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Ved 10-27-2007 02:30 PM

Selecting I/O pins
 
Hi,
I was recently asked in an interview that what points you will
consider while selecting the I/O pins in FPGA ?

One thing which came to my mind was :
1) considering board aspect that the pins corresponding to peripherals
should near to that peripheral so as to minimize the routing delay
in PCB.

2) pin should be compatible to the I/O type of peripheral device, i.e.
LV_TTL, LC_CMOS etc.

Were my points valid ?
Any other points are welcome.

Regards


John Retta 10-27-2007 04:33 PM

Re: Selecting I/O pins
 
Hi -

[3] Some I/O pins are
- Input Only
- Lack IOB FFs
- Dedicated clk input
- For DRAM interfacing there may be specific pins reserved for
specific signals

[4] In some families (Spartan 3/3E), only 2.5 V Differential signaling
supported, not 3.3 V. The important corallarys are : Banks
which contain dfferential signaling must have VCO tied to 2.5, and
this precludes single ended 3.3V signals from being used in that bank.

Sometimes the subtleties of IO are overwelming, and vary slightly from
family to family. One thing I try to do before a pwb is sent out for the
first time, is to code the I/O for the final pin selection, and synthesize.
This is pretty easy, will be the first thing done when coding anyway, AND
allows you to view the generated report files for any possible bad. or
more likely, non-optimal pin assignments.

There is a section in the .mrp file and in the .pad file which should be
reviewed to ensure what you want is what you got.

--
Regards,
John Retta
Owner and Designer
Retta Technical Consulting Inc.

Colorado Based Xilinx Consultant

email : [email protected]
web : www.rtc-inc.com


"Ved" <[email protected]> wrote in message
news:[email protected] oups.com...
> Hi,
> I was recently asked in an interview that what points you will
> consider while selecting the I/O pins in FPGA ?
>
> One thing which came to my mind was :
> 1) considering board aspect that the pins corresponding to peripherals
> should near to that peripheral so as to minimize the routing delay
> in PCB.
>
> 2) pin should be compatible to the I/O type of peripheral device, i.e.
> LV_TTL, LC_CMOS etc.
>
> Were my points valid ?
> Any other points are welcome.
>
> Regards
>




David Spencer 10-27-2007 05:53 PM

Re: Selecting I/O pins
 

> Any other points are welcome.


I would have mentioned something about in an ideal world making sure you can
achieve timing closure before committing to a PCB. These days, the effect of
I/O location on design performance is much, much less significant than it
was, say, ten years ago - largely because of better and faster on-chip
routing, but if you are pushing the limits then the I/O selection can make
all the difference.

Of course, since it was an interview question, the "right" answer would
probably some talked about working as a team with the PCB layout people and
choosing a pin-out that is acceptable to both groups.




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