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TEST & MEASUREMENT Fig. 1: DC drifts without 8b/10b encoding. Fig. 2: 8b/10b coding maintains DC level and ensures clock mands. the protocol trigger and decode software can be used recovery. to convert the waveform data into a binary format by recover- ing the clock first and comparing the voltages with a user-de- First, the coding of the 8 bit word to 10b symbol will change fined threshold and some hysteresis. A block diagram of how at the speed of the data rate and would require an adjust- this software works is shown in figure 3, with results shown in ment of the symbol rate in the trigger memory to the same figure 4. speed. second, triggering on the right 8b/10b symbol requires as shown, there are two tables that list the characters and synchronization or alignment of the 8b/10b codes to the data the protocol. the protocol is correlated to characters and to 0s stream. For real time triggering the hardware must be ca- and 1s in the acquired waveform. this makes is easy to track pable to synchronize to one of the “comma symbols” (K.28.1, errors in the protocol down to the physical layer. the displayed K.28.5, and K.28.7) which are unique and cannot be found in waveform in figure 5 can be useful to understand why wrong 0s the data stream at any bit position in the code. the synchro- and 1s have been possibly misinterpreted by the receiver. cur- nization character can be somewhere in the data stream and sors and the actual zoom window can be synchronized with the might be very infrequent or appear only once. one example scope waveform display and can helpful in locating the cause for a synchronization character is the comma symbol, K28.5 of the protocol error. (011110101). once the alignment symbol has been found, the decoding of the subsequent symbol values can proceed. Capturing specific data values Searching for a specific character in the protocol table is a software “triggering” solutions actually perform a search common method to locate protocol errors in the data stream. through the acquired data and therefore have long dead times But searching is a post-acquisition process and is limited to that cause very large gaps between the acquisitions and in- a time frame that is set by the size of the acquisition memory. crease the chances of missing the character in question. the dead time between acquisitions is quite large and is Many higher end oscilloscopes are equipped with a dedi- caused by oscilloscope and the processing time of the soft- cated trigger chip for triggering on 8b/10b data patterns in ware for interpreting the waveform into binary and then search- high speed serial signals up to 6.25 Gb/s. This enables the ing for the character. This is illustrated in figure 6. instrument to find rare events since it is now able to trigger on therefore the chance of capturing any infrequent and rare 8b/10b characters. characters are acronyms for a pattern of 10 faults is very low. For example if there are 10 million points bits of the 8b/10b code, i.e. D31.6 or K28.5. A second option sampled at 50 gs/s, the real time acquisition will stop after 200 related to a high speed serial standard’s protocol is triggering microseconds. But it will take hundreds of milliseconds before on 8b/10b words (commands), where words consist of multiple the system can capture the next block of 10 million points. characters (commonly 4 words or 40 bits). It should be noted Larger memory will even increase the problem. To find rare that every standard has its own word definitions. events it is necessary to trigger on those faults. a powerful debugging tool is the ability to trigger on 8b/10b code errors. no serial trigger would be able to trigger on all Most digital oscilloscopes provide a large portfolio of trigger- possible character errors, disparity errors or losses of byte ing capabilities. traditionally, troubleshooting is related to time synchronization, but it is usually possible to trigger on common and level qualified triggering. With the advanced trigger modes errors such as disparity or character errors. available, triggering on glitches, transitions, runts and so forth is much easier than in the past. For protocol errors, it can be Network element delay mea surements useful to trigger on commands, characters, or bit sequences. triggering on 8b/10b serial patterns can be used for measur- Unfortunately a serial trigger circuit designed for nrZ patterns ing the time delay of an active network element. one might cannot find those faults because most high speed serial data think this an easy task to solve even without special triggering signals are 8b/10b coded and require a dedicated hardware on 8b/10b. But it can be challenging when it’s necessary to solution. a standard nrZ trigger cannot trigger on words of measure the time delay under real conditions. the setup for 8b/10b coded data streams for two reasons. this measurement is shown in figure 7. The input signal of the Fig. 3: Conversion of waveform data into protocol. 16 Electronic Engineering Times Europe January 2013 www.electronics-eetimes.com


EETE JANUARY 2013
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