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A/D Conversion Causing Noise in RS232 Data

 
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weg22



Joined: 08 Jul 2005
Posts: 91

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A/D Conversion Causing Noise in RS232 Data
PostPosted: Thu Feb 01, 2007 10:50 am     Reply with quote
Hi all,

I'm reading in data from 2 sensors:

1. IR sensor using A/D conversion
2. SONAR sensor using software RS232

If I disconnect the IR sensor, the SONAR sensor is read flawlessly. However, if I connect the IR sensor then every 10th reading or so the SONAR data will jump 5-10 inches when holding the sensor perfectly still.

I'm powering everything off the same battery which I cannot change. I also have a 0.1 uF cap between the signal pin and ground of the IR sensor. Any suggestions on how I could get rid of the little bit of noise would be appreciated.

Thanks,
-weg
newguy



Joined: 24 Jun 2004
Posts: 1907

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PostPosted: Thu Feb 01, 2007 11:09 am     Reply with quote
I've used a couple of ultrasonic distance measurement sensors and they're pretty sensitive to power line variations. They tend to consume a lot of current when they're transmitting. Do you have a relatively large cap on the sonar sensor's power line? If not, try a 10 uF+, and place it close to the sensor.

What is your timing like? What I mean is how do you do your measurements, how often, and in what order? ie. read IR sensor, delay xxxx us/ms, read distance sensor, delay yyyy, repeat. Do you allow some settling time after you change a/d channel before reading the channel? Do you delay long enough to allow for the (relatively) long round trip delay of the sound wave?
weg22



Joined: 08 Jul 2005
Posts: 91

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PostPosted: Thu Feb 01, 2007 11:24 am     Reply with quote
My code is below:

Code:

#include <16F877>
#device adc=8
#include <stdlib.h>
#include <math.h>

#define LED PIN_C0

#fuses HS,NOWDT,NOPROTECT,PUT,NOLVP
#use delay(clock=10000000)
#use rs232(baud=9600, xmit=PIN_B1, rcv=PIN_B0, stream=sonar1, INVERT) // SONAR #1
#use rs232(baud=9600, xmit=PIN_E1, rcv=PIN_E2, stream=PC) // TO PC


main()
{
   // IR
   unsigned int ir_raw1=0;
   
   // SONAR
   int i=0;
    char c;
   char sonarData[4];
   unsigned int sonar_raw1=0, sonar_raw2=0, sonar_raw3=0;


   // setup to do A/D conversion
    setup_adc_ports(RA0_RA1_RA3_ANALOG);
   setup_adc(ADC_CLOCK_DIV_8);


   output_high(LED);
   delay_ms(1500);
    output_low(LED);

   while(1)
   {
      // read in value on AN0
      set_adc_channel(0);
      delay_us(50);
      ir_raw1 = read_adc();

      // SENSOR #1
      c = 'a';
        while(c != 'R') {c = fgetc(sonar1);} // sonar outputs data as "Rxyz"
        i=0;
        for(i=0; i<=2; i++) // get next 3 bytes
      {
            sonarData[i] = fgetc(sonar1);
      }
        if(isdigit(sonarData[0]) && isdigit(sonarData[1]) && isdigit(sonarData[2]))
      {
         sonarData[3] = '\0';
            sonar_raw1 = atoi(sonarData);
      }

      fprintf(PC, "%u, ", ir_raw1);
      fprintf(PC, "%u\r\n", sonar_raw1);
      
   }

} // end of main
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