RE: The Art of FPGA Design - Post 37

dubya — Sun, 31 Oct 2021 17:53:28 GMT

posting this comment on the new vhdl 2008 in Vivado 2021.2 announcement but my question is really about the prior 2018ish work.

In post 21 with the behavioral FIR model here:

A function "element()" is used in this context:

B<=ELEMENT(CI,K,N); -- register for the coefficient inputs

It takes a sfixed vector and returns an sfixed.

A few related questions:

1) I can't find this function defined in the other blogs. Maybe im missing it

2) I'm not sure why we need a function to index an array. Wouldn't CI(K) work? (Assuming vhdl 2008 support)

3) I don't understand the second argument "N". Why does indexing the vector need to know the length of the array of vectors?

Jancumps made a very useful gist here:

Of the types package. I looked there for a definition of Element () didn't find one.

Then I tried to write one myself and simulate it with xsim 2021.2. I get the hilarious error that I can't convert an sfixed vector to type sfixed.

That's funny to me because that's what the function is trying to do. Likely my poor understanding of the 3rd argument N has something to do with it.

Thanks,

Newbie