In setting up the K8055 boards from our most recent shipment we have found a few boards where the ATT1 pot does not get up to the 255 maximum setting. On those boards, if we work the pot several times across the full range they eventially allow us to reach the 255 setting.
As we use the K8055 with both the ATT1 and ATT2 pots at full scale perhaps we should be putting a fixed resistance, certainly in the place of the ATT1 pot. We have not had any problems getting the ATT2 pot to the 255.
To find out the reason to this problem you may turn both pots ATT1 and ATT2 to the maximum position and then measure the voltage on pin 2 and on pin 3 of the microcontroller IC3.
If the voltages are different then the reason is the pot or the circuitry associated to it.
If both voltages are equal then the reason is the difference between the analog input channels of the microcontroller.
[quote]On those boards, if we work the pot several times across the full range they eventially allow us to reach the 255 setting.[/quote]This may indicate there is some remaining resistance in the pot when turned to the maximum position.
If your application requires no need to adjust the input gain and a high stability is required, then you may replace the pots with fixed resistors of 100k.