ADC problem with SK40C

[MaxD]

ADCON0:
bit7 - ADCS1
bit6 - ADCS0
bit5 - CHS2
bit4 - CHS1
bit3 - CHS0
bit2 - GO/DONE
bit1 - unimplemented bit
bit0 - ADON
----------------------------------------------------------
ADCON1:
bit7 - ADFM
bit6 - unimplemented bit
bit5 - unimplemented bit
bit4 - unimplemented bit
bit3 - PCFG3
bit2 - PCFG2
bit1 - PCFG1
bit0 - PCFG0
---------------------------------------------------------
According to the registers above, I set RA3 as Vref+ and RA2 as analog input:

CODE: SELECT_ALL_CODE

TRISA = 0b11111;
TRISE = 0b111;

ADCON0 = 0b10010001;
ADCON1 = 0b10000001;

__delay_ms(2);
   ADGO=1;
   while(ADGO==1)
      continue;
   return(256*ADRESH+ADRESL);


I can't get any power supply from RA3, why? If there is no power supply from RA3, then why LCD didn't show 0 value? Any problem with my register setting?

[ABSF]

Vref+ is an input pin. You're not going to get any voltage from it.

Allen

[holy_rainman]

Doesn't that mean we have to supply voltage ranging from 0V to 5V to Vref? Does it act like a comparator? I haven't played with Vref yet...

[ABSF]

The Vref+ of the ADC is selectable to Vdd internally or externally through RA3 by the "PCFGx" bits. The external Vref+ can be 0-5V using linear pot or a voltage reference chip such as TL431.

Allen

[robosang]

Allen is right.

1st thing, PIC does not have analog output pin, only Analog Input pin. Of course some uses PWM output to get analog voltage, still you need to do some circuitry to get that

2nd, don know why you want to change the Vref(+) to RA3, just tie it to VDD which is 5V normally. For cases where Vref(+) is tie to I/O pin is they wanted the Vref(+) to be other than VDD, example 2.0V, 4.0V, 1.023V.....

[MaxD]

So in order to get voltage from Vref+, i need to add in some extra components in the circuit, else it is just an analog input with no input voltage, right?

[ABSF]

So in order to get voltage from Vref+, i need to add in some extra components in the circuit, else it is just an analog input with no input voltage, right?

No, you're not going to get voltage from Vref+ but you're going to give a reference voltage to Vref+.

1st thing, PIC does not have analog output pin, only Analog Input pin. Of course some uses PWM output to get analog voltage, still you need to do some circuitry to get that

What robosang was saying is that PIC does not have analog output PIN cos ADC output is in digital form stored in ADRESx. If you need analog output, you have to use another IO pin (CCP1 or ordinary IO pin) and use PWM method to convert the number stored in ADRESx or any other variables into analog form. But why do you want to do that ? This can be easily done with an op-amp.....

Allen

[MaxD]

Oh, i see...thank you for your well explanation allen and robosang, thank you for your help =D