on the contrary,
Especially used 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 cannot be directly connected, if they are All are driven by the same microcontroller, the initial state must be set. Prevent straight through!
2. Definition:
l Pull-up is to clamp an indeterminate signal through a resistor at a high level! The resistor acts as a current limiting device at the same time! Pull down the same reason!
l Pull-up is to inject current into the device, pull-down is output current
l Weak strength is only the resistance of the pull-up resistor is different, there is no strict distinction
l For non-collector (or drain) open-circuit type circuits (such as ordinary gate circuits), the ability to boost current and voltage is limited. The function of the pull-up resistors is mainly to output current channels for open-collector output circuits.
3. Why use a pull resistor:
l Generally, when the single button is used for triggering, if the IC itself does not have an internal resistor, in order to maintain the single button in the untriggered state or return to the original state after the trigger, another resistor must be connected outside the IC.
l Digital circuits have three states: high level, low level, and high impedance. Some applications do not want high-impedance state. They can be stabilized by pull-up or pull-down resistors, depending on design requirements. set!
l Generally speaking, I/O ports, some can be set, some can not be set, some are built-in, some need to be external, the output of I/O port is similar to C of a triode, when C is connected through a resistor and power supply When together, the resistor becomes the upper C-pull resistor, that is, if the port is high when it is normal, and C is connected to it through a resistor and ground, the resistor is called a pull-down resistor, making the port normally Is it low, what is it:
For example, when a port with a pull-up resistor is set to the input state, its normal state is high, which is used to detect the input of the low level.
l Pull-up resistors are used to provide current when the bus drive capability is insufficient. The general saying is that the current is drawn, and the pull-down resistor is used to absorb the current, which is the sink current that your classmates say.
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