Issue 160 November 2003
Timing (Analysis) is Everything
A How-To Guide for Timing Analysis

TIMING DIAGRAMMER PRO

Performing a timing analysis using Timing Diagrammer Pro is straightforward. First, you need to draw in the waveforms. Initially, there are no delays or constraints; you don’t have to be accurate at first, because the accuracy comes when you add the delays and constraints.

Second, you need to add the delay and constraint information to the waveforms; this will automatically add the delays and constraints to the parameter spreadsheet. Then, enter the exact minimum and maximum numbers for the delays and constraints in the parameter spreadsheet. Doing so automatically updates the view in the diagram editor window and shows the areas of timing uncertainty.

Timing Diagrammer Pro allows you to use libraries of timing values for parts. Thus, you can reuse some of the work that you’ve (or someone else) done already.  

DESIGN SEQUENCE

You now know how to use Timing Diagrammer Pro to perform timing analysis, but how can you use the tool in a real project? The following sequence has worked well for my team for a number of design projects.

First, capture the interface specifications in Timing Diagrammer Pro with all of the timing constraints shown. An interface is any part of the design that interacts with another part, such as a write cycle from a microprocessor to a memory and a connection to a PCI bus. These interface specifications form the basis for subsequent design decisions; they may give the designers an early indication as to whether the design is feasible, impossible, or sheer lunacy. For instance, if the interface specifications dictate that you will have to use a 34-ps SRAM, you’ll probably try to get on another design project!

As the design progresses, put real timing numbers into Timing Diagrammer Pro, which will immediately tell you if the constraints are still met. At some point, there may be more than one option. Using this tool, you can model each of the possible solutions to determine if they work. If they do not work, then redesign, whether that means simply changing to a faster component or a completely new circuit. An alternative might be to change the original requirements, assuming that your customer allows you to do so. Don’t count on it!

When it comes time to review your design prior to building your PCB, for instance, if the Timing Diagrammer Pro files are up to date, you have proof that your design will meet the timing requirements. Note that if your circuit has critical timing paths, you may want to include PCB delays in the timing analysis as well.