It shouldn't give you any implementation. When a spec deviates from the what and starts filling in the how, it stops being a spec and becomes a half-assed plan. And a half-assed plan is worse than none at all.

I don't deal with low level hardware, so my experiences are different from yours, but I've done a lot of requirements gathering and spec writing myself, and I consider the hardest part of the job to be refining out the implementation crap that every armchair quarterback wants to throw into the mix.

When you get right down to it, you're writing a spec because you don't have the time, skill or inclination to do something yourself, so you're telling someone you trust to make competent decision in how to go about doing what you need. When you go from telling them what you need to telling them how to do their jobs, you have to question why you're writing a spec instead of doing it yourself.