Ensuring signal integrity in high speed flash memory systems - Embedded.com

Ensuring signal integrity in high speed flash memory systems

Signal integrity will matter more in next-generationflash devices, and several changes in thetechnology will make managing signal integritymore critical. For example, data rates in these deviceswill range from 400 MHz to 6 GHz. To supportthe faster data rates, edge rates will have tobecome 10 to 100 times faster. Demand for increased storagecapacity will also drive the need for denser packaging and morecomplex interconnects. Pressure to reduce costs will force youto make trade-offs that affect signal integrity, such as using lower-cost materials or even eliminating the use of ground planes.As rise times become shorter, a signal’s high-frequencycomponents become more pronounced. Higher-frequencysignals are more sensitive to interconnect quality, so signal-integrityproblems tend to proliferate. As signal frequenciesincrease, signal loss also increases. Therefore, high-frequencycomponents of fast-rise-time signals experience more lossthan the low-frequency components, leading to signal distortionand ISI (intersymbol interference).

Signal-integrity problems

Understanding the causes of signal-integrity problems andtheir remedies is critical. Signal-integrity problems includereflections and distortions, crosstalk, ground bounce, and jitter.Reflections and distortions relate to signal quality on anindividual net. When a signal encounters an impedance discontinuity,it generates a reflection that becomes further distortedas it continues along the net. The reflection travelsfrom the impedance discontinuity in two directions—towardthe receiver and back to the driver. The reflections themselvesreact to other discontinuities, creating further reflectionsthat distort the true signal in complex ways, generatingeffects such as ringing, overshoot, and slope reversal. Carefuldesign to maintain well-controlled impedance along keytraces is the best way to improve signal integrity.

Managing signal integrity in tomorrow’s high-speed flash-memory-system designs figure 1Crosstalk-induced signal-integrity problems involve multiplesignal nets. If you place an active net near a quiet one,capacitive and inductive coupling can cause some of the energyfrom the signal on the active net to couple over to thequiet side (Figure 1 ).

The quality of grounding and current-return paths in yourdesign is among the biggest factors affecting crosstalk. In mostPCB (printed-circuit-board) designs, ground planes are availableas return paths. This approach is best if you can affordthe extra plane. If cost pressure forces you to eliminate using aground plane, you must use other strategies, such as placing aground trace next to the signal or using differential instead ofsingle-ended signaling.

Ground bounce also affects signal integrity and relates topower distribution. As with any network that has interconnects,inductance exists in power and ground networks. Asthe I/O signals transition from zero to one or one to zero, transientcurrent flows in the power-distribution network. Manysignals’ switching at once generates large transient currents.Any inductance or resistance in the power- and ground-distributionnetwork converts these transient currents into voltagespikes that appear as noise in other signals or even as a shiftin the ground voltage. Ground planes or multiple ground orpower connections reduce the impedance and therefore theSSO (simultaneous-switching-output) noise. Using lowervoltage swings and protocols that minimize the number of signaltransitions also helps.

Managing signal integrity in tomorrow’s high-speed  flash-memory-system designs figure 2Jitter issues also affect signal integrity. Reflections, crosstalk,and SSO all can contribute to jitter. In addition, ISI created on lossy channels, PLL (phase-locked-loop) noise, EMI (electromagneticinterference), differences in transmitting and receivingthreshold voltages, and ordinary delay mismatches ininternal logic can generate jitter. The strategy for managing jitterdiffers, depending on the cause of the jitter. Proper shieldingcan help with EMI-induced jitter, but it cannot fix a noisypower supply. Knowing whether the jitter is random, periodic,or correlated to some other event in the system helps you determinethe best ways to address the problem (Figure 2 ).

Manage signal integrity in your designs

As every RF engineer knows, everything in a circuit canaffect the signal. To manage signal integrity, it is critically importantto first identify the parts of the design that affect signalintegrity. A common approach is to start by creating amodel of your design and its components and interconnects.However, a model typically is less accurate than it needs tobe. You must make measurements of your circuits, comparethem with your model, and adjust the model to make it consistentwith your measurements. Once the model is accurate,you can use the simulator to predict which changes will improvesignal integrity. Pay particular attention to vias, wirebonds, packages, PCB traces, and connectors when consideringcomponents that will affect signal integrity.

The goal of simulation and modeling is to predict the real-worldbehavior of your design. Engineers have traditionally used modern design and simulation environments, such asAgilent’s ADS (Advanced Design System), for microwaveand RF design. As digital speeds approach microwave speeds,engineers have been applying these tools to digital design, especiallyfor evaluating signal integrity. These tools accuratelysimulate high-speed effects, such as distortion, mismatch, andcrosstalk, in your channels.

Managing signal integrity in tomorrow’s high-speed flash-memory-system designs figure 3Integration of system, circuit, and EM (electromagnetic)simulators provides accurate answers and avoids error-proneand time-consuming data transfer among point tools (Figure3 ). The ADS model in the figure can assess signal integrityand predict eye diagrams at various nodes in the backplane.You can see the eye after the daughtercard, at the high-speedbackplane connector, and after the backplane traces. Fromthese eye diagrams, you can determine where signal-integrityproblems caused the eye to degrade and at what point youshould modify the design. An accurate model such as thisone gives you insight into your design and lets you rapidlyevaluate changes that will improve its performance.

Physical-measurement tools

Making physical measurements is key to assuring the accuracyof your model and validating the final performance ofyour design. At the speeds of next-generation flash design, itis important to analyze the data in both the time domain andthe frequency domain. You can make physical measurements with a time-domain instrument, such as a TDR (time-domainreflectometer), or a frequency-domain instrument, such as aVNA (vector network analyzer).

Managing signal integrity in tomorrow’s high-speed flash-memory-system designs figure 4

If you’re new to measuring interconnect behavior, you maywant to consider starting with a TDR, such as the Agilent54754A differential and single-ended TDR module. It providesan intuitive waveform and good first-pass model. Forgreater accuracy and higher bandwidth, you’ll need to learnS parameters and use a VNA, such as the Agilent N5241A.Another possibility is to use tools such as Agilent’s PLTS(physical-layer test system). With the PLTS, you can work ineither the frequency domain or the time domain, whicheverbest suits your requirements, regardless of whether you are usinga TDR or a VNA (Figure 4 ).

Managing signal integrity in tomorrow’s high-speed flash-memory-system designs figure 5As the signal travels through the physical structures, theTDR measures and displays impedance. Ideally, the impedancetrace is a flat line, indicating no discontinuities. At the daughtercardvia, you can see a large discontinuity, so you know youmust modify your design to raise its impedance.

Managing interconnect design on PCBs

Active circuitry, packaging, and connectors all influencesignal integrity; PCBs can also influencesignal integrity, often in difficult-to-detect ways. Competing demandsfor high speed and low cost often collideon the PCB. For example, FR4 islow cost and has relatively good performanceat lower data rates. However,when data rates exceed 10 Gbps,problems increase dramatically. Figure5 should help you identify whichelements of your design might be causingproblems and suggests ways to dealwith those problems.The next generation of flash willenable next-generation performance,but next-generation speeds will requiredesigners to pay increased attentionto signal integrity. Understandingwhat can affect signal integrity andhow to model and measure it will helpyou deliver reliable, high-performanceproducts on time and on budget.


This article has also been published on EDN. http://www.edn.com/design/test-and-measurement/4363955/Managing-signal-integrity-in-tomorrow-s-high-speed-flash-memory-system-designs-item-2

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