The eye diagram is a very easy way for signal quality to be assessed in serial data. It is a standard analysis tool for high speed data signals such as PCI Express, SAS, SATA, etc. In serial buses such as I2C, LIN, CAN Bus or similar low speed buses it is far less common, but offers an easy way to assess the signal quality. In contrast to the high speed bus systems, the "Low Speed" serial standards transmit data in short bursts using blocks of a few bits of data at a time and, in addition, the clock is often a separate signal.
While it is possible to trigger the oscilloscope on these signals and create eye diagrams just using the oscilloscope’s persistence display this may not be exactly what is desired. In this mode there is no way to discriminate whether the data is from a single packet or even the same device on the bus. The ScanOverlay in LeCroy’s WaveScan search and scan analysis tools makes it very easy to create eye diagrams from low speed signals.
First, let’s take a look at how eye diagrams are generated.
The traditional eye diagram is formed by acquiring multiple segments and displaying them on a persistence display to show the history of all the acquired segments. Since each segment is acquired based on a trigger, the resulting eye diagram exhibits the effects of trigger jitter. A better way to form the eye diagram is to acquire a single long record containing all the data you wish to be in the eye diagram. If you now locate each unit interval (UI) then the long acquisition can be segmented and an eye diagram created.
WaveScan, LeCroy’s search and analysis tool, can be setup to detect each edge in a waveform using a measurement search mode based on the Time Interval Error (TIE) parameter. Note that TIE is included in the Jitter and Timing Analysis option package, JTA2. Using the virtual edge detection of the TIE parameter we can even detect missing edge locations in long, non-transitioning, data runs. If we search on these edges using WaveScan then we can create a series of segments, one for each edge location. WaveScan’s ScanOverlay trace overlaps these edges on a persistence display allowing us to create an eye diagram from a single acquisition. The setup for this analysis technique is shown in Figure 2.
In Figure 2 WaveScan has been setup to search based on the TIE parameter. Note that the TIE setup includes edges with both positive and negative slopes as well as including virtual edges. Every one of the signal edges, including the virtual edges, are detected in WaveScan and all these edges are added into the ScanOverlay, marked Sc in the figure. The time scale of the ScanOverlay is adjusted to view slightly more than one UI (1 μs) resulting in a classic eye diagram display.
Figures 3 and 4 show similar displays for both SPI and I2C serial data signals.
This is a useful technique that shows the flexibility of LeCroy’s WaveScan and its ScanOverlay trace function.