Real World Examples #4 – More on “Thinking Hardware”

I was reviewing some code not so long ago, and noticed together with the owner of the code, that we had some timing problems.
Part of the code looked something like that (Verilog):


wire [127:0] a;
wire [127:0] b;
wire [127:0] c;
assign c = select_register ? a : b;

For those not familiar with Verilog syntax, the code describes a MUX construct using the ternary operator. The two data inputs for the MUX are “a” and “b” and the select is “select_register”.

So why was this code translated into a relatively slow design? The answer is in the width of the signals. The code actually synthesizes to 128 parallel MUX structures. The “select_register” has actually 128 loads.
When a construct like this is hidden within a large code, our tendency is to just neglect it by saying it is “only” 2:1 MUX deep, but we have to look more carefully than that – and always remember to consider the load.

Solving this problem is relatively easy by replication. Just creating more versions of the “select_register” helped significantly.

A Message for the New Year

Holiday season is gone, the new year is just starting and I am into preaching mood.

I get many, many emails from people asking me to help them with their designs, interview questions or just give advice. Sometimes, if I am not fast enough in replying, I even get complains and emails urging me to supply the answer “ASAP”. This is all OK and nice, but I would like you the reader to stop for a second and think on how much YOU are contributing to our community?

Not everyone can or likes to write a technical blog, but there are other options one can utilize – one of my favorites is posting on a forum. Even if you are a beginner in the field, post your questions, this is already a big help for many. I personally post from time to time on EDA board. Just go through that forum and have a quick look, some questions are very interesting while others can be extremely stupid (sorry) – who cares! What matters in my eyes, is that the forum is building a database of questions and answers that can help you and others.

I assume that most of my readers are on the passive side of things (just a hunch). I hope this post will make you open an account on one of the forums and start posting.

p.s. please use the comments section to recommend your favorite design related forums or groups.