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The reading is not right

 
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amjad_alahdal



Joined: 19 Feb 2013
Posts: 50

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The reading is not right
PostPosted: Wed Sep 04, 2013 4:40 am     Reply with quote
Hi,
I used PIC18F2455 Crystal is 16MHz,
I used an ultrasonic range finder such as this one :
https://www.sparkfun.com/products/9491
it's very obvious that I can use 3 ways to get the reading,
I could read the voltage using the analog, but, it wasn't very accurate. So, I tried to use the RS232
I used the following Code :
Code:

#include<18F2455.h>

#device PASS_STRINGS = IN_RAM
#FUSES HS,NOPROTECT, NOLVP,NOWDT
#USE DELAY(CLOCK = 16M)

#define    Indicator_LED    PIN_A5
#define    To_LCD         PIN_B7
#define      FROM_URF      PIN_B6
#USE RS232(BAUD = 9600,bits = 1,XMIT = TO_LCD, RCV = FROM_URF,STREAM = COM_LCD,ERRORS)
#include<lcd.h>
#include<math.h>

void main()
{
   unsigned int32   distance = 0 ;
   delay_ms(1000);
   fprintf(COM_LCD ,"The Distance:");
   lcd_cursor_move(2,5);
   fprintf(COM_LCD,"MM");
   while(true)
   {
      delay_ms(250);
      distance = getc();
      lcd_cursor_move(2,1);
      fprintf(COM_LCD,"%ld",distance);
   }
}


but the reading is totally wrong

I will be happy if you help me
temtronic



Joined: 01 Jul 2010
Posts: 9244
Location: Greensville,Ontario

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PostPosted: Wed Sep 04, 2013 6:39 am     Reply with quote
You've got to read the datasheet again....page two
When using 'RS-232..serial' the data from the sensor is sent as a 'datastream' consisting of ASCII 'R', then 3 numbers, then a Carriage return.

Knowing this you can either use the GETS() CCS function or use the 'ex_sisr.c' example and make a buffered serial ISR.

How you proceed is up to you..
Personally , I'd modify the ex_sisr.c program as it allows capturing the sonar data 'in the bakground' while your 'main' program does 'other things'.

hth
jay
Ttelmah



Joined: 11 Mar 2010
Posts: 19545

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PostPosted: Wed Sep 04, 2013 7:45 am     Reply with quote
Provided you have good smoothing, and are not doing anything silly, the analog output from these is very good. It is ratiometric to the supply, so isn't affected by supply variations, and can give accuracies nearly as good as the sensor gets....
Remember the sensor _will_ give instantaneous spurious readings as 'noises off' are picked up. Were you selecting the ADC clock correctly?. Were you allowing for the acquire time?. Was the line from the sensor properly shielded?.

For reading the serial, search here for threads about 'state machines'. A very simple one in the serial ISR, is the best way to do this.

Best Wishes
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