In recent columns I have identified the shortcomings of today's FPGA packages in terms of their origins and specifics. This month I'll talk about the ways in which these problems can be addressed from both a vendor and user perspective.

Since the vendors are responsible for these problems, it's best to start with what they can do to solve them. From a technical standpoint, there are three areas that can be addressed. First, for those devices that have programmable slew rate drivers, the edges can be slowed down. Second, the output drivers can be spread around the periphery of the device rather than clustered together. Third, the timing of the device can be staggered so that all members of a bus don't switch at the same time. Any one of the above actions will help to reduce simultaneous switching noise or Vcc and ground bounce.

Action listM

In some instances, depending on the severity of the problems occurring within the device, all three actions are required in order to reduce the problems enough to make the device usable. For better-designed parts, none or perhaps only one of the actions may be required. It should be noted, however, that for many current FPGA parts, none of the foregoing actions is a choice. So the burden is then placed on the engineer to determine whether the part will work in his or her design.

So, what must you as an engineer do when selecting a component? First, you must insist that the FPGA vendor show you a functional test circuit that is doing precisely what is intended in your design. If such a functional test circuit does not exist, you have to be prepared to build one yourself. Next, when you are presented with the application notes, you must insist on seeing the engineering work used to support the rules that are called out in the notes.

Finally, you should be suspicious of any application note that tells you to put ferrite beads in the power leads of a part. The use of ferrite beads degrades the performance of the power subsystem as well as the component itself. Even though this degradation has been demonstrated many times, some app notes still call for their use.

Lee Ritchey is the founder and president of Speeding Edge (www.speedingedge.com), a consulting firm specializing in the high-speed pc-board and system design disciplines.