Register Ring

I want to start in on some sequential logic. We have a few combinational modules that we can build on already, so the est thing would be to make a sequential-only circuit. Once we’ve clarified how the concept works, we can add in combinational logic in.

By ‘sequential-only’, I am referring to a setup with only registers, it just passes it’s initial input in a loop forever, not changing it. I’m hoping it’ll help me with the concept a bit more, and flush out any issues with the registers.

Here’s a diagram of a three stage loop, just imagine the outputs loop back (drawing it would be messy):

ring.png

Runtime Behavior

Let the initial state of the first stage’s outputs be DATA, with the first stage requesting NULL. The second and third stages are outputting NULL, and requesting DATA. Now let red be DATA, and blue be NULL.Slide1

After the gate delay for the register, the DATA wave is passed, and a request for NULL is sent back.

Slide2Slide3Slide4Slide5Slide6

And, we’re back where we started

Slide7

The ring will continue indefinitely. The VHDL source is available here, though without the test script, all stages remain at NULL, requesting DATA. I simulated this, and t turns out that it’s harder to see the pattern in graph form. To make things easier, I raised the number of stages.

Capture

At 8 stages, I start to be able to see it clearly as distinct wavefronts going through the pipeline. To make it really obvious, ramp it up to 12.

Capture

Not all stages shown.

And finally, if set at three, the pattern is harder to see, but it’s there. In the 3-stage case, the time spent requesting NULL and requesting DATA is the same for each stage.

Capture

If you use 2 stages, the system locks. A slideshow version of the ring pictures from above. As you can see, the transition to NULL only occurs while there are 2 DATA stages, and the transition to DATA only occurs while there are 2 NULL stages. This is so that no wavefront is ever ‘overwritten’. The second instance of that state saves the value.

This slideshow requires JavaScript.

One Reply to “Register Ring”

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s