Hi Boris,
I could do that or several other things including buffers etc, however I find this A bit contradictory as the atmega does not function as per the best practice described in the interfacing section see http://cq.cx/interface.pl#3 it also does not behave the same as the PIC's as you would expect.

So I fell the best solution is to aline all the parts so they are consistent with each other and the interfacing best practice section.

David.

Hi thanks got all that, but I stall don't see why they need to be turned on in the first place as that thread is A negative result of the pullup.

To quote Jonathan "It's better not rely upon the on-chip pull-ups, since they are so weak."
Then I need to add my own pull up/down resistors anyway so I feel it is best to turn them off and let the user add there own resistors.

Can they be turned off or is there some programing issue I don't understand ?

David.

These are the problems.

first I can't use PIC not enough memory.

I am using for A PLC input +24vDC for high this is an industry standard so I am not changing that, so pull low does not work.

I arranged my circuit as per Jonathan's interfacing recommendations which does not work due to the pullup.

The circuits I am building are all modular so I have Analog in, Digital in, Digital out, serial interface and special out cards.

I have also designed some different types of processor boards.

The problems are the need for strong pull down to combat the pull up, This then requires A lower value of series resistor driving the micro, this intern reduces the protection from voltage spikes and other things that go bad.

The voltage that appears across the pull down resistor subtracts from the noise immunity of the input.

When changing from an AVR to PIC processor there needs to be component changes made to what is meant to be standard cards
David.

Hello Friend, I also work with industrial PLC. if I understand your problem, I have a suggestion. Design the circuit to work with positive voltage input (+24) exciting an optical coupling, which would lead to negative potential pin of the microcontroller (GND). Then the pull-up resistors are no longer a problem to be essential.

Hi David R . This is the schematic of hardware i use for years in industry enviroment and is ok . you need to set the CKOPT bit on Atmega to function normaly in noisy enviroment .

(por Gérard)

Resistances can be active or deactivated, low level or high level, when the program is done.
Explanation; When we place a new X contact, a box will appear, in that box, we will mark it if you want pulup to be activated, if the box is not market, the resistors will be deactivated.

Thank you for answering the solution, and I'm glad it was fixed.

Gracias por contestar la solución, y me alegro que se haya solucionado.