STC timer initial value calculation

One 、 computing method

   Are subject to 6MHz Crystal oscillator , timing 20ms For example ：
1. Method 1：

  The formula ：N=65536-(Fosc/FD/(Freq))
  N: 		 Timer 16 Bit counter 

  Fosc:  	 Crystal oscillator frequency , System frequency 

  FD: 		 frequency division   （Frequency division）, The timing range of the counter can be increased by frequency division .

  Freq : 	 frequency   （Freq=1000000( Microsecond )/TIME( Microsecond ),TIME: Set time ）

  1、 First calculate the frequency corresponding to the time of the timer required for calculation Freq, Such as timing 20ms, Corresponding Freq by （1000000/20000）=50Hz.
  2、 Calculate the value that the SCM timer needs to count through the crystal vibration frequency of the SCM ：（Fosc/12/Freq）=( 6000000Hz/12/50)=10000【 Use here 12 frequency division 】, So the timer counts 10000 Times to reach the time 20ms.（ Here is 12T And traditional 89C52 equally , Count once, so divide by 12）
  3、 Timer 16 position , Up to... Can be counted 65536 Time , Overflow is interrupt , So use （65536-10000）=55536 For the initial value in the timer .
  
  
2、 Method 2：
  here STC Set the timer T0 by 12T, And traditional 51 It's like a single chip computer
  1、6MHz It shows that the single chip microcomputer oscillates every second 6000 000 Time , Every time 1 Millisecond oscillation 6000 Time .
  2、 Single chip microcomputer 12 An oscillating clock timer counts once , We need timing 20ms, SCM every 1ms The total count is ：6000/12=500, It means that every single chip computer 1ms Conduct 500 Times count , We need timing 20ms So the total count of the timer is 500*20=10000 Time .
  3、 We have got the timer timing 20ms Required count value , Because the timer counts to 65536 Just interrupt , So we need to assign the initial value to 65536-10000=55536;

Two 、C Language use

Method 1：

#define FOSC 6000000L // System frequency 
#define T0_TIM (65536-(Fosc/12/50))
union VarS16{
    
	U16 mU16;
	U8 mU8[2];
};

void main()
{
    
	union VarS16 tmp;
	tmp.U16 = TO_TIM;
	
	TMOD = 0x00;
	
	// High in the former 
	TL0 = tmp.mU8[1];
	TH0 = tmp.mU8[0];
	TR0 = 1;		//T0 Start counting 
	ET0 = 1;		// Can make T0 interrupt 
	EA = 1;		// Open total interrupt 
	while(1)
	{
    
		;
	}	
}

void tm0_isr() interrupt 1 using 1
{
    
	/* Write the code to be executed by the interrupt */
}

Method 2：

#define FREQ_KHZ 6000L // System frequency 
union VarS16{
    
	U16 mU16;
	U8 mU8[2];
};

void main()
{
    
	union VarS16 tmp;
	tmp.mU16 = (FREQ_KHZ/12L) * 20;					//
	tmp.mU16 = (~tmp.mU16) + 1;					// Take the opposite  +1  amount to  ( 2^16 - tmp.mU16 ) 
	TMOD = 0x00;
	
	// High in the former 
	TL0 = tmp.mU8[1];
	TH0 = tmp.mU8[0];
	TR0 = 1;		//T0 Start counting 
	ET0 = 1;		// Can make T0 interrupt 
	EA = 1;		// Open total interrupt 
	while(1)
	{
    
		;
	}	
}

void tm0_isr() interrupt 1 using 1
{
    
	/* Write the code to be executed by the interrupt */
}