GPIO_Mode_AIN： Analog input mode 
GPIO_Mode_IN_FLOATING： Floating input mode 
GPIO_Mode_IPD： Drop down input mode 
GPIO_Mode_IPU： Pull up input mode 
GPIO_Mode_Out_OD： Universal open drain output mode 
GPIO_Mode_Out_PP： Universal push-pull output mode 
GPIO_Mode_AF_OD： Multiplex open drain output mode 
GPIO_Mode_AF_PP： Multiplexed push-pull output mode 

The first four are input modes , The last four are output modes .

1. Input and output

from 51 The people who came here , Unfamiliar with input and output .51 SCM does not need to set input and output modes , Automatic change , No program configuration required , When we write a program to read the level of the pin , In fact, it is used as an input mode , When we write a program to let the pin output level , In fact, it is used as an output mode . in other words , Not at all 51 SCM has no concept of input and output , But you have used it for a long time but haven't found it .

STM32 And 51 The difference is that ： When we want to read the pin level , That is, as input , We must first set the pin to the input function , Before you can read it . When we want to control the high and low levels of pins , That is, as output , We must first set the pin to the output function , You can set .

2. Pull up and pull down

The concept is more abstract , You just need to remember ： When there is no external interference on the pin , Pull up is to set the pin to high level , You use a multimeter to measure the pull-up pin , It's high level ; Pull down is to set the pin to low level , You use a multimeter to measure the pull-down pin , It's low level , That is to say 0V.

3. Push pull and open drain

The concept is more abstract , You just need to remember ： Set to push-pull , It improves the output current capability of the pin . Set to open drain , That is, the input current capability of the pin is improved . Then you have to know , When set to open leakage , If there is no external pull resistance , The default output is low .

4. General and reuse

General purpose is used as ordinary input and output pins . Multiplexing means that this pin can be used as SPI Pin I2C Pins and other functional pins .

5. Introduction

Combined with the above , You can get some simple application settings .

For example, check the key , It should be set as input , If there is no external pull resistance , You need to set the pin as the pull-up input , In this way, if it is detected that the pin is low , Just press the button . If an external pull-up resistor is added , At this time, you can set the pin as the pull-up input , It can also be set as floating input , Set to these two , Due to external pull-up resistance , The default is high , Again , Low level detected , Just press the button .

When you want to control the high and low levels of pins for application , It is necessary to set the pin as the output function . For example, drive a device , Drive a light-emitting diode, etc . If you want to use high-level light-emitting diode , You need to set the pin to push-pull output , To increase the current output capacity , If you set open drain output , The LED must not be on . But if you use low-level light-emitting diode , Then set to open drain and push-pull output can normally light up the LED .