Pull-up resistor

(i) pull-up resistor:
1, when the TTL circuit drives the COMs circuit, if the TTL circuit output high level is lower than the COMs circuit's lowest high level (generally 3.5V), then the output of the TTL will need to connect the pull-up resistor to improve the output high level value.
2, OC Gate Circuit must be coupled with a pull resistor, in order to use.
3, in order to increase the driving capacity of the output pin, some single-chip microcomputer pin also often use the pull-up resistor.
4, on the COMs chip, in order to prevent damage caused by static electricity, the use of the pin can not be suspended, generally connected to the pull resistor to reduce the input impedance, provide a relief path.
5, the chip pin and pull resistor to improve the output level, so as to improve the chip input signal noise tolerance to enhance anti-jamming ability.
6. Improve the ability of the bus to resist electromagnetic interference. It is easier to accept external electromagnetic interference when the pin is floating.
7, the long-term transmission of resistance mismatch easily caused by reflection wave interference, coupled with the pull-down resistor is resistance matching, effective suppression of reflected wave interference.

(b) The selection principles for the resistance of the pull-up resistor include:
1, from the power consumption and the capacity of the chip sink should be large enough to consider, the resistance is large, the current is small.
2, from ensuring enough drive current considerations should be small enough, small resistance, large current.
3, for high-speed circuit, too large pull-up resistor may be flat edge. Comprehensive consideration
The above three points are usually selected between 1k and 10k. There's a similar reason for the pull-down resistor.

(c) The choice of pull-up resistor and pull-down resistor should be combined with the characteristics of the switch tube and the input characteristics of the lower circuit to set, the main need to consider the following factors:
1. Balance of drive capability and power consumption. The above pull resistance for example, generally, the smaller the pull-up resistance, the higher the driving capacity, but the greater the power consumption, the design should pay attention to the balance between the two.
2. The driving requirements of the lower circuit. As an example of the same tensile resistance, when the output is high, the switch tube is disconnected and the pull-up resistor should be selected to provide sufficient current to the lower circuit.
3. High and low level setting. The threshold levels for different circuits are different, and resistors should be set appropriately to ensure that the correct level is output. For example, when the output is low, the switching tube is on, the pull-up resistor and the on-resistance voltage divider of the switch tube should be guaranteed below the 0 level threshold.
4. Frequency characteristics. As an example of the above pull resistance, the capacitance between the pull-up resistor and the drain source stage of the switching tube and the input capacitance between the lower circuit will form an RC delay, the greater the resistance, the greater the delay. The setting of the pull-up resistor should take into account the needs of the circuit in this regard.

(d) The setting principle of the pull-down resistor is the same as the pull-up resistor.
OC Gate output High level is a high impedance state, its pull-up current to be provided by the pull-up resistor, set the input port is not more than 100uA, set the output drive current of about 500uA, the standard operating voltage is 5V, the input port threshold is 0.8V (below this value is low); 2V (High level threshold).
When the pull-up resistor is selected:
500uA x 8.4k= 4.2 is selected greater than 8.4K output can pull down to 0.8V below, this is the minimum resistance, and then small can not pull down. If the output drive current is larger, the resistance can be reduced to ensure that the pull down can be less than 0.8V.
When the output is high, the leakage current of the pipe is ignored, and the two input ports need 200uA
200uA x15k=3v is the pull-up resistor voltage drop of 3V, output can reach 2V, this resistance is the maximum resistance, and then the large pull less than 2 v. Select 10K is available. COMs Door Reference 74HC series
Design of the leakage current of the pipe can not be ignored, the actual IO port current at different levels are different, the above is just the principle, a word summed up: output high power to feed the back of the input, output low level do not feed the export to support (otherwise the excess current fed to the cascade input, higher than the low threshold is unreliable)

The input pins that are not used in the digital circuit are connected to a fixed level and are connected by a high level or ground through a 1k resistor.
1. Resistance effect:
* The receiving group is to prevent the input terminal from floating
* Reduces interference caused by external current to the chip
* Protection diode in CMOS, general current not less than 10mA
* Pull-up and drop-down, current limit
* Change the level of the potential, commonly used in Ttl-cmos matching

2. There is a definite state when the pin is dangling
3. Increase the drive capability when the high level output.
4. Provide current for OC Gate
* That depends on what the output driver is, if the device requires a high voltage, and the outlet output voltage is not enough, you need to add a pull resistor.
* If there is a pull-up resistor that its port is high on the default value you want to control it must be low level to control such as the Collector of the Tri-State gate transistor, or the diode positive to control the pull-up resistor of the current pulled down to become low level. Instead
* Especially used in the interface circuit, in order to obtain a certain level, the general use of this method, in order to ensure the correct circuit status, in order to avoid accidents, for example, in the motor control, the inverter bridge up and down the bridge arm can not direct, if they are all using the same microcontroller to drive, must set the initial state. Prevent direct

2. Definition:
* Pull-up is to embed the indeterminate signal through a resistor at high level! Resistance at the same time to limit the flow of action! Pull the same!
* Pull-up is the injection current to the device, and the drop-down is the output current
* Weak strong is only the resistance of the tensile resistance of different, there is no strict distinction
* For non-collector (or drain) open-circuit output circuits (such as ordinary gate circuit) the ability to raise current and voltage is limited, the function of the pull-up resistor is mainly for the open-collector output circuit output current channel.

3. Why to use pull resistance:
* Generally for single-key trigger use, if the IC itself does not have an internal resistor, in order to maintain a single key is not triggered by the state or after the trigger to return to the original state, must be outside the IC to connect another resistor.
* Digital Circuit has three states: high-level, low-level, and a high-impedance state, some applications do not want to appear in the state, can be a pull-up resistor or pull-down resistor in a stable state, depending on the design requirements!
* Generally speaking is I/O port, some can be set, some may not be set, some built-in, some need external, I/O port output is similar to a transistor C, when C is connected through a resistor and power supply together, the resistor becomes the upper C pull resistor, that is, if the port is high when normal, c when connected by a resistor and ground, the resistor is called a pull-down resistor, so that the port is usually low-level, action:
For example, when a port with a pull-up resistor is set to a state of loss, his normal level is high, which is used to detect low-level inputs.
* The pull-up resistor is used to provide current when the bus drive capacity is insufficient. The general argument is to pull the current, the pull-down resistor is used to absorb the current, that is, the sink current.

Pull-up resistor