I use this code :
Code:
    vmax = 0
    for cnt =1 to 70       			
        adcin 3, v_cal
        vmax =vmax max v_cal
        pause 100
    next cnt

    v_cal=vmax * 5000            ' I use VDD as Vref
    v_cal=div32 1023

write 0, v_cal.LowByte
pause 10
write 1, v_cal.HighByte
pause 10