Questions on ultrasonic distance finder

[lilimike]

Hi,

I am at the begining of my project, actually I spent 1 week trying to make the LCD work properly so that tels me that I have a long way to go.

I am using a 16F627A (just because that's what I have on hand) if a better chip is suggested I can change it.
I have found a number of designs on the Web and read quite a few treads on forums, not that I want to reinvent the wheel but I would rather make my own design although I am taking some parts of interesting code here and there.

I do have a few questions to get me going. Based on the attached file, this is pretty much the simplest flowchart I could come up with and next to it is my understaning of what needs to happen as far as signal however I am no sure about the following details:

I presume that between T1 and T2 there has to be a burst of 40k times per second but I don't know how long it should last.

Ditto for the listening time between T3 and T4 and I guess the length of time it is listening is partly related to the range I can detect as long as I have enough transmitting and receiving power.

Please correct me if I am wrong but I figure I only want to start listening at T3 because I don't want the receiver to pickup vibrations from the transmiter since they are both on the same PCB.

What I have seen so far can take care of a ranges from a few centimeters to a few meters but I would like my range to be 6 cm to 6 meters so I understand I have to add some amplification at the receiver using op-amps and probably I have to increase power to the transmitter but how do I increase the Tx power? by raising the voltage?

and one last question (for now) when I detect an echo, do I calculate the time from T1 or T2?

I don't really need more than +/-20 cm of precision at 6 meters but I do need a minimum of 2 or 3 cm at 6 cm.

Here is where I am at with the code which basically I can see 40Khz on the scope and I know the LCD works. Like I said, I have a long way to go...

@ __config _XT_OSC & _WDT_OFF & _MCLRE_ON & _LVP_OFF & _CP_OFF

DEFINE OSC 4
TRISB = 0                   ' Make PORTB all outputs
TRISA = %10                 ' A0 = send burst, A1 = listen to echo
CMCON = 7                   ' Disable comparator
FLAGS = 0
T1CON = %00000001           ' enable Timer1 prescaler 1:1

disTance    var byte
ECHO        var PORTA.1
Cycles      var byte

LCD_DB4     VAR PORTB.0       ' Set port for 4 bits bus
LCD_DB5     VAR PORTB.1
LCD_DB6     VAR PORTB.2
LCD_DB7     VAR PORTB.3
LCD_RS      VAR PORTB.5       ' Set RS bit port
LCD_E       VAR PORTB.4       ' Set Enable bit port
LCD_Lines     CON 2         ' # of Lines on LCD,  1 or 2 (Note: use 2 for 4 lines)
LCD_DATAUS    CON 50        ' Data delay time in us 
LCD_COMMANDUS CON 2000      ' Command delay time in us 
INCLUDE "LCD_AnyPin.pbp"    ' Must go after LCD initialization 
Pause 500: LCDOUT $FE,1: pause 250   ' clear screen and relax
LCDOUT $FE,$80,"Distance Finder"  
                            
goto start

asm
_Pulse
   bsf  PORTA,0 ; change to whatever port pin you have available
   goto $+1    ; 2uS per GOTO $+1
   goto $+1
   goto $+1
   goto $+1
   goto $+1     
   bcf  PORTA,0
   goto $+1 
   goto $+1
   goto $+1
   goto $+1
   goto $+1
   decfsz _Cycles,f
   goto _Pulse  ; 25uS total for 40kHz carrier
   RETLW 0      ; return to CALLing routine
ENDASM	  

Start:                      

    ' start timer ...

    ' send burst ...
    Cycles = 8    ' number of carrier cycles to generate
    CALL Pulse
    PAUSEUS 24

    ' Listen for echo ...

    ' timed out...

    ' count number of time outs ...

    ' update display
    gosub display_upd
    
    GOTO start


Display_UPD:
    LCDOUT $FE,$C0,"Distance = ",dec disTance,"  "  
return
                            
end