ADCON0=0x00;// ADCON0.VCFG=0; Vref=VDD是5V沒錯

第一次使用 12F675 ADC, 依照手冊設定 MCU SFR後, 若設定無誤, VDD=5V, ANALOG 輸入如果在1.3V以上應該可以得到1024/4以上的 AD值, 可是實際測試卻要接近5V左右, 不知問題出在哪裡
以上, 還請不吝指教
CODE如下

#include <pic12f675.h>
//ADC utilities

void init_a2d(void){
ANSEL=0x34;//ADC CLOCK INTERNAL, Analog Select only AN2
//ADCON0=0x80; // right justify result.
ADCON0=0x00; // left justify
ADON=1; // turn on the A2D conversion module
}

//unsigned int read_a2d(unsigned char channel){// for 10bit ADC
unsigned char read_a2d(unsigned char channel){// for 8bit ADC
channel&=0x03; // truncate channel to 2 bits
ADCON0&=0xc3; // clear current channel select
ADCON0|=(channel<<2); // apply the new channel select
GO_DONE=1; // initiate conversion on the selected channel
while(GO_DONE);
//return (ADRESH<<8)+ADRESL; // return ADC value of the result
return ADRESH;
}



void main(void)
{


//internal RC oscilator calibration
_asm
bsf STATUS, 5 ;Bank 1
call 3FFh ;Get the cal value
movwf OSCCAL ;Calibrate
bcf STATUS, 5 ;Bank 0
_endasm;

TRISIO=0x04;// set AN2(GP2) as "input" direction pin
CMCON=0x07;
init_a2d();

/*
// for debug and test code
while(1)
{
delay_ms(1000);
GPIO=GPIO^0x30;
}
*/
while(1)
if(read_a2d(2)>63) GP5=1;
else GP5=0;

}