It seems that the quality of the clock source is very important. My clock generator is an oscillator made with two inverter buffers and capacitors and resistors. It works only if I take 600us or more to read the ADC ports. And that was quite an improvement. The original routine and circuit that I received would take around 200ms.

To test it I've made a small conector with a resistive ladder that I pluged in the ADC, conecting all the 8 ports in differents points of the ladder. I could read in the LCD 8 values from 00h to FFh, if the routine had a correct timing.

A wrong timing would present random values, usually the same value in all positions and, if the timing was "almost there", the byte flipping between the 1st and 8th positions.

The hands-on experience is the best teacher... I really don't understand when someone ask questions here and refuses to do some pratical experiences because they are "too simple". Before start walking and running we all crawled first.