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Real World Examples #1 – DBI Bug Solution

January 7, 2008

In the previous post I presented the problem. If you haven’t read it, go back to it now cause it will make this entire explanation simpler.

Given the RTL code that was described, the synthesizer will generate something of this sort:

dbi_bug_sol1.png

A straight forward approach, to solve the problem, would be to try to generate the MSB of the addition logic and do the comparison on the 4-bit result. This logic cloud would (probably) be created if we would make the result vector to be 4-bit wide in the first place. It would look something like this:

dbi_bug_sol2.png

This looks nice on the paper, but press the pause button for a second and think – what is really hiding behind the MSB logic? You could probably re-use some of the addition logic already present, but you would have to do some digging in the layout netlist and make sure you got the right nets. On top of that, you would probably need to introduce some logic involving XORs (because of the nature of the addition). This is quite simple if you get to use any gate you wish, but it becomes complex when you got only NANDs and NORs available. It is possible from a logical point of view, but since you need to employ several spare cells, you might run into timing problems since the spare cells are spread all over and are not necessarily in the vicinity of your logic. Therefore, a solution with the least amount of gates is recommended!

So let’s rethink the problem. We know that the circuit works for 0-7 “1”s but fails only for the case of 8 “1”s. We also know that in that case the circuit behaves as if there were 0 “1”s. Remember we go 4 input NANDs and NORs to our disposal. We could take any 4 bits of the vector, AND them and OR them with the current result. It’s true, we do not identify 8 “1”s but in a case of 8 “1”s the AND result of any 4 bits will be high and together with the OR it will give the correct result. On other cases the output of this AND will be low and pass the correct result via the old circuit! There is a special case where there are exactly 4 bits on and these are the bits that are fed into our added AND gate, but in this case we have to anyway assert the DBI bit.
The above paragraph was relatively complicated so here is a picture to describe it:

dbi_bug_sol3.png

It is important to notice that with this solution, the newly introduced AND gate is driven directly from the flip-flops of the vector. This makes it much easier to locate in the layout netlist, since flip-flop names are not changed at all (or very slightly changed).

Here is the above circuit implemented with 4 input NAND gates only (marked in red). This is also the final solution that was implemented.

dbi_bug_sol4_fix.png

Closing words – this example is aimed to show that when doing ECOs one has to really put effort and try to look for the cheapest and simplest solution. Every gate counts, and a lot of tricks need to be used. This is also the true essence of our work, but let’s not get philosophical…

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