where to place registers in VHDL modules

Question

I'm a software guy by trade and I have been dabbling in digital design on FPGA using the open source toolchain. I have made a few designs and generally understand how the handle verilog and VHDL.

One of the things I'm wondering is not really a black and white question but rather I'm wondering whether there is a "best practice".

I generally divide the designs I make into modules. For example I have a uart that I can include in a design. Now the transmit part of the uart has a feedback mechanism that indicates whether it is ready to receive another 8 bit word to transmit.

Now I could choose to make either the input or the output registered essentially forcing a pipeline. However this would have some impact on users of the design. If I register the input or the output the feedback is always delayed by 1 clock cycle. This is not so bad and might even be what you want. But If both input and output are registered the feedback is delayed by 2 clock cycles which might be confusing for users.

Is there some kind of best-practice to follow here. As in most of the time register your input or your output?

Any help is appreciated!

Answer 1

UART might not be the best example to drive this question, since the external activities associated with a UART are generally so slow relative to the other activities internal to the FPGA that an extra clock or two in a handshake hardly makes any difference at all.

However, in the high-speed video signal processing pipelines that I build (the bulk of my work these days, it seems), I generally register the outputs of each module. Then any modules receiving these signals can use combinatorial logic on them without worrying about excessive input delays, glitches, etc.

Inputs get registered only if they are known to be coming from a different clock domain or a completely asynchronous source, in which case, I have a library of CDC (clock domain crossing) modules that I use.