adc problems

[guest*]

Wondering if any body can help me,

i'm trying to read from the a/d however the rading keeps jumping between 0.00 and 244.99

vref- =0v
vref+ =2.5v

vin= 394mv-412mv it jumps a bit but shouldn't cause the reading to jump this much should it.
my code is below.

*****************************************************
include <16f877.h>
device ADC=10; // setting it to 10 bit a/d default is 8
#include <stdlib.h>
#include <math.h>

#use i2c(MASTER, SDA=PIN_C4, SCL=PIN_C3)
#use delay(clock=20000000)
#use rs232(baud=9600,xmit=PIN_C6,RCV=PIN_C7)


#include <lcd_d.c>
#define button PIN_B0 //defines b0 as an input button

main()
{

float value=0.00;
setup_timer_1(T1_INTERNAL|T1_DIV_BY_8);
delay_ms(50);
lcd_init();
delay_ms(10);

SETUP_ADC_PORTS(ALL_ANALOG);

setup_port_a( ALL_ANALOG ); //use with 20mhz
//setup_adc( ADC_CLOCK_INTERNAL ); only use with less than 1mhz colck

setup_adc(ADC_CLOCK_DIV_32);
set_adc_channel( 1 ); //specify whcich a/d channel to use


printf(lcd_putc,"\fDesigned by\nguest!! 2004"); //boot up screen
delay_ms(250);
printf(lcd_putc,"\f");

while(1)
{
value = read_adc();
delay_ms(50);
output_c(value);
printf(lcd_putc,"\n%3.2f value ",value);
delay_ms(50);
}
return 0;
}
***********************************************

[PCM programmer]

Your problem is that you have not setup the ADC module
in the PIC to use the Vref pins.

You have it setup like this:
setup_adc(ALL_ANALOG);

and you really need this:
setup_adc(RA0_RA1_ANALOG_RA3_RA2_REF);

You can find the information on how to properly setup
the ADC port by looking in the 16F877 data sheet.
Look in Section 11 of the data sheet, at the table for:
PCFG3:PCFG0: A/D Port Configuration Control bits:
This table gives a list of all possible settings for the ADC port.

Also, look in the 16F877.H file, in this section:
Constants used in SETUP_ADC_PORTS()
This section shows you the constants used by CCS for
the setup_adc_ports() function. These constants correspond
to the table in the data sheet, that I referred to above.
You should study them to understand how to use the ADC.

------------

You have a few other strange things in your code.

These two statements do the same thing,
so get rid of one of them.
SETUP_ADC_PORTS(ALL_ANALOG);
setup_port_a( ALL_ANALOG );

At the end of main(), you have return(0);
That line of code won't really do anything.
It just moves 0 into a temporary register.
Remove that line of code.

[Guest]

ok so instead of,

i use
SETUP_ADC_PORTS(ALL_ANALOG);
setup_port_a( ALL_ANALOG );


i use: setup_adc(RA0_RA1_ANALOG_RA3_RA2_REF);

forgot to mention i'm using a 20mhz clock.
is it also possible to set vref- to -2.5, its not quite clear on the datasheets.

[PCM programmer]