[Fpga-synth] Very Odd Error Message from WebPACK ISE

[Eric Brombaugh]

Scott Gravenhorst wrote:

> Quite honestly - it started when I got the thing to compile and make sound.  Until then, there were
> issues with the code that either prevented it from finishing the compile or it wouldn't make sound.
>  I noticed the error after I got sound out of it and honestly hadn't paid much attention to the
> summary display.  I use a set of scripts to process .syr and other files that give me up-front
> information about really bad things I may have done.  And of course, this is the only copy of the
> source, I don't have previous nonworking attempts archived.
> 
> Thus I shall plod and slog as usual until the beast is revealed...

I've had pretty good luck in searching for issues by commenting out 
large blocks of logic until the problem disappears. Bear in mind that 
this means:

* Problem is in the deleted logic
* Problem is in the interface between deleted and surrounding logic
* Problem is resource or overall size related.

> BTW, this is for a Spartan-3A DSP 1800.

Yup. Which means you need to be careful about the way your using the 
DSP48 blocks. XST is pretty smart about inferring them, but sometimes 
makes a real mess of it. These new blocks require more I/O pipeline 
registers than the older MULT18x18 blocks, and if you don't explicitly 
provide them in your code, XST will try to borrow them from upstream & 
downstream logic with often hilarious results.

If you're instantiating then you don't need to be concerned about this. 
I typically don't instantiate though, because it's easier to maintain 
the code, understand the functions and port to new architectures.

Eric