7.5.17 Overriding the Figure of Merit for Adaptation

Register 0x2c, bits 5:4, Register 0x31, bits 6:5, Register 0x6b, Register 0x6c, Register 0x6d, and Register 0x6e, bits 7 and 6

The default figure of merit for both the CTLE and DFE adaptation in the DS125DF410 is simple. The horizontal and vertical eye openings are measured for each CTLE boost setting or set of DFE tap weights and polarities. The vertical eye opening is scaled to a constant reference vertical eye opening and the smaller of the horizontal or vertical eye opening is taken as the figure of merit for that set of equalizer settings. The objective is to adapt the equalizer to a point where the horizontal and vertical eye openings are both as large as possible. This usually provides optimum bit error rate performance for most transmission channels.

In some systems the adaptation can reach a better setting if only the horizontal or vertical eye opening is used to compute the figure of merit rather than using both. This will be system-dependent and the user must determine through experiment whether this provides better adaptation in the user's system. For the DS125DF410, the DFE figure of merit type can be set using register 0x2c, bits 5:4. The value of this two-bit field versus the configured figure of merit type is shown in Table 9.

The CTLE figure of merit type is selected using the two-bit field in register 0x31, bits 4:3, with the same effect as in Table 9.

For some transmission media the adaptation can reach a better setting if a different figure of merit is used. The DS125DF410 includes the capability of adapting based on a configurable figure of merit. The configurable figure of merit is structured as shown in Equation 1.

In this equation, HEO is horizontal eye opening, VEO is vertical eye opening, FOM is the figure of merit, and the factors a, b, and c are set using registers 0x6b, 0x6c, and 0x6d respectively.

In order to use the configurable figure of merit, the enable bits must be set. To use the configurable figure of merit for the CTLE adaptation, set bit 7 of register 0x6e, the en_new_fom_ctle bit. To use the configurable figure of merit for the DFE adaptation (in the DS125DF410), set bit 6 of register 0x6e, the en_new_fom_dfe bit. The same scaling factors are used for both CTLE and DFE adaptation when the configurable figure of merit is enabled.