Key microcontroller key jitter

1 //write by:cyt

2 //Time:2017-2-10

3 //Porject Name:key shake_destory

4 #include

5 #define GPIO_KEY P0 ///

6 void Delay1ms(unsigned char n);

7 unsigned char Key_Scan() // GPIO_KEY is a global variable, input here

8 {

9 unsigned char i=0,Key_Value=0;

10 if(GPIO_KEY!=0xff)

11 {

12 Delay1ms(10);

13 if(GPIO_KEY!=0xff)

14 Key_Value = GPIO_KEY;

15 while(GPIO_KEY!=0xff&&i<50)

16 {

17 Delay1ms(10);

18 i++;

19 }

20 }

21 return Key_Value;

twenty two }

Why does the key jitter?

The switch used for common buttons is a mechanical elastic switch. When the mechanical contacts are opened and closed, due to the elastic effect of the mechanical contacts, a button switch will not be immediately and stably connected when closed, nor will it be disconnected all at once when disconnected. Therefore, there will be a series of jitters at the moment of closing and disconnecting. The measure to avoid this phenomenon is to eliminate the jitter of the button.

Jitter Time

The length of the jitter time is determined by the mechanical characteristics of the key, generally 5ms to 10ms. This is a very important time parameter and is used in many occasions. Click to view the picture The length of the key stable closing time is determined by the operator's key action, generally a few tenths of a second to several seconds. Key jitter can cause a key to be misread multiple times. To ensure that the CPU processes a key closure only once, key jitter must be removed. Read the key status when the key is closed stably, and it must be determined that the key is released stably before processing.

Key debounce

To debounce a key, you can use either hardware or software.

Hardware debounce

When the number of keys is small, key jitter can be eliminated using hardware methods.

Two NAND gates form an RS flip-flop.

When the key is not pressed, the output is 0; when the key is pressed, the output is 1. At this time, the mechanical properties of the key are used to make the key disconnected instantly due to elastic jitter (jitter jump B). The key does not return to the original state A, the state of the bistable circuit does not change, the output remains 0, and no jittering waveform is generated. In other words, even if the voltage waveform at point B is jittering, after passing through the bistable circuit, its output is a regular rectangular wave. This can be easily verified by analyzing the working process of the RS trigger.

Software debounce

There are many buttons, and software methods are commonly used to debounce

That is, after detecting that the key is closed, a delay program is executed, with a delay of 5ms to 10ms, so that the leading edge jitter disappears and the key state is detected again. If the closed state level is still maintained, it is confirmed that the key is really pressed. When the key is released, a delay of 5ms to 10ms is also given, and the key processing program can only be transferred after the trailing edge jitter disappears.