51 MCU - Timer working mode 2

1. Use of working mode 2

Here we will explain the working mode 2 of timer 0, which is to use the "small test tube" method. The initialization of TMOD is "TMOD=0x02;".

We know that the longest overflow time of working mode 2 is only 256*(12/11059200)=0.000277 seconds=277 microseconds.

We want to implement a running light experiment with an interval of 51ms. First, we define a variable cnt to record the number of overflows each time. Then our timing is 51 microseconds (because the maximum timing is only 278 microseconds). When the value recorded by cnt is equal to 1000, it proves that 51ms has passed, so we can execute the running light task again.

We already know that working mode 2 does not need to fill the initial value after the overflow. For example, if our timing time is 51 microseconds, then the calculated "distance overflow scale" is 47 (the actual calculated value is 47.0016, so the timing is 51 microseconds and the error is not large).

So the initial value of TL0 is 256-47=209=0xD1. Then the initial value of TH0 is also 0xD1, which ensures that the new initial value is equal to the value of TH0 after each TL0 overflow. Therefore, timer working mode 2 is also called 8-bit automatic reload mode. Readers need to learn these theoretical knowledge separately for related concepts.

2. Code for working mode 2

#include  

#include //See Chapter 6, Lecture 8 for details

void main()

{

    u8 i;

    u16 cnt;

    LED_Init(); //Initialize LED hardware module 

    TMOD = 0x02; //Set timer 0 to working mode 2

    TH0 = 0xD1;  

    TL0 = 0xD1; //timing 51us

    TR0 = 1; //Start timer 0

    while(1)

    {

        if (TF0 == 1) //This if statement can be entered every 51us

        {

            TF0 = 0; //After timer 0 overflows, clear the interrupt flag

            cnt++; //Record overflow times

        }   

        if(cnt>=1000) //51ms has passed

        {

            cnt=0;   

            P0=~(0x01<            i++;

        }      

        if(i>=8)i=0;

    }

}

If you compare the code of working mode 1 in the previous lecture, you will find that TH0 and TL0 no longer appear in the while loop of working mode 2. After each overflow of TL0, the initial value becomes the value of TH0 at the beginning, so the timing time has been determined before the while loop.

You can try changing the 1000 in "if(cnt>1000)" to 10000, which will be a running light with an interval of 510ms.

In the future, the timing time of the questions we do will generally be at least 1 millisecond, so there will be many opportunities to use working mode 1, while working mode 2 will be rarely used in our previous tutorials. We will review this knowledge point when we learn about serial communication.