7.5.14 Overriding the DFE Tap Weights and Polarities

Register 0x11, bits 3:0, Register 0x12, bit 7 and bits 4:0, Register 0x15, bit 7, Register 0x1e, bit 3, Register 0x20, Register 0x21, Register 0x23, bit 6, Register 0x2f, bit 0, and Registers 0x71–0x75

For the DS125DF410 the DFE tap weights and polarities are normally set automatically by the adaptation procedure. These values can be overridden by the user if desired.

Prior to overriding the DFE tap weights and polarities, the dfe_ov bit, bit 6 of register 0x23, should be set. This bit is set by default. In order for the DFE tap weights and polarities to be applied to the input signal, bit 3 of register 0x1e, the dfe_PD bit, must be set to 0. It is necessary to change the default settings of these registers, because the DFE is powered down by default.

It is also necessary to set bit 7 of register 0x15 in order to manually set the DFE tap weights. This bit is cleared by default.

Bits 4:0 of register 0x12 set the five-bit weight for DFE tap 1. The first DFE tap has a five-bit setting, while the other taps are set using four bits. Often the first DFE tap has the largest effect in improving the bit error rate of the system, which is why this tap has a five-bit weight setting.

The polarity of the tap weight for tap 1 is set using bit 7 of the same register, register 0x12. The polarity is set to 0 by default, which corresponds to a negative algebraic sign for the tap.

The other four taps are set using four-bit fields in registers 0x20 and 0x21. The polarities of these taps are set by bits 3:0 in register 0x11. These tap polarities are all set to 0 by default.

As is the case for the CTLE settings, if changing the DFE tap weights or polarities causes the DS125DF410 to lose lock, it may readapt its CTLE in order to reacquire lock. If this occurs, the CTLE settings may appear to change spontaneously when the DFE tap weights are changed. The mechanism is the same as that described above for the CTLE boost settings.

When the DS125DF410 is set to adapt mode 2 or 3 using bits 6:5 of register 0x31, it will automatically adapt its DFE whenever its CDR state machine is reset. This occurs when the user manually resets the CDR state machine using bits 3:2 of register 0x0a, or when a signal is first presented at the input to the channel when the channel is in an unlocked state.

Regardless of the adapt mode, DFE adaptation can be initiated under SMBus control. Because the DFE tap weight registers are used by the DFE state machine during adaptation, they may be reset prior to adaptation, which can cause the adaptation to fail. The DFE tap observation registers can be used to prevent this.

Prior to initiating DFE adaptation under SMBus control, write the starting values of the DFE tap settings into the DFE tap weight registers, registers 0x11, 0x12, 0x20, and 0x21. The values can be read from the observation registers, registers 0x71 through 0x75. For each DFE tap, read the current value in the observation register. Both the polarities and the tap weights are contained in the observation registers as shown in Table 14. For each DFE tap, write the current tap polarity and tap weight into the DFE tap register. Once all these values have been written, DFE adaptation can be initiated and it will proceed normally. If the DS125DF410 fails to find a set of DFE tap weights producing a better adaptation figure of merit than the starting tap weights, the starting tap weights will be retained and used.

CTLE adaptation can also be initiated manually. Setting and then clearing bit 0 of register 0x2f will initiate adaptation of the CTLE. As with the DFE, if the DS125DF410 fails to find a set of CTLE settings that produce a better adaptation figure of merit than the starting CTLE values, the starting CTLE values will be retained and used.