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Old 10-06-2003, 05:48 PM
Panic
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Default Design question (Working with Altera EPXA1F484C1)


I have a question about how I best should implement my I/O
in an FPGA (Altera EPXA1F484C1) . Being a student, and
not having a whole lot of people to ask for advice, I have no
idea how I best should solve my problem.

I initially wanted to have 128 + a few inputs and 128 + a few
output pins, so that I could work in parallell on 128 bit blocks.

This didn't work, apparently because I had used to many pins.
Quartus (II 3.0) gave said in it's report that I had only used 258
of in all 292 available pins, but I suppose I had used all available
general purpos I/O-pins or whatever. I scaled down my design,
until I got to compile it. At that time I was using 186 pins. And
I when looked at the floorplan, every row and column I/O-pin
was in use. So I guess 186 pins is the maximum limit.

So my first question is: Why can I only use 186 pins, when Quartus
tells me that there are 292 pins available?

So I had to decise how I would send and recieve my data. And this
is my main question, my design question:

Would I be best off using say 64 pins for input and 64 pins for output,
and solve my problem that way, or could I use a bidirectional solution,
where I alternate on recieving and sending data over a 128 bits bus?

I put together a test-design wich utilizes this solution, and it compiled
fine.
I put an image of the schematics on a website, so you can take a look:
http://www.battlefield.no/bilder/EPXA1_PinTest3.gif

But is this an sound solution?! Or should I rather go for separat in- and
out-pins? Since I have no clue, I have to ask someone. And you guys
drew the shortest straw ;-)

-"Panic"


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  #2 (permalink)  
Old 10-06-2003, 11:11 PM
H. Peter Anvin
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Default Re: Design question (Working with Altera EPXA1F484C1)

Followup to: <[email protected]>
By author: "Panic" <[email protected]>
In newsgroup: comp.arch.fpga
>
> So I had to decise how I would send and recieve my data. And this
> is my main question, my design question:
>
> Would I be best off using say 64 pins for input and 64 pins for output,
> and solve my problem that way, or could I use a bidirectional solution,
> where I alternate on recieving and sending data over a 128 bits bus?
>


Impossible to tell without knowing what your application is, i.e. what
"SubBlock" contains. Typically, a bidirectional bus will provide
higher throughput in any one direction, but it costs time to turn
around. Thus, if you are continuously processing data (let's say that
you're doing an AES stream encryptor), you'd be better off with
separate input and output busses, even at half width, but if you will
be communicating in one direction at any one time then one
bidirectional bus will serve you better.

Also, in the case of the split bus solution, consider whether or not
it is practical in your application to double-clock the 64-bit busses.

-hpa




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