51 MCU - Input Capture

1. Capture high level time

We use the counting function of timer 0 to capture the high level time of the external pin. Timers are called differently in different usages. For example, if we want to know how long a process lasts, the counting method of the timer is called a counter.

The external pin we choose this time is still P1.6, and initially let this pin output a low level.

We have not mentioned before what happens when the third bit GATE in the lower four bits of TMOD is 1. Here is an explanation. When this bit is set to 1, if "TR0=1;" and P3.2 must be high, the count of timer 0 will be triggered (it will not be triggered when P3.2 is low), that is, TL0 will increase by 1 every (12/11059200) seconds, and after adding to 256 and becoming 0, TH0 will increase by 1. When it continues to add to 65535, there will be "TL0=255;" and "TH0=255;", and adding 1 will turn all to 0. Here we review the knowledge we have learned before.

So we connect P1.6 and P3.2. When TR0 is set to 1, the counting function will be turned on as long as P1.6 outputs a high level, and the counting function will be stopped when P1.6 outputs a low level. We then use the three digital tubes on the left to display the number of TH0, and the three on the right to display the number of TL0.

So we're going to do this

P1.6=0;

TR0=1;

P1.6=1; //Start counting

delay_ms(30); //delay for a while

P1.6=0; //Stop counting

Code First use software to test how much time "delay_ms(30);" actually takes.

Therefore, P1.6 maintains a high level time of 41.784ms.

After connecting P1.6 and P3.2 with Dupont wire, please note that our delay time should not exceed 71ms, that is, the duration of high level should not exceed 71ms, which is the same as the timing time cannot be set to 71ms at a time. Because the maximum count can only reach 65535.

2. Code

Download the code and observe the value displayed on the digital tube.

#include  

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

//Please use Dupont wire to connect P1.6 and P3.2

void main()

{     

    LED_Init(); //Initialize LED hardware module

    TMOD=0x09; //lower four bits 1001 

    BEEP=0; //Let P1.6 output low level first

    TR0=1;

    BEEP=1; //Start counting  

    delay_ms(30);

    BEEP=0; //Stop counting  

    LedBuff[0]=LedChar[TL0%10];

    LedBuff[1]=LedChar[(TL0/10)%10];

    LedBuff[2]=LedChar[(TL0/100)%10]; 

    LedBuff[3]=0x7F&LedChar[TH0%10]; //Add decimal point for easy distinction

    LedBuff[4]=LedChar[(TH0/10)%10];

    LedBuff[5]=LedChar[(TH0/100)%10];

    while(1)

    {     

        SEG_Scan(); //Read the values ​​of TH0 and TL0  

    }   

}

The digital tube shows 150.109, so

(150*256+109)*(12/11059200)= 0.0417849s

The captured high-level time is 41.7849ms, which can be said to be quite accurate.

If you have forgotten why it is “150*256”, please go back and review the previous content of this chapter.

The function of the seventh bit GATE of TMOD is the same as that of the third bit GATE, except that it uses timer 1 to count, and the pin that triggers the counting is P3.3. For details, please refer to section 5.2.2 of the document "Teaching You to Learn 51 Microcontrollers Step by Step"