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Design007-Apr2020

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38 DESIGN007 MAGAZINE I APRIL 2020 ing to cost an estimated $235 each in quan- tities of 1,000 at a time. Again, most of the boards didn't work, and the few that did work failed EMI testing and were expensive. I looked at the board and realized what the problem was. I asked the engineer why he put that note on the schematic. And he said, "It was in the app note. I figured they knew what they were talking about." I asked, "Did you do a timing analysis?" He replied, "You know what management around here is like. We nev- er have time for things like that." I said, "Do you have time now to do a timing analysis?" The engineer came back to me a day later and said, "By my calculation, we have about ±200–300 picoseconds of available skew." I asked him, "Do you know how much that is in terms of length? It's somewhere between two and three inches. Let's even call it an inch. That means you could have made all these memory lines the same length ± 1inch, and they would have all worked." He exclaimed, "Bull. I don't believe it." I said, "Well, we are going to prove it to you!" We stripped away the routes, repositioned the components to make things route prop- erly, routed the board with no line matching at all, and checked them when we were done, and they were all within an inch of the same length. We did no serpentining at all. We made a 10-layer board out of it instead of an 18-lay- er board with six routing layers and four plane layers. It worked perfectly. It passed EMI testing, and the price dropped to $34 per board from $235 per board. That's what happens when people blindly follow app notes. DESIGN007 Rick Hartley is the principal engineer at R Hartley Enterprises. Figure 1: Close-up of the original 18-layer board showing an abundance of tromboning.

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