Eye Scan Theory

The eye scan architecture can support different types of eye margin analysis, including the following:

• Statistical eye view: The eye scan block can count all data errors continuously over some period for a calculation of BER and generation of eye diagrams, as in the right side of RX Margin Analysis.
• Waveform view: Given a data pattern known to be prone to errors (or any other pattern of interest), the eye scan block can statistically determine voltage levels per bit for that pattern, allowing generation of analog waveforms for the recovered pattern.
• Scope view: Data collected by the eye scan block can be post-processed for the generation of standard scope displays such as derivative-based displays, as in the left side of RX Margin Analysis.
• Diagnostic mode: Under a variety of programmable trigger conditions, the instantaneous contents of the data buses are captured and available to be read out. This can be used, for example, to examine the pattern of burst errors due to DFE behavior.

RXDATA is recovered from the equalized differential waveform by sampling after the RX equalizer. The horizontal sampling position is determined by the CDR function and the vertical position is differential zero. This is indicated as data sample in RX Margin Analysis.

To enable eye scan functionality, an additional sampler is provided with programmable (horizontal and vertical) offsets from the data sample point. This is indicated as offset sample in RX Margin Analysis.

A single eye scan measurement consists of accumulating the number of data samples (sample count) and the number of times that the offset sample disagreed with the data sample (error count). The bit error ratio (BER) at the programmed vertical and horizontal offset is the ratio of the error count to the sample count. The sample count can range from tens of thousands to greater than 10¹⁴.

Repeating such BER measurements for the full array of horizontal and vertical offsets (or a subsampled set of offsets) produces a BER map as shown in RX Margin Analysis, commonly referred to as a statistical eye, where the color map represents log₁₀(BER). In this view, the eye is apparently smaller than a traditional oscilloscope view (as in RX Margin Analysis) because it has been closed by very low probability jitter and noise that does not show up in the much lower number of samples of an oscilloscope.

Because this functionality puts no restrictions on the data patterns being received nor requires any changes in the RX settings, it can be performed while application data is being received without error. Furthermore, no interconnect logic is required—only the ability to read and write attributes.