what's wrong with this simple A/D code?

[eliberg]

Hi to everyone,
I'm trying to simply convert an analog input using the ad module of the PIC18F4431.
The analog input is on the AN0 pin, the range of the voltage is 0-5V (Vdd-Vss).
The problem is that whatever I put on the AN0 pin (0V, 1V, 3V...5V) I always read adc3=1023.
Trying to visualize adresh and adresl I obtained that adresh=3 and adresl=255.
I'v also tried setting manually each of the AD registers, but it was the same. Always 1023.

What's wrong?
The code is the following:

#include <18F4431.h>
#device adc=10
#use delay(clock=20000000)
#fuses HS, NOPROTECT, NOBROWNOUT, NOWDT, NOLVP, PUT, NOSTVREN, NOFCMEN, NOIESO, MCLR
#use rs232(baud=19200,xmit=PIN_C6,rcv=PIN_C7,parity=N,bits=8)

#define led_status PIN_D7


void main()
{ unsigned int16 adc3=0;

output_bit(led_status,1);
delay_ms(2000);
output_bit(led_status,0);
printf("\n\r adc alive");

setup_adc_ports(sAN0|VSS_VDD);
setup_adc(ADC_CLOCK_INTERNAL);

while (1)
{
set_adc_channel(0);
delay_us(20);
adc3=read_adc( );

printf("\n\r adc=%lu",adc3);
delay_ms(1000);
}
}

Thank you.
Eli

[dr]

setup_adc_ports(sAN0|VSS_VDD);

I think your syntax is wrong here. In my usage, setup_adc_ports( ALL_ANALOG) always works. I don't know how that would pan out with the
18F4431 though.

[rnielsen]

[eliberg]

Thanks for the replays.
First of all I need just one AN pin, so I cannot use ALL_ANALOG, also because the other possible AN pins are used as digital I/O, in my project.
In the 18f4431.h "setup_adc_ports(sAN0|VSS_VDD)" is the right syntax to set AN0 as analog, from 0 to 5V.

Second, I'm not sure I can use #device adc=16, because the AD module is a 10bit AD converter, in the 18F4431, so I can set adc=8 or adc=10.
Anyway I will try, thanks for the idea.

In the meanwhile I think I solved the problem by myself. Inserting "set_tris_a(0b00011001)" now the adc works. The only strange thing is that the digital value becomes 255 when the input voltage is 4V, not 5V.

Any idea for this new problem?
Thank you very much!
Eli

[Humberto]

[eliberg]

Ok, with #device=10 I obtain 1023 at 4V, instead of 5V.
The same problem with #device=8 (255 at 4V).
I don't know where is the problem!
eli

[rberek]

Do you know for a fact that your supply rail is 5V?

If it has drooped to 4V, then you will get the full reading at the lower voltage.
_________________
The difference between genius and stupidity is that genius has its limits...

[eliberg]

Yes, I'm sure my voltage is 5V, but if I set my Vdd, for example, to 5.5V, the ADC gets 1023 to 4.5V.
Do you think it cuold be a "broken" PIC?
Thanks
eli

[Ttelmah]

Double check that the supply is actually 'right' at pin11. This is the pin used for the analog reference.
Yes, it could be a faulty PIC, possibly in the MOSFET used to connect the reference to the ADC.

Best Wishes