What should I do with the unused IOs of CPLD which are physically connected to other devices (such ucontrollers address and data buses).

1- Should I declare them in the port list or not at all?

2a. If I declare them in the port list, what should they be... output? or input? and what to do with them after that?

2b. If I dont declare them in the port list, then should I enable "programmable grounds" on them? In this case I think they will be grounded which may cause a problem on the other device(ucontroller etc).

What is the best approach.?

I have gone through xapp ug445 but still confused what to do with these pins.

Regards

M. Adnan 

If you set the default action for unused pins to "float" then you don't need to connect

them in your circuit.  If you also have unused pins that are not connected to an

otherwise driven net however, you may want to ground those pins explicitly by

setting them as outputs and assigning them to zero.

Alternately you can set the default action to "programmable ground" unused pins,

but then all your signals that need to float will  need to be added to the project

and assigned to a high impedance state.

Finally if your external signals have sufficient drive capacity, you can just set the

default action for unused pins to pull up, which could work for all open or otherwise

unused pins.

Under no circumstances should you apply "programmable ground" to pins that are

driven by external logic.

HTH,

Gabor

Thanx gszakacs

My questions were realy clear but I could not get ur answer which is implicit with respect to my questions.

Anyways. ... where can I find an option to set unused pins to pullup? Is it available in Xilinx 9.1i foundation?

In my case these pins are going to be drived externally by other devices such as microconrollers etc.

Regards

M. Adnan

I guess it's been a while since I last used this family.  I was thinking of

Coolrunner II.  Your choices for the unused pins are "keeper" or "float".

This is called "I/O Pin Termination" in the properties for "fit".  Keepers

are actually good for most applications.  The one place I wouldn't recommend

it is if you have an open-drain signal with a pullup.  The keeper would fight

the pullup on the rising edge of the signal, causing you to need a smaller pullup resistor.

If you're worried about the keeper interfering with other logic, I would suggest to change

the option to "float" and then explicitly ground any I/O pin that is not used and not connected

to an active signal on the board.  Just don't check the box "Create Programmable GND Pins on Unused I/O".

HTH,

Gabor