Pic18f4520: Adc-> Using An0, An1 For Vref

[Fizzel 0]

Hello there,

 

I'm trying to use the AN3, AN2 as reference voltage for the 10bit ADC. On AN3 i have 2.5V and on AN2 i have 2V.

But i don't get any valid data. When I use VSS and VDD as Vref everything works fine...

This is my code:

 

void Ini_ADC(void)	//Initialisierung des 10bit-ADC (siehe DB)
{
trisa.0=1;
trisa.1=1;
trisa.2=1;
trisa.3=1;	

adcon2	=0x94;		
adcon1	=0x3b;		//AN2, AN3=Vref
adcon0	=0x01; 		//use AN0, ADC=on	
}

void start_ADC()
{	 	
adcon0	|= 0x2; 			//start conversion
while((adcon0 & 0x02)>=1);	
tmp_h=adresh;
tmp_l=adresl;
tmp_h=tmp_h << 0x08;
tmp_value_m=tmp_h | tmp_l;
delay_ms(1);

  _EEPROMWrite(0x39,tmp_value_m);
}

 

The value of tmp_value_m is always FFFF, so where is my failure?

 

THX for help

Fizzel

[davidb 0]

The value of tmp_value_m is always FFFF, so where is my failure?

 

THX for help

Fizzel

You haven't told us which part you are using or shown your variable definitions.

Also the maximum result from the ADC should only be 0x03ff.

 

Your code for converting adresl and adresh to 16 bits is long winded and unless both tmp_h and tmp_value_m are defined as shorts then it will not work.

 

If you use:

tmp_value_m = (tmp_h <<8) | tmp_l;

instead then that should work. However, the code would be more efficient if you re-define adresl as a short or alternatively use a union:

unsigned short adresult @adresl;

then simply copy the result:

tmp_value_m = adresult;

 

Regards

 

davidb

[IanM 0]

Hello there,

 

I'm trying to use the AN3, AN2 as reference voltage for the 10bit ADC. On AN3 i have 2.5V and on AN2 i have 2V.

But i don't get any valid data. When I use VSS and VDD as Vref everything works fine...

This is my code:

 

void Ini_ADC(void)	//Initialisierung des 10bit-ADC (siehe DB)
{
 trisa.0=1;
 trisa.1=1;
 trisa.2=1;
 trisa.3=1;	

 adcon2	=0x94;		
 adcon1	=0x3b;		//AN2, AN3=Vref
 adcon0	=0x01;		 //use AN0, ADC=on	
}

void start_ADC()
{		 
 adcon0	|= 0x2;			 //start conversion
 while((adcon0 & 0x02)>=1);	
 tmp_h=adresh;
 tmp_l=adresl;
 tmp_h=tmp_h << 0x08;
 tmp_value_m=tmp_h | tmp_l;
 delay_ms(1);

   _EEPROMWrite(0x39,tmp_value_m);
}

 

The value of tmp_value_m is always FFFF, so where is my failure?

 

THX for help

Fizzel

 

For MOST PIC16 and PIC18 parts, the MINIMUM VREF (VREF+ - VREF-) is approx VDD_MAX/2. Check your datasheet for the minimum Vref for YOUR PIC! It will NEVER work properly with only 0.5V difference.

Edited February 3, 2011 by IanM

[Fizzel 0]

ok what i found in the datasheet is:

 

ΔVREF Reference Voltage Range

(VREFH – VREFL)

min=1.8V @ VDD < 3.0V

max=3V @ VDD ≥ 3.0V

 

I have VDD=3.3V so in my understanding delta_Vref has to be min 3V, right?

 

The problem is that i have a temperature sensor with a resolution 1,48mV/Kelvin, so with 3V for delta_Vref i have a resolution of nearly 2Kelvin/count, what isn't very detailed. Do I have any other solution for getting a resolution like 0.5Kelvin/count?

 

THX for Re

Fizzel

[trossin 0]

You could always add an external Operation Amplifier to multiply your input signal by a constant. There are many cheap ones out there to choose from. The end result will be a more accurate measurement than trying to play with the LSBs of the A/D as you will have too much noise in your readings without an amplifier when trying to measure tiny voltage differences.