VHDL: integers for synthesis?

Question

I'm a bit confused on if I should be using integers in VHDL for synthesis signals and ports, etc.

I use std_logic at top level ports, but internally I was using ranged integers all over the place. However, I've stumbled across a few references to people saying you should only use signed/unsigned for synthesis-targeted code.

I've gone and reworked my current project to use unsigned... and, well, it's noticeably uglier.

Is it a bad practice to use integers? What's the problem? Is there some uncertainty on what width the tool will map integers to?

Answer 1

Integers are fine in synthesis, I use them all the time.

I use std_logic at top level ports, but internally I was using ranged integers all over the place

That's fine!

Be aware:

• You are simulating first aren't you :) - Integer types don't automatically "roll-over" in simulation - it's an error to go out of the range you've specified for them. If you want roll-over behaviour, you have to code it explicitly.
• They are only specced to go from $-2^{32}-1$ to $+2^{31}-1$ (i.e. not quite the full range of a 32-bit integer, you can't use $-2^{32}$ and remain portable), which is a bit of a pain at times. If you need to use "big" numbers, you'll have to use unsigned and signed.
• If you don't constrain them, you can sometimes end up with 32-bit counters where less would do (if the synth and subsequent tools can't "see" that they could optimise bits away).

On the upside:

• They are much faster to simulate than unsigned/signed vectors
• They don't automatically roll-over in simulation (yes, it's in both lists :). This is handy - for example you get early warning that your counter is too small.

When you do use vector types, you are using ieee.numeric_std, not ieee.std_logic_arith aren't you?

I use integers where I can, but if I explicitly want "roll-over n-bit counters", I tend to use unsigned.

Answer 2

Jan Decaluwe wrote an entire white paper on the problems of integers versus bit vectors. I expect his answers would be to use integers whenever possible. http://www.jandecaluwe.com/hdldesign/counting.html