What is the use of entering the critical point? How to realize STM32 single chip microcomputer?

Hello everyone , I am boundless .

Once I was broadcasting live to internal students , A few friends asked me , Why enter the critical , What's the use of criticality ？

At that time, I didn't expect a good explanation , Today, let me give you a few examples to answer .

If any boss thinks I'm wrong, please talk privately and I'll correct it , Don't secretly spray me in the comment area , Can't I change it yet .

Enter critical and exit critical , My first contact was in ucos System , At that time, I also felt that this professional term was awesome and complicated .

When I finish talking , You'll find it's actually quite low.

Entering the critical point is to shut down the MCU and interrupt , Exit critical It is to resume the interruption of single chip microcomputer , Remember, it's best to recover , Not open , Because before entering the critical point, the total interruption of MCU may not be on .

Then why enter the critical ？ What's the role ？

Let's suppose a scenario ：

Let's look at this piece of code .

Suppose our program runs to 12 That's ok , That is to say a = 0 The location of ,*p The value of must be equal to 2 Right? ？

Suddenly a timer interrupts , Then in the timer interrupt handler , We did p++.

After execution, return to the main program to continue execution , That is to say, I will implement 13 Line code , This is the time *p Guess the value of ？

you 're right , Definitely not 2, It is 3.

Obviously, the condition judgment is 2, Finally, the value becomes 3, such bug It's estimated that it can make you cry .

At this time, entering the critical point is very stable , It can be changed to the following code .

STM32 Unlike 51 SCM is so direct EA=0 You can turn off the total interrupt , But through __get_PRIMASK and __set_PRIMASK To do it , You can also use it __disable_irq and __enable_irq.

Specifically, you can search in the project , Read notes .

Every single chip microcomputer turns off the total interrupt in a different way , So there is also a difference between entering the critical code and exiting the critical code .

You just need to remember The essence is to shut down and interrupt , After executing the procedure , Just restore the total interruption .

Don't worry too much about how to switch when the single chip microcomputer is always interrupted , You can find it on the Internet .

Then continue to talk about our above program , A lot of people would say , I avoid this problem when writing programs , Not in the timer right p Just operate the pointer ？

If you are doing some big projects , Almost impossible to avoid , For example, our queue algorithm .

We will receive the data in the serial port interrupt , Put the data in the queue , Then analyze in the main function .

After using the queue , The stability of data transmission can be greatly improved , Even if the data transmission is fast , There is a large amount of data, and there will be no packet loss .

Let's take a look at the listing function of the queue algorithm .

such , If you don't use a pointer , It is almost impossible to achieve , Even the flexibility and portability are not high .

Just imagine , If this kind of pointer is frequently operated in the main function and interrupt , Without criticality, the program is easy to crash due to abnormal pointer pointing or program logic problems caused by data disorder .

Of course , In addition to protecting global variables 、 Array 、 Structure and other data are not bad, except , It can also be hardware data , such as IO、SCI、SPI、flash.

Of course ,RTOS Maybe it's more complicated , I won't explain it here , Just understand the essence .