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EETE JANUARY 2013

TEST & MEASUREMENT Troubleshooting and verifying 8b/10b encoded signals with a real-time oscilloscope By Dean Miles Few serial technologies have become more widely this can indicate where to perform further measurements and adopted than 8b/10b coding, which is now used in standards suggest actions to solve the problem. if this does not solve like PCI-Express, Serial ATA, SAS, Fibre Channel, InfiniBand, the problem, the engineer can look at a composite of all data Firewire, MiPi M-PhY, hDMi, DisplayPort, ciPri, oBsai, XaUi, values and transitions on the bus using an eye diagram. UsB3.0 and others. therefore, any designer will eventually the eye diagram can show issues related to noise, jitter, and need the ability to efficiently analyze 8b/10b encoded signals signal integrity. it can also be used to check for violations of an using common instrumentation such a real-time oscilloscope. eye diagram mask which are specified in many industry stan- the intent of 8b/10b line coding is to achieve Dc balance dard compliance tests. any kind of degradation of the signal and provide enough state changes to ensure stable clock will cause less margin or more hits in the eye mask. this deg- recovery. since Dc balance is maintained, 8b/10b signals radation can indicate significant problems in the physical layer can be transmitted through transformers, optical channels or ac coupled links which have DC offsets at the pins of their integrated circuits. ac coupled data signals would have Dc drifts depending on the data content. a long sequence of 1s will lead into positive drift and many 0s will drift toward negative voltage, as shown in figure 1. Without correction it will cause errors at the receiver side since a fixed threshold is being com- pared to the drifting voltage level of the data signal. 8b/10b line coding will compensate for these effects by mapping 8 bits of data to 10 bit symbols (or characters). each 8 bit word corresponds to two 10b characters to ensure the long term ratio be- tween 1´s and 0´s is nearly 50 percent, as outlined in figure 2.The difference in numbers between 1s and 0s is called “running disparity” (rD) and it is either +1 or -1. therefore the encoding of one 8 bit data word (PhY) design. examples of signal integrity issues that can lead will change depending on the preceding symbol at the speed to mask test failures include slow signal rise time (bandwidth), of the data rate. small signal amplitude (attenuation), large overshoot (induc- with high-speed serial signals now delivering multiple tance), or large jitter and noise components such as cross talk gigabits per second, they require very high bandwidth in the and intersymbol interference (isi). physical layer for their links. one way to verify the performance of serial links is compliance testing. Usually compliance tests Debugging protocol errors are used for characterization at a final state of the design. If the issues with the PhY layer will often cause intermittent faults. compliance test passes everything is fine. If not, debugging of Usually PHY verification and protocol testing are done with the physical layer might become necessary. different test equipment and under different conditions using an A first step is often to look for measurements that are out oscilloscope. of range related to the appropriate standard’s specifications. To ensure best signal fidelity and highest timing resolution, the engineer should evaluate the link at the compliance test Dean Miles is a senior technical Marketing Manager at point with an oscilloscope and convert the acquired “analog” tektronix - www.tek.com waveform into binary values or even characters and com- www.electronics-eetimes.com Electronic Engineering Times Europe January 2013 15


EETE JANUARY 2013
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