MCU IO explanation (pull-up pull-down quasi bidirectional input / output push-pull open drain)

General SCM will provide pull-up and pull-down functions :

Pull up ： The signals of uncertainty , Fixed at high level , Power Supply The resistance to the device pin is called pull-up resistance , The function is to use this pin for high level at ordinary times , Pull up is to inject current into the device , Instant current

The drop-down : The signals of uncertainty , Fixed to site level , The earth The resistance to the device pin is called pull-down resistance , The function is to make the pin low at ordinary times , Pull down is the output current from the device , I.e. pull current

Input ：

Pull up input : The uncertain signal is embedded at the high level through a resistor

Drop down input : Pull the voltage down , Pull to GND, Pull the uncertain signal to the low level

Enter the float : Susceptible to interference , The detection level is variable

simulation : Traditional input , Digital to analog conversion

Schmidt input ： Prevent circuit interference

The disadvantage of the basic circuit is that jitter will occur when reading the jump edge of the external signal , Schmidt trigger solves the above jitter problem

Three state input ：

Three state circuit can provide three different output values ： Logic “0”, Logic “1” And high resistance state

High level      Low level     High resistance

Output :

The difference between multiplexed push-pull and push-pull output :

Push pull multiplexing Without going through the output data register (ODR). If push-pull output is used , Then the pin level is directly controlled by ODR control , For example, if the serial port is not a multiplexed push-pull interface, it will directly have a ODR Not controlled by usart Peripheral control

Push pull output ： Generally, it is push-pull output , True output high and low

Push pull output features :

The pull-down of this configuration is the same as the quasi two-way port and open drain configuration , It has strong pulling current ability , The difference is , Have a lasting Strong pull up , Whether it has strong high-level or low-level driving ability

Open drain output ：

Less common , If as a logic output , May be I2C, You need to pull up the resistor

Turn off all pull-up transistors , Only the pull-down transistor is driven , The pull-down configuration is the same as that of the quasi two-way port , Therefore, only low level can be output （ Absorbing current ）, And high resistance state . Cannot output high level （ The current is also transmitted ）

Quasi two-way port :

When IO The output is high , Its driving ability is very weak , External load can easily pull it to low level .

When IO The output is low , Its driving ability is very strong , It can absorb considerable current

The quasi two-way port can only effectively read 0, And yes 1 The method of reading non-zero , When reading in, you should first ask io Write on 1, read

True two-way io It can be read directly without any pre operation

When the quasi two-way port is used as input , Through a Schmidt trigger and a non gate , For interference and filtering .

 

When the quasi two-way port is used as input , The button can be grounded , Here's the picture 1, Of course, you can also remove R1 Directly connect the button , When the key is closed , Port is pulled to low level , When the key is released , The port is internally “ Extremely weak upper

PULL ” The transistor is pulled to the high level . When the port is output , It shall not be externally connected to the ground LED Such as graphic control , In this way, the drive capability of the port is very weak ,LED Only a very weak light , If you want to drive LED, Use figure

3 Methods , In this way, when the output of the quasi two-way port is low , Absorbable 20mA Current , Therefore, it can drive LED. chart 4 The method of , however LED When it doesn't glow , The port should absorb and receive a large amount of current .