Current sinking, current sinking, sinking current, pull-up resistor, high resistance state analysis

Sink current, pull current output, sink current output

Pull and discharge, active output current, output current from output port;

Irrigation, passive input current, slave output port Inflow;

Suck is the active suction current that flows in from the input port.

The current sinking and sinking current is the current flowing from the off-chip circuit through the pin into the chip; the difference is that the sinking current is active, and the current flowing from the input end of the chip is called sinking current. The sinking current is passive, and the current flowing from the output is called the sinking current; the pulling current is the output current provided by the digital circuit output high level to the load, and the output low level when sinking the current is the external input current to the digital circuit. These are actually the input and output current capabilities.

The current output of the inverter can only output a few milliamps of current for the inverter. It is unreasonable to use this method to drive the diode to emit light (because the normal operating current of the LED is 5~10mA).

Upper and pull-down resistors

1. Definition

1. Pull-up is to clamp an indeterminate signal through a resistor at a high level! “The resistance also acts as a current limiting action”! Pull down the same reason!

2, pull-up is to inject current into the device, pull-down is output current

3, weak strength is only the resistance of the pull-up resistor is different, there is no strict distinction

4 For non-collector (or drain) open-circuit output type circuits (such as ordinary gate circuits), the ability to boost current and voltage is limited. The function of pull-up resistors is mainly to output current channels for open collector output type circuits.

Second, pull resistance

1, generally used as a single button trigger, if the IC itself does not have an internal resistor, in order to maintain the single button in the untriggered state or after the trigger To return to the original state, a resistor must be connected to the outside of the IC.

2, digital circuit has three states: high level, low level, and high resistance state. Some applications do not want high resistance state, and can be stabilized by pull-up resistor or pull-down resistor. Status, depending on design requirements!

3, generally speaking, I /O port, some can be set, some can not be set, some are built-in, some need to be external, I /O port output is similar to a triode C, when C When connected through a resistor and a power supply, the resistor becomes an upper C-pull resistor, that is, if the port is normally high; when C is connected to the ground through a resistor, the resistor is called Pull-down resistor, so that the port is normally low, role: For example: "When a port with a pull-up resistor is set to the input state, his normal state is high, used to detect low level Enter ”.

4, pull-up resistor is used to solve the 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 sink the current, which is what we usually call the sink current

5. The resistor is used to prevent the input from floating

6, weakening the outside. Current interference on the chip

7, protect the protection diode in cmos, the current is generally no more than 10mA

8, increase or decrease the drive current by pull-up or pull-down

9, change the level of potential, commonly used in TTL-CMOS matching

10, when the pin is floating, there is a certain state

11, increase the driving ability when the high level output .

12.Provide current for OC gate

3. Application principle of pull-up resistor

1. When TTL circuit drives COMS circuit, if TTL circuit outputs high level Below the minimum high level of the COMS circuit (typically 3. 5V), it is necessary to connect a pull-up resistor at the output of the TTL to increase the value of the output high level.

2, OC gate circuit "There must be a pull resistor to use".

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 resistors are connected to reduce the input impedance and provide a discharge path.

5, the chip's pin plus pull resistor to increase the output level, thereby improving the noise margin of the chip input signal to enhance 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.

8. The input pins that are not used in the digital circuit should be connected to a fixed level, and connected to a high level or ground through a 1k resistor.

Fourth, pull-up resistor value selection principle

1, from the power saving and chip sinking capacity considerations should be large enough; large resistance, small current.

2, from the consideration of ensuring sufficient drive current should be small enough; small resistance, large current.

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 choice of pull-up resistors and pull-down resistors should be combined with the characteristics of the switching transistor 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.