Dr. Howard Johnson last public seminar

Ask For It

We digital engineers aren't getting the information we need to design quality products. Specifically, most ICs today lack guaranteed specifications for the minimum rise and fall times. This specification is absolutely critical to proper functioning of digital hardware. The minimum rise and fall time determines the worst-case conditions for crosstalk, ringing, SSN (simultaneous switching noise), EMI, and power-supply noise. In short, it affects every aspect of high-speed engineering. Yet, the specification of minimum rise and fall times remains, for most chips, unfulfilled.

Chip manufacturers shy away from guaranteeing minimum rise- and fall-time specifications for two reasons. First, such guarantees limit their flexibility. A chip vendor, you see, wants to retain the right to shrink the die at any time, for any reason, without warning. Shrinking the die is one of the primary means of reducing product costs and thereby maintaining margins in an atmosphere of falling product prices.

Shrinking the die helps reduce costs because the cost of producing the new chips relates mostly to the number of silicon wafers processed. Reducing the feature size of all the circuits within a chip allows the manufacturer to fabricate more chips on each wafer. Unfortunately, shrinking a die affects the rise and fall times, sometimes dramatically reducing them. If a manufacturer guaranteed minimum rise and fall times, it might restrict its ability to use the newest, smallest, most cost-effective lithographic dimensions.

The second reason for not guaranteeing the minimum rise and fall times really floors me.

As a highly placed marketing manager at a major chip manufacturer puts it, "Why guarantee something your customers aren't asking for?" This manager is repeating the same industry line I've heard over and over at shows, conferences, and behind the scenes. Chip manufacturers aren't hearing the call for minimum rise- and fall-time specifications.

Note that I didn't say we digital folks aren't asking for minimum specifications. I said the chip people aren't hearing us. It's time to ask louder.

Have you ever worked on a perfectly good product, something that's been in the field for years, only to find that some new batch of chips causes total product failure? You are not alone. Engineers all over the world are finding that new chips with faster rise and fall times often break old products designed in an era of relatively slow components. Engineers then need to redesign these old products to accommodate the faster components. For system-level products already in production, introducing a new version of an old chip can spell disaster. In some cases, especially for products with big backplanes, a completely backward-compatible redesign may be impossible. In simple terms, without a guarantee on the minimum rise and fall times, you have no guarantee that you will be able to continue to successfully manufacture your product.

From the perspective of a chip manufacturer, shrinking a die is a winning proposition, because the new chip is almost certain to meet or exceed its published specifications at a lower cost. From the perspective of a digital designer, shrinking a die in an existing product design can be a terrifying prospect, because the new rising and falling edges are almost certain to switch considerably faster.

If you work for a chip manufacturer, I understand your perspective and appreciate the importance of continuing to shrink dies. However, I would like you to consider taking one important step. Please make the process more transparent. Let your customers know in advance when you plan to shrink a die. Give them a last buying opportunity to purchase as many as they want of the old versions before you begin shipping them new ones. Remind them to check with engineering to make sure that the new products will work in old designs. These terms should be in every standard purchasing contract.

Given a choice, I always buy from a vendor that guarantees fair advance warning of a die shrink. Any company that has used a new batch of chips to build a few thousand dysfunctional products and then found out that old chips weren't available any more and had to shut down production while the engineers redesign the product will agree.

If you are a digital designer, I have a special request for you. Next time you talk to a chip vendor, please ask for the minimum rise-and fall-time specification. Let them know it's important. Let them know you need it. And, if you are planning to buy a lot of high-speed parts, let them know that you value working with a vendor that understands the importance of signal integrity in high-speed digital designs. If we all ask for it, maybe they'll listen.