[STC MCU Learning] Lesson 10: Timers and Counters of MCU

1.10.4.3、TLx、THx

TL0+TH0: 16-bit register of timer 0

TL1+TH1: 16-bit register of timer 1

Suppose our pulse width is 0.5us, and we want to set the timing to 0.5ms and 4.444ms, then the counts are:

1000 = 0x3E8 = High 0x3 Low 0xE8 => TL0 = 0xE8 TH0 = 0x3

8888 = 0x22B8 = High 0x22 Low 0xB8 => TL0 = 0xB8 TH0 = 0x22

Correct a previous mistake, overflow means exceeding the maximum number of bits of the timer!

1.10.5. Timer Programming Practice 1

1.10.5.1 Experimental tasks

The delay function is used to implement the flashing delay. During the delay process, the CPU has to be used here and cannot do other things. This is the disadvantage of the previous

Interrupt + Timing:

The CPU can do the main task (static digital tube displays 0-F), the timer is set to 500ms, and an interrupt is generated when the timer expires. In the interrupt handler isr, the LED is made to flash! !

1.10.5.2 How to program

1. Ideas:

(1) Timing (timer initialization)

Assign a value to TMOD to determine the operating mode of T0.

Calculate the initial value of the timer count and write it into TH0 and TL0

Open the timer interrupt and assign values ​​to ET0 and EA

Set TR0 to start the timer.

(2) What should the main program do?

(3) Interrupt Handler

You can modify it directly in the key interrupt program in the previous lesson! [MCU Learning] Lesson 9: Key

2. Question to think about: How to calculate 0.5s?

     The machine cycle is the time required for the CPU to complete a basic operation. Machine cycle = 1/MCU clock frequency.

Our microcontroller's external crystal oscillator is 12MHz and works in 12T mode, so the internal clock frequency is 1MHz and the machine cycle is 1us. A maximum of 65535 (in 16-bit timer working mode) pulses can be set, which means the maximum timing time is 65535*1us=65535us=65.535ms.

If you need to set a longer time (for example, 500ms), the timer cannot meet this requirement directly. The solution is to add up multiple timings to form a long time.

    Timing method: first use the timer to set 50ms, then *10

    What is the initial value of our timing 50ms?

12T mode, external crystal oscillator 12MHz, internal clock frequency 1MHz, internal machine cycle: 1us

The number of timings is: 50ms/1us=50000

Initial value: Start counting from 65535-50000+1 (because the counter actually overflows when it counts to 65536) = 15536 ​​= 0x3CB0, and the initial value is equal to 0x3CB0

TL0 = 0xB0, TH0 = 0x3C (You can also understand it this way: TH0 = 15536/256, TL0 = 15536%256)

Let’s look at these two pictures together to analyze how we need to program!

MCU works at 12T, no choice

We use timer 0

Working mode configuration

Because it is an internal clock count, it is equivalent to the timer mode, GATE = 0

Because it is timer mode, C/T = 0

Because the 16-bit timer mode is selected, M1 = 0, M0 = 1

Combining the above three points: TMOD = 0x01

Set the timer initial value (note the manual, 16-bit timer mode, TH0 and TL0 need to be reloaded)

TH0 = 0x3C

TL0 = 0xB0

Interrupt Enable

ET0 = 1

EA = 1

Timer/Counter Control Configuration

Enable counting, TR0 = 1

No external interruption involved, no need for IT and IE

IF will automatically change when the count overflows, so don't worry about it.

Timer Interrupt Handler Reference Manual P141

The final procedure is as follows

#include

/**************Code description**************************

This program uses the timer interrupt method to realize the static digital tube display 0-F and LED1 flashes once every 0.5s.

P0--Static digital tube

P1.0 LED1

*************************************************/

sbit led = P1^0; //LED is connected to P1.0

unsigned char CNT = 10; //Count 50ms each time, repeat 10 times

unsigned char  count;

//Segment code table of independent digital tube

unsigned char val[16] = 

{

0xc0, 0xf9, 0xa4, 0xb0, 

0x99, 0x92, 0x82, 0xf8, 

0x80, 0x90, 0x88, 0x83,

0xc6, 0xa1, 0x86, 0x8e

};

/*************Delay function*************/

void delay()

{

unsigned char i = 200,j = 100;

while(i--)

while(j--);

}

/******************************Timer interrupt service routine*************************/

void tm0_isr() interrupt 1 using 1

{

TH0 = 0x3C;

TL0 = 0xB0; //Manual reload

if (count--== 0)

{

// It means that the interruption has been 10 times and 0.5s has expired

led = !led; // LED inverted

count = CNT;

}

}

/***************************Main function************************************/

void main(void)

{

unsigned char i = 0;

/************Timer initialization*************/

TMOD = 0x01;

TH0 = 0x3C;

TL0 = 0xB0;

ET0 = 1;

EA = 1;

TR0 = 1;

led = 1;

count = CNT; 

while (1)

{

//main mission

for (i=0; i<16; i++)

{

P0 = choices[i];

delay();

}

}

}

1.10.6.3, Use of 51 Timer Widget