（17） DAC conversion experiment

        In the last section, we wrote about ADC Related content of , In this section, we will write about DAC Related content of ,DAC That is to convert digital signals into analog signals . Its function and ADC contrary , In this section, we convert the output digital signal into analog signal led The light turns on gradually and then turns dark to realize the corresponding function .

About DAC

        We know ADC In fact, there are several important features , and DAC There are also several particularly important features .1 Also resolution , and ADC They have exactly the same characteristics .2 Is linearity , Linearity means the maximum deviation between the actual conversion characteristic curve and the ideal straight line characteristic . It is usually expressed as a percentage of full scale .3 It is absolute precision and relative precision , Absolute accuracy refers to the whole scale range , The maximum error between the actual output value of analog quantity corresponding to any input data and the theoretical value . Relative precision and absolute precision have the same meaning , The absolute accuracy is due to DAC Gain error （ When the input code is all 1 when 、 The difference between the actual output value and the ideal output value ）、 Zero error （ Digital input is all 0 when .DAC Output value ）、 Caused by nonlinear error and noise .4 Is the establishment time , Setup time refers to the time when the input digital quantity changes to full scale , When the output analog signal reaches the full scale value 1/2 Time of scale value , We can set up time to DAC classification , Super high speed , High speed , Medium speed , Low speed, etc .

About DAC

        In the actual research and development process, for the sake of cost , We don't usually use DAC, We can use PWM To simulate the DAC Output , Then you need to connect PWM Of · Related content .PWM Chinese means pulse width modulation , Short for pulse width modulation . We can look at the picture directly, which should be easier to understand .

         In fact, we can adjust the pulse width to indicate the size of our voltage , This is actually PWM True meaning , And about the PWM The important meaning of the square wave is that our square wave occupies a period , The larger the duty cycle , The higher the voltage , We can go through PWM To simulate analog quantity .

About hardware ：

         It's about PWM Hardware circuit of the circuit , For our software, there is basically no need to understand , We just need to know R21 As our input

About software ：

        We use PWM Output , Finish let led The process of light first on, then off and then on .

#include "reg52.h"
int DIR;
int val = 0;
int count = 0;
int timer1 = 0;
sbit pwm = P2^1;
void t0_init(){
	TMOD |= 0x01;
	TL0 = 0xff;
	TH0 = 0xff;
	EA = 1;
	ET0 = 1;
	TR0 = 1;

}
void main(){

	t0_init();
	while(1){
		if(count >= 100){
			count = 0;
			if(DIR == 1){
				val++;
			}
			if(DIR == 0){
				val--;
			}
		}
		if(val >= 1000){
			DIR = 0;
		}
		if(val <= 0){
			DIR = 1;
		}
		if(timer1 >= 1000){
			timer1 = 0;
		}
		if(timer1 < val){
			pwm = 1;
		}
		else{
			pwm = 0;
		}
	}
}

void t0_timer() interrupt 1{
	TH0 = 0xff; 
	TL0 = 0xff;
	timer1++;
	count++;
}