Interview Question – BCD Digit, Multiplied by 5

A while back, someone sent me the interview question I am about to describe, asking for help. I think it serves a very good example of observing patterns and not rushing into conclusions.
I will immediately post the answer after describing the problem. However, I urge you to try and solve it on your own and see what you came up with. On we go with the question…

Design a circuit with minimum logic that receives a single digit, coded BCD (4 wires) and as an output gives you the result multiplied by 5 – also BCD coded (8 wires).

So, I hope you got a solution ready at hand and you didn’t cheat .

Let’s first make some order and present the input and required outputs in a table (always a good habit).

Looking for some patterns we can see that we actually don’t need any logic at all to solve this problem!!

You will be amazed how many people get stuck with a certain solution and believe it is the minimal one. Especially when the outcome is one or two single gates. When you tell them it can be done with less, they will easily find the solution. IMHO there is nothing really clever or sophisticated about this problem, but it demonstrates beautifully how it is sometimes hard for us to escape our initial ideas and approaches about a problem.

Coming to think of it, this post was more about psychology and problem solving than digital design – please forgive…

A Coding Tip for Multi Clock Domain Designs

Multi clock domain designs are always interesting, but almost always hide some synchronization problems, which are not that trivial. There are tools on the market that identify all(??) clock domain crossings within a design. I personally had no experience with them, so I can’t give an opinion (although I heard some unflattering remarks from fellow engineers).

Seems like each company has its own ways of handling this problem. One of the oldest, easiest and IMHO one of the most efficient ways, is to keep strict naming guidelines for your signals, whether combinatorial or sequential !!

The most common way is to add a prefix to each signal which describes its driver clock e.g. clk_800_mux_32to1_out or clk_666_redge_acknowledge.

If you don’t use this simple technique, you won’t believe how useful it is. Many of the related problems of synchronization are actually discovered during the coding process itself. Moreover, it even makes life easier when doing the code review.

If you have more tips on naming convention guidelines for signals in RTL – post them as a comment!