pic18f67k90 problem adc

[alain-debutant]

hello everyone
i am a beginner in programming, my problem is: to acquire a value from 0 to 5V on ANA0 and send it to my screen with a pic18f67K90 or a pic18f67k22
the principle works but the problem is the linearity of the reading
attached is a raised curve, I think I forgot something!!
i have already made an identical assembly with a pic26k22 and I did not have this problem, the 67k90 or 67k22 must be special??
if someone could tell me where my mistake is, thank you in advance
cordially

I can't insert an image, sorry, explanation: input voltage
AN0 = 0v --> screen reading or debugger 234
AN0 = 1.2V --> screen reading or debugger 255
AN0 = 1.21V --> screen reading or debugger 0
An0 = 1.21v -> 4.9 v screen reading or linear debugger
An0= 5v --> screen reading or maximum debugger 188

/* compilateur CCS V5.116
PIC18F67K90 ( idem si pic18F67k22 )
*/



#include "main.h"

void main()
{

setup_timer_0(RTCC_INTERNAL|RTCC_DIV_32|RTCC_8_BIT); //2,0 ms overflow
setup_timer_2(T2_DIV_BY_16,250,10); //1,0 ms overflow, 10,0 ms interrupt


disable_interrupts(INT_TIMER0);
disable_interrupts(INT_TIMER2);


disable_interrupts(GLOBAL);

setup_adc_ports(sAN0, VSS_VDD);
setup_adc(ADC_CLOCK_INTERNAL | ADC_TAD_MUL_20);

set_adc_channel(0);
delay_us(20);

while(TRUE){

bouton = input(PIN_E7);
if(bouton == 1){

potard1= read_adc(adc_start_and_read) ;
delay_us(100);


//////// tacho metre
fputc(0XAA,PORT1);
fputc(0X3D,PORT1); // 3D -> N16 write / 42 -> str write
/// adresse ////
fputc(0X00,PORT1);
fputc(0X08,PORT1);
fputc(0X00,PORT1);

fputc((potard1 & FILTRE_INT16_H ) >>8,PORT1) ;
fputc(potard1 & FILTRE_INT16_L ,PORT1);

//Copie suffixe
fputc(0XCC,PORT1);
fputc(0X33,PORT1);
fputc(0XC3,PORT1);
fputc(0X3C,PORT1);

delay_ms(5);

output_high(PIN_B3);
}
else
{
output_low(PIN_B3);


} // while true

}

[temtronic]

quick comments
start putting comments at the end of most lines of code ! It costs nothing yet allows EVERYONE to understand what should be happening. It'll really help 3 weeks from now !

As a basic test, I'd JUST send the RAW data from the pot position.
This allows you to confirm PIC does read the pot and send the correct data.
Since it is a pot, I suggest a small (.1-1mfd ) cap on the wiper to ground as a 'filter' and sample at say a 1Hz rate

also add 'errors' to the USE RS232 (.... options...... )

[Ttelmah]

I'm very puzzled why you are trying to access the upper 8 bits of an 8bit
value?..
However key difference is that the ADC on this chip is a differential ADC.
The top bit is a sign bit.

[alain-debutant]

hello
thank you Ttelmah for your clear answer, I suspected that this chip was special but I would not have imagined that it was a differential input, sorry to abuse your knowledge how to do the code for this kind of application? for information I also put a capacitor for filtering on this input and the signal is very clean, I am starting with chip programming by passion and I have no training as a programmer!
thank you again for your help, you are a genius

[alain-debutant]

for Mr Ttelmah: originally I am on a 16 bit value, but as I had the same problem I wanted to simplify my real value that I would like to use is indeed in 16 bit

thanks

[alain-debutant]

Mr Ttelmah
how to configure this chip in single channel mode?? I looked at the datacheet page 373 but I don't know how to do it, or how to use it to have a linear scale from 0 to 4095??
thanks again for your help

[Ttelmah]

The point is it is always a differential ADC module. Even when you run it
so Vss is the second input. The top bit is always ADSGN. The result is
12bits plus sign. Because you were trying to select 8 bit mode, this switches
to returning the high 7 bits plus sign. If you just switch to 12bit mode,
and read the low twelve bits of the value returned, if you want 8 bits, just
rotate this right 4 bits.

[alain-debutant]

thank you very much Ttelmah, I will do like that.
good luck on the forum you are great
best regards