IO buffers

[Altera_Forum]

Hi guys i am doing synthesis at module level and IO buffers are being implemented by the tool.. 

I dont want these buffers to be implemented. Can anybody help me with the constraints for not inferring a buffer at input and output ports

[Altera_Forum]

Add the assignment Virtual Pin to the pins you don't want to go to I/O. I recommend minimally not doing this to the clock though, otherwise it gets placed internally and uses local routing to get to a global.

[Altera_Forum]

Hii thanks for the reply. 

But just wanted to know one thing like the output ports of my module are being buffered and are causing the delays for example the one in the image below...   So just clarifying that can i define my output ports as virtual??

[Altera_Forum]

Yes. 

The 2ns IO buffer delay will go away. I also wonder if that long IC delay will go away too. If that FF feeds a lot of I/O that are placed around the die, it will have terrible timing. That being said, you currently have a timing constraint on the I/O for it to be analyzed. I find it difficult to analyze I/O on a sub-module that won't be I/O once it's all hooked together. Your timing depends so much on what it drives and where it's placed, it's really hard to do correctly. I tend to leave that for integration(but try to make sure the connections are as short as possible, i.e. register the outputs)

[Altera_Forum]

Yes the flip flop does feed too many mux and mux only and then the mux out is being drived to the output and yes i am facing timing issues to a bigger extent. 

Actually the code wasnt written by me i have to do the synthesis only. Just tell me one thing can a path such as  Flop->mux->output a desirable logic. i doubt that since a combo logic is feeding the outputs it maybe a reason for larger delays. And ya the clock to the module i am constraining is 225Mhz. It will be helpful for me if u can comment on this. :)

[Altera_Forum]

I'm not following. I assumed this block really fed something else internally(another hierarchy). But it sounds like it's real? For 225MHz you almost have to push the register into the I/O cell, which means there will be no logic between the output reg and the IO.

[Altera_Forum]

Ok I get you. 

I will try duplicating the registers in order to reduce the fanout. And thanks for your suggestion you reduced my work a lot. :)