ADC problems!

[achacko]

What's wrong with the following code? It just displays values that starts with some value like 700 and then keeps on decreasing to near zero.
I have checked PIN_B4 and has a 3V dc signal applied on it. I used to get this ok on older PCMs. My current version is PCH 3.168. Tried it also on 16F877 with same results!

Thanks for your help.


#include <18F258.H>
#DEVICE ADC=10
#fuses HS,NOPROTECT,BROWNOUT,PUT,NOLVP,NOWDT
#use delay (clock=8000000)
#USE rs232(baud=9600,bits=8,parity=N,xmit=pin_C6,rcv=pin_C7,ERRORS)


//MAIN
//-------------------------------------------------------------
main()
{
int16 value;

printf("hello World!\n\r"); //show something

SETUP_ADC_PORTS(ALL_ANALOG);
SETUP_ADC(ADC_CLOCK_INTERNAL);
SET_ADC_CHANNEL(4); //Read from Chnl 4 only


do
{
value=read_adc();
printf("%lu\n\r", Value);
delay_ms(1000);
}while(TRUE);

}

[verba]

I don�t know a lot about this, but why don�t you put the delay between the adc and the printf, then you give time to adc to read correctly.
Then, I think you need divide the frecuency of clock. In the datasheet you can read than for speeds greaters 1 Mhz, RTC only is recomended to sleep mode. Try with div_32.
I will post now a problem of changes in values of adc very stranges, maybe it can help in your problem.
Verba

[Ttelmah]

I hope the comment that you have tested pin B4, is a missprint, since the analog input is pin RA5 - note this is not RA4...
ADC_CLOCK_INTERNAL will work, but may not give good accuracy. On some chips, the data sheet recommends not using this, or using 'sleep', over 4MHz. Use div_32 instead.
Your first reading will be wrong (you are not allowing any time between selecting the ADC channel and reading it. Put the delay at the start of the loop, so the capacitor in the ADC has time to charge.
Obviously, the stability of the reading, will depend on th quality of your supply rail. Noise here will degrade the performance significantly.
I suspect your signal is connected to the wrong pin, and the real pin is discharging with successive readings.

Best Wishes

[achacko]

Thanks everyone. Obviously I was looking at the wrong pin. Got it fixed and all working now