Hi
I recently started using the PIC micro (having previously used the
68HC11) mainly for hobby purposes. This week I tried to use the A2D
converter on the 18F452 and hit a problem. I initialise the A2D as
follows:

InitializeAD
        movlw   B'11111111'             ; Make PortA all  inputs
        movwf   TRISA,0
                =09
        movlw   B'11000001'             ; Select RC osc, AN0 selected,
        movwf   ADCON0,0                ; A/D enabled
                =09
        movlw   B'10001110'             ; Make RA0 analog input
        movwf   ADCON1,0                ;Use Vdd and Vss as reference
for A2D

                                        ;Not using interrupts(polling
GO/DONE bit)
        return=09

In this configuration, the A2D result reads full scale (3F) when the
input voltage is only about 3.52V. I first had a MPX5114A connected and
couldn't get anything but  '3F' so to test it I have a 5K pot connected
between 5V and ground with the wiper connected to AN0. I have a 0.1uF
cap to ground to prevent noise. If I vary the pot from 0V upwards, the
reading starts at '00' and increases till the input voltage reaches
3.52V where it reads '3F'. I tried setting the ADCON1 register so that
(PCFG0:3 =3D '1111') an external reference is used and physically
connected Vdd and Vss to AN3 and AN2 respectively and in this
configuration it works. I have read the documentation through a few
times and can find no reference to Vref/2 or something like that and a
friend who also uses the PIC has done the exact same thing but with a
16Fxxx chip and has no problem. Is there something that I am missing or
do I just have a defective chip (I hope not as I've used an SMD device)
I have searched the list for previous posts on this subject but haven't
found any. Please could anyone shed some light on this.
Regards
Mark
Capetown SA



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