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Design007-June2019

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68 DESIGN007 MAGAZINE I JUNE 2019 mission lines. The easiest way is to use a re- sistive element. Termination can be made at the source or at the load. Series termination is excellent for point-to-point routes (such as data signals)—one load per net. It works well for traces that are electrically short and is also used to fanout multiple loads radially from a common source whereas parallel termina- tion, which has a resistance value equal to the transmission line impedance, is preferred for address buses. The impedance of the trace is extremely im- portant as any mismatch along the transmis- sion path will result in a reduction in signal quality and possibly the radiation of noise. Mismatched impedance causes signals to re- flect back and forth along the lines, which causes ringing at the load. This ringing reduc- es the dynamic range of the receiver, eats into the noise budget, and can cause false trigger- ing. In Figure 5, the attributes required to deter- mine the source impedance of the driver are extracted from an IBIS model IV curves. Then, the required series termination resistance is calculated based on a distributed system to match the transmission line for the selected layer in the iCD Stackup Planner. The final attribute that determines the prod- uct's reliability is AC impedance of the power distribution network (PDN). One needs to en- sure that the power planes and associated de- coupling can handle the high switching cur- rent demanded by the processor and memory devices. Inadequate power delivery can exhibit intermittent signal integrity issues. These in- clude high crosstalk and excessive emission of electromagnetic radiation, degrading the per- formance and reliability of the product. The PDN must accommodate variances of current transients with as little change in power sup- ply voltages as possible. So, the goal of PDN planning is to design a stable power source for all of the required power supplies. The same PDN connections (planes) used to transport high transient currents are also used to carry the return currents for critical signal transmission lines. If high-frequency switch- ing noise exists between these planes, cou- pling may occur, resulting in ground (supply) bounce, bit failure, or timing errors. Many of the failures to pass electromagnetic compli- Figure 5: Series terminator extracted from the IBIS model. (Source: iCD Termination Planner)

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