A/D from 16F873

[Francesc]

Hi all, is very strange for me, we try to read 4 ports analogs from 16F873, but the readings not are corrects.
We try to enter differents voltages, here a sample:
0,0 V..0000 bits
0,5 V..0131 bits
1,0 V..0240 bits
2,0 V..0174 bits
3,0 V..0130 bits
3,5 v..0229 bits
4,0 v..0060 bits
4,5 v..0129 bits
5,0 v..1023 BITS

I include an small part of my code to try found the error.
Please, help me..

#include <16f873.h>
#device ADC=10
#fuses xt,nowdt,put,nolvp
#use delay(clock=4000000)
#use RS232(BAUD=9600,BITS=8,XMIT=PIN_C6,RCV=PIN_C7)
#use standard_io ( a )
#use standard_io ( b )
LONG T,K1,K2,K3,K4,K;
main()
{
while(1)
{
SET_TRIS_A(0x27);
//SETUP_ADC(ADC_CLOCK_DIV_8);
SETUP_ADC(ADC_CLOCK_INTERNAL);
SET_ADC_CHANNEL(0);
delay_ms(10);
T=READ_ADC();
K1=T/64; //0 to 1023
delay_ms(50);

SET_ADC_CHANNEL(1);
delay_ms(10);
T=READ_ADC();
K2=T/64;
delay_ms(50);

SET_ADC_CHANNEL(2);
delay_ms(10);
T=READ_ADC();
K3=T/64;
delay_ms(50);

SET_ADC_CHANNEL(5);
delay_ms(10);
T=READ_ADC();
K4=T/64;
delay_ms(50);

display();
}
}

[KerryW]

Add this line

setup_port_a( ALL_ANALOG );

Also, you should take these lines

SET_TRIS_A(0x27);
//SETUP_ADC(ADC_CLOCK_DIV_8);
SETUP_ADC(ADC_CLOCK_INTERNAL);

out of the while loop.

It should look like this

main()
{
SET_TRIS_A(0x27);
//SETUP_ADC(ADC_CLOCK_DIV_8);
SETUP_ADC(ADC_CLOCK_INTERNAL);
setup_port_a( ALL_ANALOG );
while(1)
{
SET_ADC_CHANNEL(0);
delay_ms(10);
T=READ_ADC();
K1=T/64; //0 to 1023
delay_ms(50);
.
.
.

[PCM programmer]

Here are some additional comments on your code.

------------
I noticed that you have this line of code:

SET_TRIS_A(0x27);

You are setting pins A3 and A4 as outputs. If I look at the
chart in the 16F873 data sheet, of permissible Analog and
Digital pin combinations on Port A, I don't see a setting
that would allow you to do this. You should look at that
chart, and also look at the constants (such as A_ANALOG)
given in the 16F873.H file. The constants are used with
setup_adc_ports(), and correspond to the settings shown
in the data sheet.

---------------------
After you choose the proper setting for your Port A
configurations (between Analog and Digital pins), you
should change your SET_TRIS_A() function so it fits
that configuration.

---------------------
I also noticed that you are dividing the ADC result by 64,
to compensate for the result being left-justified in a 16-bit word.

You don't have to do that. The PIC has a bit which allows
you to set the ADC result to be right-justified. See this
post which shows how to do it:

http://www.ccsinfo.com/forum/viewtopic.php?t=2771&highlight=adfm_bit

[wedilo]

Hello Francesc,
Why does you devides the value from ADC by 64?
I think READ_ADC() returns an 10bit value (0...1023)

But that is not the problem, I mean you have forgotten the 'setup_adc_ports' command