How to give ADC reference voltage for PIC16F877A?

[vinodstanur]

I want to give ADC reference volts Vref+ (VDD/1000) and Vref-(VSS) .
HOW? (i want to give above vref in program.(not in AN3 and AN2)

my program with Vref+(VDD) and Vref-(VSS) is below and its working;
(most significant 8 bits are shown iin port b)
#include <htc.h>
#define _XTAL_FREQ 4e6 // 4MHz
__CONFIG(0x3F3A);
void main()
{
TRISB=0;
TRISA=1;
ADCON1=0b00001110;
__delay_us(10);

ADCON0=0b01000001;
__delay_us(10);
while(1)
{
ADGO=1;
while(ADGO);
PORTB=ADRESH;
__delay_ms(50);
}
}

 

[xxtigerxx]

it is simple if you set the PCFG = 0000 or 0010 ir 0100 or 1001 or 1110 the vref+ is VDD .

 

[vinodstanur]

it is simple if you set the PCFG = 0000 or 0010 ir 0100 or 1001 or 1110 the vref+ is VDD .

BUT my question is how to scale +vref to (VDD/10) or (VDD/1000) or like that , without using AN3 and AN2 by an external voltage divider circuit..
In above program PCFG is set as 1110. There the +vef is VDD.
I mean , is it possible to include the vref other than VDD and VSS in the program itself?:-(