What is a pull-up resistor? What is a drop-down resistor?

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What is a pull-up resistor? What is a drop-down resistor?

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!
Pull-up refers to the injection of current to the device, and the drop-down refers to the output current. The weak strength is only the difference in the resistance of the pull-up resistor, so there is no strict distinction between them. For non-collector (or drain) open-circuit output circuits (such as common door circuits) provide limited capability to increase current and voltage. The pull-up resistance function is mainly used to output current channels of Open-collector output circuits.

Ii. pull-up and pull-down resistors:

1. Improve the accuracy of the power supply:. 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 must be connected at the TTL output end, to increase the output height. B. A pulling resistor must be added to the OC door circuit to increase the output level.

2. Increase the drive capability of the output pins. Some single-chip microcomputer pins often use the pull-up resistor.

3. N/A pin protection against electrical and dry cables: On the COMs chip, in order to prevent damage caused by static electricity, unused pins cannot be suspended. Generally, the cables are connected to the Pulling Resistance to reduce the input impedance, provides the leakage path. At the same time, pin suspension is easier to accept external electromagnetic interference.

4. Resistance matching to suppress interference from reflected waves: interference from reflected waves is easily caused by resistance mismatch during long-line transmission. In addition, the drop-down resistance is used to effectively suppress interference from reflected waves.
5. Set the default potential to the default potential. when you do not need these pins, these inputs are pulled down to 0 or pulled up to 1. On an I2C bus or other bus, the idle state is obtained by the up/down resistance.

6. Increase the noise tolerance of the input signal of the chip: If the input is in the high-impedance state or the input is in the suspended state, add a pull or drop-down to avoid the impact of the random level on the circuit operation. Similarly, if the output end is passive, you need to add a pull or drop down. For example, the output end is only the collector of a transistor. This improves the noise tolerance of chip input signals and Enhances Anti-interference capabilities.

Iii. selection principles of the pull-up resistance values 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.
The above three points are generally selected from 1 K to 10 K. The drop-down resistance is similar.

Iv. Principles

The uplink resistance is actually the load resistance of the collector output. Whether it is in the application of the switch and analog amplification, the selection of this resistance is not the head. There is not much to say about working in the linear range. Here we discuss the use of transistor as a switch, so we only talk about the switch mode. Find the data of a TTL device and simply look at the last level. The internal load resistance varies depending on the drive capacity and speed requirements. The low-power resistance value is large, and the fast resistance value is small. However, it is difficult for chip manufacturers to meet the needs of applications. There are not many types of chips with the same function. Therefore, simply do not use this load resistance, and users are free to choose external ports, therefore, the OC and OD output chips appear. Because the transistor works in the saturation and cut-off zones for digital applications, the requirement on the load resistance is not high, and the resistance value is small as long as it is not small to damage the last transistor, as long as the output increase time meets the design requirements, you can select one of them to work normally. However, whether a circuit design is excellent or not must be considered. The switch circuit output from the Collector is always on or off to the ground. When the transistor is turned on, the current goes from the load resistance to the ground through the conducting transistor, the current changes from the load resistance to the ground through the load input resistance. If the load resistance is selected as a small point, the power consumption will increase, this should be avoided in battery power supply and system design that requires low power consumption. If the resistance is large, it will lead to the delay of the rising edge of the signal, because the rising edge of the load input capacitor is charged by the passive pull-up resistor, the longer the resistance increases, and the longer the descent edge is discharged through the active transistor, depending on the device itself. Therefore, when selecting the pull-up resistance value, the designer should consider both power consumption and speed based on the actual situation of the system.

5. Explain the input/output pins from the IC (MOS process) perspectives:

1. if the chip Input Pin is left blank on the system board (not connected to any output pin or driver), it is dangerous. at this time, it is very likely that the capacitor charge inside the input pin is accumulated to reach the intermediate level (for example, 1.5 V), so that the PMOS and NMOS tubes of the input buffer are turned on at the same time, in this way, a direct channel is formed between the power supply and the ground to generate a large leakage stream, and the chip may be damaged after a long time. in addition, being in the intermediate level will lead to confusion in the internal circuit's logic (0 or 1. after the pull-up or drop-down resistor is connected, the internal point capacity is charged (Put) to the high (low) level, and the internal buffer only has the NMOS (PMOs) pipe conduction,
The DC path from the power supply to the ground will not be formed. (as for the prevention of damage caused by static electricity, because the Chip Pin Design will generally add a protection circuit, but it is not necessary ).

2. For output pins:
1) normal output pin (push-pull type), generally there is no need to connect to the pull or drop-down resistor.
2) OD or oC (open drain or open collector) type pin. This type of Pin requires an external pull-up resistor to achieve the line and function (multiple outputs can be directly connected at this time. typical applications: the INT (interrupt signal) outputs of multiple chips on the system board are directly connected, and then the LEDs are pulled up, and the int pin of the MCU is input to enable the interrupt alarm function ).
Its working principle is:
In normal operation, the NMOS tube inside the OD pin is closed, which is in a high resistance state for the outside, and the output is in a high level (invalid interrupt state) due to the external pull-up resistance ); when there is a need for interruption, the NMOS tube in the OD pin is connected, because its on-resistance is far less than the upper-resistance, so that the output is in a low level (valid interrupt State ). it is recommended that the upstream and downstream resistor values of the mos circuit be dozens to hundreds of KB. (Note: This answer does not involve the TTL chip, nor does it consider the effects of Impedance Matching and electromagnetic interference when designing high-frequency PCB .)
1. the pull-up or drop-down resistor indicated on the Chip Pin refers to a resistor or equivalent resistor designed inside the Chip Pin. the purpose of designing this resistor is to set this pin to the default state without adding components when the user does not need to use this pin function. instead of suspending the CMOS input. note that if the default value is not what you want, you should directly connect the input to the desired status.
2. If this pin is pulled up, it can be used for "line" or "logic. other chips with open or open collector output. form a negative logic or input. if it is a drop-down, it can form a positive logic "line or", but the external side can only be the CMOS High-level open-circuit chip output, this is because of the high CMOS output, low-level current is given by the drain poles of PMOS and NMOS respectively, which can be p-open or N-open. the high level of TTL is output by the source pole follower, which is not suitable for "line or ".
3. In principle, the drop-down resistor is not recommended to change the level of the driver from TTL to CMOS. It is best to enable the flat conversion circuit. if the power supply on both sides is 5 V, you can directly connect but affect the performance and stability, especially when the CMOS driver TTL. when the logic levels on both sides are different, you must use level conversion. when the power supply voltage is 3 V or below, do not use direct connection or do not use resistance to pull the level.
4. The chip's external resistor is determined by the application, but it is not feasible to use the resistor to pull the level or improve the drive capability in the logic circuit. driver circuit should be added for Driver Improvement. flat conversion circuit should be used to change the level. dedicated chips are available for long-line reception.

 

The pull-up resistor is to clamp uncertain signals at a high level through a resistor, which also throttling. Similarly, the pull-down resistor is used to clamp uncertain signals at a low level. The pull-up resistance refers to the input current of the device, while the drop-down refers to the output current. So when can I use the top and drop-down resistors?
1. When the TTL circuit is used to drive the CMOS circuit, if the high level output by the TTL circuit is lower than the lowest high level (generally 3.5 V) of the CMOS circuit, the pulling resistance must be connected at the TTL output end, to increase the output height.
2. A pulling resistor must be added to the OC door circuit to increase the output level. 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 CMOS chip, in order to prevent damage caused by static electricity, unused pins cannot be left blank. Generally, the cables are connected with a pull resistance to reduce the input impedance and provide a load leakage path.

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.
In addition, 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.
The above three points are generally selected from 1 K to 10 K. The pull-down resistance is similar to the pull-up resistance, see the figure. VCC is equal to 1 as input, And Gnd is equal to 0.

If the short-circuit (Press) resistance of the key is zero, press the key, out = 0. when the key is disconnected, out =? Obviously, when out is suspended, VCC can be output, which can be measured by a gauge,
This VCC is produced by R1 "pull-up". As the name suggests, R1 is the pull-up resistance. The size of the pull-up resistor depends on the requirements of the output load. Generally, the logic circuit has a large Impedance for the high-level output, and the output current is required to be small. The pressure drop on the pull-up resistor can be ignored, of course, the pull-up resistance cannot be too large, otherwise it cannot be ignored.

The actual circuit also has this structure

The R1 is also the pull-up resistor.

The drop-down resistance is rarely used. The principle is the same as above, but the resistance is pulled down to Gnd.

The output structure of the P0 port of the single chip microcomputer is similar to that of some circuits. The field effect tube may be used.

When Q1 and Q2 are turned on separately, 0 and 1 can be output externally. When Q1 and Q2 are not turned on? What should I do if I want to output 1? External pull-up resistor! Why do we need to use the pull resistance:

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. 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! 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. 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, that is, the filling current. Sometimes, when the keyport of the Main Board is repaired, the power supply of the measurement keyport is normal when the load is connected, but it is not easy to use. After the surrounding resistance components are excluded, you may consider changing the I/O chip. It will work well after the change. however, I found in the repair practice that sometimes I can fix it without changing I/O, as long as the pull resistance of 472 is changed to a small value, the keyboard port is also easy to use. for example, change to 102,272,222, but the minimum value cannot be less than 102. if the resistance is less than 102, it seems easy to burn the keyboard. this is true in practice. this experience is a reference for everyone. i/O can only be changed if I cannot change it after I/O.