Pull-up resistance:
1. When the TTL circuit is used to drive the COMs circuit, if the high level output by the TTL circuit is lower than the lowest high level (generally 3.5 V) of the COMs circuit, the pulling resistance should be connected at the TTL output end, to increase the output height.
2. Open resistor must be added to the OC door circuit before it can be used.
3. In order to increase the drive capability of the output pin, some single-chip microcomputer pins often use the pull-up resistor.
4. On the COMs chip, in order to prevent damage caused by static electricity, unused pins cannot be left blank. Generally, the pulling resistance is connected to reduce the input impedance and the leakage path is provided.
5. Increase the output level by adding a pull resistor to the Chip Pin, thereby improving the noise tolerance of the chip input signal and enhancing the anti-interference capability.
6. Improve the anti-electromagnetic interference capability of the bus. When the PIN is left blank, it is easier to accept external electromagnetic interference.
7. During long-line transmission, the interference of the reflected wave is easily caused by the mismatch of the Resistance. In addition, the drop-down resistance is used to effectively suppress the interference of the reflected wave.
The selection principles of the pull-up resistance include:
1. Considering Power Consumption Reduction and chip current filling capacity, it should be large enough; the resistance should be large and the current should be small.
2. Make sure that the driving current is sufficient, the resistance is small, and the current is large.
3. for High-Speed Circuits, excessive pull-up resistance may smooth the edge. Comprehensive Consideration
The preceding three points are usually selected from 1 K to 10 K. Similar to the drop-down Resistor
The selection of the pull-up and drop-down resistors should be set based on the characteristics of the switch and the input characteristics of lower-level circuits. The following factors must be taken into account:
1.
Balance between drive capability and power consumption. The above pull resistance is used as an example. Generally, the smaller the pull resistance, the stronger the driving capacity, but the larger the power consumption, the design should pay attention to the balance between the two.
2.
Driver requirements of lower-level circuits. In the same case, 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-level circuit.
3.
Set the level. The threshold levels of different circuits are different. The resistance should be properly set to ensure that the correct levels can be output. The above pulling resistance is used as an example. When the output is low, the switch turns on. The uppulling resistance and the switch turn on resistance values must be under the zero threshold.
4.
Frequency features. The above pulling resistance is used as an example. The input capacitance between the upper pulling resistance and the drain source level of the switch will form an RC delay. The larger the resistance, the larger the delay. The requirements of the circuit should be considered for the setting of the pull-up resistor.
The Setting principle of the drop-down resistor is the same as that of the top-down resistor.
The high-voltage output of the OC gate is usually a high-impedance mode. The pull-up current must be provided by the pull-up resistor, so that each port at the input end cannot exceed UA, and the output port drive current is about UA, the standard operating voltage is 5 V, the input port's high and low level threshold is 0.8 V (below this value is low), and 2 V (high level threshold ).
When selecting the pull-up Resistor:
500ua x 8.4 K =
4.2 means that the output end can be pulled down to 0.8v when the value is greater than kb. This is the minimum resistance value and cannot be pulled down when the value is small. If the output port drive current is large, the resistance value can be reduced to ensure that the drop-down time can be lower than 0.8v.
When the output power is high, the leakage current of the pipe is ignored, and the two input ports need 200ua
200ua
X15k = 3 V: the pressure drop of the pull-up resistance is 3 V, and the output port can reach 2 v. the maximum resistance value is 2 v. Select 10 K. For more information about coms door, see 74hc series.
During the design, the leakage current of the pipe cannot be ignored, and the actual current of the IO port is different at different levels. The above is just the principle. In a word, the output high-power supply usually needs to feed the rear input port, do not feed the output port to the output low level (otherwise, the excess current is fed to the cascade input port, which is unreliable if it is higher than the low level threshold)
The input feet that are not used in the digital circuit must be connected to a fixed level and connected to a high level or ground through a 1 K resistor.
1. Resistance:
L The power-on group is used to prevent the input from being suspended.
L weaken the interference of external current on the chip
L protection of protection diodes in CMOS, the general current is not greater than 10mA
L pull up, pull down, and throttling
L 1. The potential of changing level, often used in TTL-CMOS matching
2. Fixed status when pin is suspended
3. added the driving capability for High-level output.
4. Provide current for 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 pulling resistance is required.
L
If there is an upper-tension resistor, its port is at a high level by default. You must use a low level to control the collector of a three-state gate circuit transistor, or the diode cathode is used to control the current of the pull-up resistor to a low level. Conversely,
L
This method is especially used in the interface circuit to obtain the determined level, so as to ensure the correct circuit status, so as to avoid accidents, for example, in the motor control, the lower axle arm of the inverter bridge cannot be directly connected. If they are driven by the same single-chip microcomputer, the initial state must be set. prevent passthrough!
2. Definition:
L
Pulling up is to embed uncertain signals through a resistor at a high level! The resistor can throttling at the same time! The same is true for the drop-down!
L The pull-up mode injects current into the device, and the drop-down mode is the output current.
L weak and strong only have different resistance values for the pull-up resistor. There is no strict distinction between them.
L
The ability of non-collector (or drain) open-circuit output circuits (such as common door circuits) to increase current and voltage is limited, the pull-up resistance function is mainly used for the output current channel of open collector output circuit.
3. Why do we need to use the pull resistance:
L
Generally, when a single key is used for trigger, if the IC itself does not have an internal resistance, in order to make the single key remain in the not triggered status or return to the original status after the trigger, it must be connected to another Resistance Outside the IC.
L
There are three States for a digital circuit: high level, low level, and high resistance. In some applications, high resistance is not desired. In this case, you can use the pull-up or drop-down resistor to make it stable, depends on the design requirements!
L
Generally, I/O ports can be set, some cannot be set, some are built-in, and some need external connections. The output of I/O ports is similar to that of a transistor C, when C is connected to the power supply through a resistor, the resistor becomes the upper C resistor, that is, if the port is normal, it is high, when C is connected to the ground through a resistor, the resistor is called a drop-down resistor, so that the port is usually at a low level. Does it work:
For example, when a port connected with an active/standby resistor is set to an input state, its normal state is high, which is used to detect low-level input.
L
The pull-up resistor is used to provide current when the bus drive capability is insufficient. Generally speaking, it is the pulling current. The drop-down resistor is used to absorb the current, which is also called the filling current.
