51单片机pwm波的频率是怎么算的？

  bit Right_moto_stop=1;

bit Left_moto_stop =1;

unsigned char pwm_val_left =0;//变量定义

unsigned char push_val_left =0;// 左电机占空比N/20

unsigned char pwm_val_right =0;

unsigned char push_val_right=0;// 右电机占空比N/20

unsigned int timer=0;

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

/* PWM调制电机转速 */

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

/* 左电机调速 */

/*调节push_val_left的值改变电机转速,占空比 */

void pwm_out_left_moto(void)

{

if(Left_moto_stop)

{

if(pwm_val_left<=push_val_left)

{

Left_moto_pwm=1;

}

else

{

Left_moto_pwm=0;

}

if(pwm_val_left>=20)

pwm_val_left=0;

}

else

{

Left_moto_pwm=0;

}

}

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

/* 右电机调速 */

void pwm_out_right_moto(void)

{

if(Right_moto_stop)

{

if(pwm_val_right<=push_val_right)

{

Right_moto_pwm=1;

}

else

{

Right_moto_pwm=0;

}

if(pwm_val_right>=20)

pwm_val_right=0;

}

else

{

Right_moto_pwm=0;

}

}

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

///*TIMER1中断服务子函数产生PWM信号*/

void timer1()interrupt 3

{

TH1=(65536-1000)/256; //1ms定时

TL1=(65536-1000)%256;

timer++;

pwm_val_left++;

pwm_val_right++;

pwm_out_left_moto();

pwm_out_right_moto();

}