Puzzle #3

OK, you seem to like them so here is another puzzle/interview question.

In the diagram below both X and Y are n-bit wide registers. With each clock cycle you could select a bit-wise basic operation between X and Y and load it to either X or Y, while the other register keeps its value.
The problem is to exchange the contents of X and Y. Describe the values of the “select logic op” and “load XnotY” signals for each clock cycle.

Big Chips – Some Low Power Considerations

As designers, especially ones who only code in HDL, we don’t normally take into account the physical size of the chip we are working on. There are many effects which surface only past the synthesis stage and when approaching the layout.

As usual, let’s look at an example. Consider the situation described on the diagram below.

Imagine that block A and B are located physically far from one another, and could not be placed closer to one another. If the speeds we are dealing with are relatively high, it may very well be that the flight time of the signals from one side of the chip to another, already becomes too critical and even a flop to flop connection without any logic in between will violate setup requirements!
Now, imagine as depicted that many signals are sent across the chip. If you need to pipeline, you would need to pipeline a lot of parallel lines. This may result in a lot of extra flip-flops. Moreover, your layout tool will have to put in a lot of buffers to keep sharp edged signals. From architectural point of view, decoding globally may sound attractive at first, since you only need to do it once but can lead to a very power hungry architecture.

The alternative is to send as less long lines as possible across the chip, As depicted below.

With this architecture block B decodes the logic locally. If the lines sent to block B, need also to be spread all over the chip, we definitely pay in duplicating the logic for each target block.

There is no strict criteria to decide when to take the former or the latter architectures, as there is no definite crossover point. I believe this is more of a feeling and experience thing. It is just important to have this in mind when working on large designs.

Synchronization of Buses

I know, I know, it is common knowledge that we never synchronize a bus. The reason being the uncertainty of when and how the meta-stability is resolved. You can read more about it in one of my previous posts.

A cool exception of when bus synchronization would be safe, is when you guarantee that:

1. On the sender side, one bit only changes at a time – Gray code like behavior
2. On the receiver (synchronized bus) side, the sampling clock is fast enough to allow only a single bus change

Just remember that both conditions must be fulfilled.

It is important to note that this can still be dangerous when the sender and receiver have the same frequency but phase is drifting! why???

Are there any other esoteric cases where one could synchronize a bus? comments are welcome!