7.5.8 Overriding the CTLE Boost Setting

Register 0x03, Register 0x13, bit 2, and Register 0x3a

To override the CTLE boost settings, register 0x03 is used. This register contains the currently-applied CTLE boost settings. The boost values can be overridden by using the two-bit fields in this register as shown in the table.

The final stage of the CTLE has an additional control bit which sets it to a limiting mode. For some channels, this additional setting improves the bit error rate performance. This bit is bit 2 of register 0x13.

If the DS125DF410 loses lock because of a change in the CTLE settings, the DS125DF410 will initiate its lock and adaptation sequence again. Thus, if you write new CTLE boost values to register 0x03 and 0x13 which cause the DS125DF410 to drop out of lock, the DS125DF410 may, in the process of reacquiring the CDR lock, reset the CTLE settings to different values than those you set in register 0x03 and 0x13. If this behavior is not understood, it can appear that the DS125DF410 did not accept the values you wrote to the CTLE boost registers. What's really happening, however, is that the lock and adaptation sequence is overriding the CTLE values you wrote to the CTLE boost registers. This will not happen unless the DS125DF410 drops out of lock.

if the adapt mode is set to 0 (bits 6:5 of channel register 0x31), then the CTLE boost values will not be overridden, but the DS125DF410 may still lose lock. If this happens, the DS125DF410 will attempt to reacquire lock. if the reference mode is set appropriately, and if the rate/subrate code is set to permit it, the DS125DF410 will begin searching for CDR lock at the highest allowable VCO divider ratio – that is, at the lowest configured bit rate. At divider values of 4 and 8, the CTLE boost settings used will come not from the values in register 0x03, and 0x13, but rather from register 0x3a, the fixed CTLE boost setting for lower data rates. This setting will be written into boost setting register 0x03 during the lock search process. This value may be different from the value you set in register 0x03, so, again, it may appear that the DS125DF410 has not accepted the CTLE boost settings you set in registers 0x03 and 0x13. The interactions of the lock and adaptation sequences with the manually-set CTLE boost settings can be difficult to understand.

To manually override the CTLE boost under all conditions, perform the following steps.

1. Set the DS125DF410 channel adapt mode to 0 by writing 0x0 to bits 6:5 of channel register 0x31.
2. Set the desired CTLE boost setting in register 0x3a. If the DS125DF410 loses lock and attempts to lock to a lower data rate, it will use this CTLE boost setting.
3. Set the desired CTLE boost setting in register 0x03.
4. Set the desired CTLE boost setting in register 0x40.
5. If desired, set the CTLE stage 3 limiting bit, bit 2 of register 0x13.

If the DS125DF410 loses lock when the CTLE boost settings are set according to the sequence above, the DS125DF410 will try to reacquire lock, but it will not change the CTLE boost settings in order to do so.