Trunk Routing vs. Bus Routing: They’re the Same Thing, Right?
Recently, a discussion flared up in our office after someone questioned the use of the term ‘trunk routing’. They wondered whether it’s really a term in its own right – or just a fancy name for bus routing?
The question was raised by someone who’s not that familiar with our high-speed design tools – so we’ll let them off the hook! But it begs the question: do people really understand the difference between the two?
Pioneering the trunking concept
But before we get into the technical detail, here’s another question: did you know that Zuken invented the trunking concept for bus planning? We launched it a while back in 2005 with the trunking tool – but we often find that people think it originated with one of our competitors.
Back to the routing question
Bus planning with trunking, the detail is handled for you – Final routing is being done upfront as you go, which means the final plan will work. Various design scenarios can be seen here: layer exchange (3), lengthening in trunk end routing (4), obstacle avoidance (5) and trunk end routing (6).
So, to set the story straight: trunk routing allows you to handle different bus routes collectively as one entity. Trunk routes contain bus routes; they’re not the same thing.
Trunk routing comes into play much closer to the planning stage of the design process; whereas bus routing is the physical layout of the signals within the bus. Often involving detailed, complex routing to avoid obstacles and match lengths, bus routing is very close to the final routing patterns that will be used in the design.
“Trunking” refers to the notion of handling sets of signals, such as buses and differential pairs, as single routing entities. This mirrors the way you might treat such sets of signals as composites that need a high degree of routing symmetry to minimize skew and distortion.
Time and place
“Bus planning” is the early stage of laying out a PCB. It is a kind of channel routing where the designer will try to visualize paths between critical components. Often bus planning is tied in with component placement, and several iterations of component placement/bus planning are required before a suitable starting layout is found. You can think of bus planning as a “sketch” stage where the general outlines of a picture are drawn, then more detail can be filled in later.
Trunking allows the user to perform this sketch stage quickly and easily, by putting the bus in as one object which they can move around and adjust at will until the sketch is complete. However, one of the advantages of trunking over competitors’ bus planning tools is that underneath it’s calculating all of the detail for you automatically, so you know that the “sketch” will work as a final picture.
What this means for you, the PCB designer, is that you can quickly sketch routes, making it much quicker and easier to do up-front planning and manipulate a collection of signals as a one group.
Compare this with the competitors’ tools which have much more of a separation between the sketch and detail stage. This can mean that making slight changes in the sketch will have a much more dramatic effect on the detail – which you unfortunately don’t find out until later on.
How can I try this out?
Click the product names to go to web pages with more information.
By John Berrie and Paul Clewes, Zuken, 26 April 2007
This article was written by Kyle Miller from the development team at Zuken’s Technology Centre in Bristol, UK. He is an expert in CR-5000 Lightning and trunk routing technology. If you have any questions on this topic, please post a comment below and Kyle will get back to you.