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Get a Lock on Your Position

By David Kelly, Software Engineer -- Tektronix

When debugging a new design, one of the most useful capabilities on a modern digital oscilloscope, such as the MSO70000 series, is the ability to trigger on a symbol or bit pattern in a serial data signal. This is not so different from an edge trigger, except the scope triggers when it sees the user-defined pattern in the serial stream. The natural result is that the oscilloscope stabilizes the waveform on the display, allowing you to examine it in detail. If you have other channels connected, you can correlate the behavior of the signals to identify issues such as crosstalk or timing.

But what if the signal has excessive inter-symbol interference (ISI), bit errors, or other problems that prevent a serial pattern trigger from reliably finding the symbol you’re looking for? 

One alternative is to apply the Serial Position Lock trigger. This trigger may be used when you have a fixed length, repeating test pattern data signal, as is commonly specified in communication standards.  The clock signal may be on a second channel or recovered from the data signal. This trigger is standard on high performance Tektronix MSO and DSA oscilloscopes (optional on the DPO models), and is selectable from the trigger drop down menu under Serial Pattern Setup. Note that “Trigger On Lock” is selected on the menu.

To use the Position Lock trigger, you just specify the bit rate and pattern length (in bits) of the repeating signal, and the scope triggers on your signal, stabilizing the waveform. The neat part is that you can scan forward and back through the waveform by pressing the arrow buttons on the menu, viewing the updating waveforms in real-time at hardware speed. 

You can scan one bit at a time, or specify the “shift amount” on the menu to skip as many bits as you like. The screen shot below shows an example using a 6 Gb/s, 2640-bit repeating 8b10b-encoded CJTPAT signal with a 57-bit shift amount (about a screen full at the given settings).

Think of Position Lock Trigger as a page of Google results -- you can quickly scan through your waveform to find problem areas even when you’re not exactly sure what you’re looking for. To aid your search, you can also enable a decoder for the signal: the example uses an 8b10b Serial Bus setup in the Vertical Bus Setup drop down.

When you’re looking for clues to help solve a tough serial signal problem, the position lock trigger can be great place to start. Give it a try sometime. If you have worked with Position Lock Trigger, it would be great to hear from you in the comments section below.

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