Other Parts Discussed in Thread: DS250DF410
Tool/software:
Introduction:
One common debug question when using TI's 25G/28G is why is the PRBS generator not working? This FAQ will review the key criteria for the retimer PRBS generator to function.
Retimer Architecture:
TI's 25G/28G retimers implement a "referenceless" CDR architecture. This means that the clock used for retiming data in the retimer is solely based on the recovered clock from the data the retimer receives. The 25 MHz calibration clock required for these retimers is just used to set the VCO to the correct approximate frequency for CDR lock, but is not used as a reference clock for retiming the data.
The recovered clock signal from the retimer is required for the PRBS generator to function. This can be seen in the block diagram of DS250DF410, shown below.
Using Retimer PRBS Generator:
A key requirement of using the retimer PRBS generator on a given retimer channel is for that channel to have CDR lock. There are two primary means of accomplishing this:
1) Provide a data input to the retimer, such as 25 GbE data.
2) Provide a sub-rate clock input to the retimer.
A sub-rate clock input is a low frequency clock signal intended to look like a series of repeated 0s and 1s to the retimer. As an example, in order to create a 25.78125 Gbps PRBS output from the retimer, a 1.611328125 GHz clock signal can be supplied to the receiver. This frequency is derived from 25.78125 Gbps / 16, which looks similar to a data pattern of 0000000011111111. For the clock amplitude, use an amplitude that matches supported receiver input amplitude range. TI often uses 800 mVppd clock input.
Beyond ensuring that a valid signal is present on the retimer receiver, it is important that the retimer CDR data rate and PRBS generator are configured correctly. Details on configuring these parameters can be found in the 25G/28G retimer programming guide, found in the corresponding retimer secure folder. Additionally, the SigCon Architect GUI combined with the appropriate device profile can be used to accelerate device configuration.