8.3.13.3 Eye Opening Monitor

The DS250DF810’s Eye Opening Monitor (EOM) measures the internal data eye at the input of the decision slicer and can be used for 2 functions:

1. Horizontal Eye Opening (HEO) and Vertical Eye Opening (VEO) measurement
2. Full Eye Diagram Capture

The HEO measurement is made at the 0V crossing and is read in channel register 0x27. The VEO measurement is made at the 0.5 UI mark and is read in channel register 0x28. The HEO and VEO registers can be read from channel registers 0x27 and 0x28 at any time while the CDR is locked. The following equations are used to convert the contents of channel registers 0x27 and 0x28 into their appropriate units:

• HEO [UI] = ch reg 0x27 ÷ 32
• VEO [mV] = ch reg 0x28 x 3.125

A full eye diagram capture can be performed when the CDR is locked. The eye diagram is constructed within a 64 x 64 array, where each cell in the matrix consists of an 16-bit word representing the total number of hits recorded at that particular phase and voltage offset. Users can manually adjust the vertical scaling of the EOM or allow the state machine to control the scaling which is the default option. The horizontal scaling controlled by the state machine and is always directly proportional to the data rate.

When a full eye diagram plot is captured, the retimer will shift out four 16-bit words of junk data that should be discarded followed by 4096 16-bit words that make up the 64 × 64 eye plot. The first actual word of the eye plot from the retimer is for (X, Y) position (0,0), which is the earliest position in time and the most negative position in voltage. Each time the eye plot data is read out the voltage position is incremented. Once the voltage position has incremented to position 63 (the most positive voltage), the next read will cause the voltage position to reset to 0 (the most negative voltage) and the phase position to increment. This process will continue until the entire 64 × 64 matrix is read out. Figure 14 below shows the EOM read out sequence overlaid on top of a simple eye opening plot. In this plot any hits are shown in green. This type of plot is helpful for quickly visualizing the HEO and VEO. Users can apply different algorithms to the output data to plot density or color gradients to the output data.

Figure 14. EOM Full Eye Capture Readout

To manually control the EOM vertical range, remove scaling control from the state machine then select the desired range:

Channel Reg 0x2C[6] → 0 (see Table 3).

The EOM operates as an under-sampled circuit. This allows the EOM to be useful in identifying over equalization, ringing and other gross signal conditioning issues. However, the EOM cannot be correlated to a bit error rate.

The EOM can be accessed in two ways to read out the entire eye plot:

• Multi-byte reads can be used such that data is repeatedly latched out from channel register 0x25.
• With single byte reads, the MSB are located in register 0x25 and the LSB are located in register 0x26. In this mode, the device must be addressed each time a new byte is read.

To perform a full eye capture with the EOM, follow these steps below within the desired channel register set: