Ultrasonic sensor code explanation

[Hasan Malik]

Can someone explain this comment of that code, how does timer0 one cycle calculated to be 4.352 cm>>???



//speed of sound 340m/s
//1 TMR0 cycle = (340/1000000)*256*0.5*100 = 4.352 in cm


#define TRIG PORTD.F0
#define ECHO PORTD.F1

double distance;
void main() {

TRISD.F0 = 1;
TRISD.F1 = 0;
TRISC=0;

OPTION_REG.T0CS = 0;
OPTION_REG.PSA = 0;
OPTION_REG.PS0 = 1;
OPTION_REG.PS1 = 1;
OPTION_REG.PS2 = 1;

while(1){

TRIG = 0;
delay_us(10000);
TRIG = 1;
delay_us(10);
TRIG = 0;

while(ECHO==0);
TMR0=0;
while(ECHO==1);
distance = (double)TMR0 * 4.352;
PORTC = (char)distance;//to check
Delay_ms(100);
}

}

 

[honeybadger]

This might be a little bit late but the answer is fairly simple. If you consider the clock frequency as 2Mhz, the person who wrote this code is taking an 8 bit counter (256 counts) to find out the time it takes to fill up the counter. He then multiplies that with the speed of sound in air, giving the distance traveled. All the 10's in the equation are just conversions from one unit to the other.

 

[jayanth.devarayanadurga]

It is better to use Timer Interrupt to generate the trigger pulse instead of delays between toggling the pin.

 

[Shrikant Vyas]

hey..i've been trying to use the interrupt capture unit to get the time..... you can even look into that as well.