Really, this question is still a little slow in responding worth studying, I started learning a board GPIO input pin to the key block circuit is a pull-down resistor on the outside , as shown below: When KEY1 pick V3.3 , thereafter a pull-down resistor connected to it, can be pressed to ensure that the key input is active high, active low input was contrary KEY2,3,4
So the natural question is no need to consider what the drop-down IO ports, most of the pit that is simple to understand that he would attend the drop-down to whether or not connected with pull-down resistors, and did not go deep elegant study this issue, which will lead to himself in generating program when confronted with someone doubts, it's not on the schematic pull-down resistors, why when software configurations to be pulled down, ah, so I began to control different between the two, then more numerous doubts. I will now organize their own understanding of others and their own income share out, and hope to have the people I have the same doubt can be clearly recognized on the drop-down configuration problems on GPIO port, which extended to the following additional circuit design or software development, but also some inspiration.
In fact, object of the pull-up resistor is to ensure that the input terminal is high level when no signal is input. When the signal is input is low level inputs should also be low. If there is no pull-up resistor,the input terminal without outside input case is vacant, its level is unknown can not be guaranteed, the pull-up resistor is to ensure the input level of the terminal when no signal input is high,the same there are pull-down resistor which is to ensure that the level of the input terminal when no signal input is low. I do not know that they have no clear, now to include a reference link their own search bar.
1.https://blog.csdn.net/xuw_xy/article/details/95514042
2.http://bbs.eeworld.com.cn/thread-1070283-1-1.html
Want to simply understand a friend, I ended up writing just need to know the meaning of what you can, and want to get to the bottom of it, then you can look at the link to the content, they can also search for knowledge!