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16f877 adc problem

 
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anjan
Guest
16f877 adc problem
PostPosted: Wed Feb 04, 2009 1:01 am     Reply with quote
I am using 16f877 for measuring voltage. It measures accurate voltage, but when I vary voltage, display does not show the voltage instantly, it shows voltage changing in steps. Here is the code.
Pls help.

Code:

#include <16F877.h>
#fuses XT,NOWDT,NOPROTECT,NOLVP
#use delay(clock=20000000)

#include <flexlcd.c>

[color=green]
void main() {
   char k;
   int current_temp, max_temp, min_temp,value;
   int16 current_ccp_delta;
int16 frequency; int count_inc=0;

setup_port_a( ALL_ANALOG );
   setup_adc( ADC_CLOCK_DIV_32);
   set_adc_channel( 1 );


   lcd_init();
   lcd_putc("\fReady...\n");

      delay_ms(500);
   while (TRUE)
   {
   set_adc_channel( 3 );
   delay_ms(10);
   value = Read_ADC();
   current_temp=value;
   value=0;
   
   delay_ms(20);
   set_adc_channel( 1 );
   delay_ms(10);
   value = Read_ADC();
   min_temp=value;
   value=0;
   
   delay_ms(20);
   set_adc_channel( 0 );
   delay_ms(10);
   value = Read_ADC();
   max_temp=value*2;
   value=0;


         printf(lcd_putc,"\fCu: %U F\nMin: %U Max:%U",                        current_temp,min_temp,max_temp);
          delay_ms(20);
   }
}[/color]
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Wed Feb 04, 2009 2:10 am     Reply with quote
Try a more simple program. See if this one works:
http://www.ccsinfo.com/forum/viewtopic.php?t=32168&start=1
This assumes you have a 5K trimpot connected to the AN0 input pin
(A/D channel 0). A voltage between 0v and 5v can be applied
to pin AN0 by turning the trimpot screw (or knob).

The program in the link was tested with a 4 MHz crystal, so it uses an
ADC divisor of 8. You're using a 20 MHz crystal, so you need to modify
the #use delay() statement to fit that, and also change the ADC divisor
to 32. (ie. make those two things be the same as in your posted code).
anjan
Guest
16f877 adc problem1
PostPosted: Wed Feb 04, 2009 4:33 am     Reply with quote
Dear Mr PCM,
My program measurement is Ok, But it update voltage very slowly, suppose there is a change of voltage from 4.0 to 2.6 or say 5.7, It first display 3.9,3.7 ...... then it reaches to 2.6 after sometime.like capacitor charging and discharging.
I need instant change of parameters.
I think You have understand my problem
Waiting for ur reply
Ttelmah
Guest
PostPosted: Wed Feb 04, 2009 5:58 am     Reply with quote
How are you actually feeding the voltage to the chip?. What does the voltage derive from?
Provided your source mets the impedance spec for the PIC ADC input (<10KR on the chip you are using), the PIC will happily change from one extreme of the detected voltage range to the other, in just one reading time. If your signal is not changing like this, look at the circuitry you have to generate/drive the voltage.

Best Wishes
psypersky



Joined: 06 Jul 2008
Posts: 13

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PostPosted: Thu Feb 26, 2009 3:03 pm     Reply with quote
Use Read_ADC(ADC_READ_ONLY) instead of Read_ADC(),
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