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16 DESIGN007 MAGAZINE I JULY 2021 A switchback is a 180° bend in a road, rail, or path, especially one leading up the side of a mountain. I used to enjoy driving the Great Ocean Road (Figure 1), which is a National Heritage-listed 243-km stretch of windy road along the southeastern coast of Australia. e tight serpentine bends certainly delay your journey but also make it a more enjoyable trip. Switchback also refers to a long trombone bend in a tuned serpentine trace. But rather than increase the delay of the signal, the switchback actually speeds it up due to the near (NEXT) and far-end (FEXT) crosstalk effects. In this month's column, I will look at why long, parallel switchbacks should be avoided. Designing a memory interface is all about timing closure. Each signal's timing needs to be Switchbacks in Tuned Routing compared to the related clock or strobe signal in such a way that the data can be captured on both the rising and falling edge of the strobe, hence the term double data rate (DDR). How- ever, the constant increase in data rate has made the timing margin associated with each rising and falling edge much tighter. To match the delay of critical signal timing, adjustments are required to the length of the individual sig- nals within a group. is is accomplished by adding serpentine (accordion) bends in the traces to decrease the velocity of the signal to match the longest delayed signal. However, the opposite occurs—the velocity of the signal is sped up by the serpentine. When an electromagnetic (EM) wave is guided by a serpentine trace, with coupling between the bends, there is an increase in the speed of the signal. at is, the EM wave negotiates the serpentine section faster than that of a straight trace of the same length. is acceleration is caused by crosstalk coupling (NEXT and FEXT) between the parallel trace segments of the serpentine traces. e amount of acceleration is directly proportional to the cou- pling strength between the bends and to the rise time of the signal. For long, coupled lengths (those longer than the critical length), signals may become distorted as they pass the serpentine section. Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: The Great Ocean Road in southern Australia.

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