ultra sound with pic [closed]

Question

Why does this code divide the time over 29? This code is for connecting ultrasound with PIC. I'm working with a 4 MHz clock and the pic type is P16F877A .

program HCSR04

dim  duration_us as word
dim  distance1   as integer
dim  length_cm   as string[5]

main:

  TRISC.0=  0
  TRISC.3=1
  TRISD.7=0                             ' B8 to B2 for LCD, B1=trig, B0=echo
  TRISB=0x00
  T1CON = 0x10

run:
  duration_us = 0
  distance1   = 0
  TMR1L    = 0                   ' Clear the TMR1L value
  TMR1H    = 0

  PORTC.0 = 1                  'initialize the trigger
  delay_us(100)

  PORTC.0=0
  T1CON.0  =1                ' Enable TMR1

  while PORTC.3= 0              ' Waiting for the echo
  wend

  T1CON.0  =1                ' Enable TMR1

  while PORTC.3= 1              ' While echo received, let the TMR1 count
  wend

  T1CON.0  =0                ' disnable TMR1

  duration_us =  TMR1L+ (TMR1H<<8 )' Retain TMR1 value = ON pulse
  distance1   = (duration_us)/29
  PORTD.7=1
  PORTB=distance1
  delay_ms(1000)
  goto run

end.

Answer 1

You divide the duration by 29 because that is evidently how you convert the unspecified time period into a distance of unspecified units.

Without knowing what the time period is measured in, nor the units of the distance you expect, why 29 was chosen is anyone's guess.

While I am not familiar with the language you are using, I can kind of follow the flow of it, however that is a completely pointless exercise since without knowing what pins are connected to what and in what way we can't know what the program is supposed to be doing.

My understanding:

• Timer is set to 1:2 prescale, so ticks at 0.5MHz (\$F_{OSC}/4/2\$).
• Set port C3 (receiver, which is an input) HIGH - what is this supposed to achieve?!
• Set port D7 to an output
• Clear timer.
• Wait 1ms (why?!)
• Wait while C3 is low
• Start the timer
• Wait while C3 is high
• Turn off the timer
• Divide the time (in 2µs counts) by 29
• Set port D7 high
• Output the distance to PORTB

Now, is it me, or does that program make no sense whatsoever? At no point is the trigger being activated. You're randomly setting other port pins to input or output or high or low for no apparent reason.