ADC reading simple circuit?

[Oinquer]

Hello,

im doing an ADC reading of an 4.2 lithium battery, and i have a doubt about the circuit...
since my pic is 5V and im using a 4.2 max input im just connecting a small resistor to between VDD and AN2 from PIC to read it.
Im doing it right since i can read it... but the reading seems to slowly go up...should be oposite....do i need to put a weak pulldown to prevent voltage from "building up" or what?

someone with some experience migh help??

thank you.

 

[tpetar]

Use zener 5,1V to protect ADC.

 

[Oinquer]

Care to explain why adc needs protection? The cell is 4.2v, micro is 5v tolerant.

 

[jayanth.devarayanadurga]

You don't need zener if your adc input is <= 5V. Post your code and circuit. There might be problem in your code.

 

[tpetar]

Care to explain why adc needs protection? The cell is 4.2v, micro is 5v tolerant.

uC is 5,5-6,5V tolerant depends from particular uC spec. Jayanth.devarayanadurga is right check code.

 

[Oinquer]

, made a fast circuit in eagle...

program LED

' Declarations section 
  dim tmp as word
  dim sas as string[10]
main:
    OSCCON= 0x7f
    ansela = 0x00
    anselc = 0x00
    FVRCON = 0xc1
    ADCON1 = 0xe3
    ADCON0 = 0x09
'   Main program 
    TRISC = 0x00
    TRISA = 0x00
    PORTC.2 = 0
    delay_ms(1000)
    PORTC.2= 1
    UART1_init(9600)
    UART1_Write(0xff)
while true

      ADCON0.1 = 1
      while (ADCON0.1)
      delay_ms(500)
      wend
      tmp =  ADRESH  << 8
      tmp = tmp +  ADRESL

      WordToStr(tmp, sas)
      UART1_Write_Text(sas)
      delay_ms(500)
wend
end.

Also...for about 5 hours i had the battery draining, and found that the readings are going up....
maybe im doing something wrong with the reading....
microcontroller datasheet **broken link removed**


Probably found error...
ANSELA.2 = 1
...testing now.