DS280DF810: Follow up the 25Gbps can't link up issue

Hi Rannie,

RX path:

Your understanding here is correct.  The customer can use the retimer prbs check and eye opening as FoM to optimize CPU TX settings.  We actually have an application note discussing this, please see below.

Please also keep in mind that each time a new CPU TX setting is tested, the retimer should re-adapt to the signal.  To trigger re-adaptation, customer can do CDR reset on the retimer.

https://www.ti.com/lit/an/slyt678/slyt678.pdf

TX path:

Rannie Zhou said:

DF810 send PRBS31 to CPU and adjust the parameter like CTLE, DFE and pre-/post- cursor, and then CPU check the FoM... So, DF810 need to generate PRBS and adjust the parameters. I noticed that in programming guide, pre-/post-/main- cursor can be set in Table5-4/ Table7-17.  Also, the PRBS generator can be enabled in 7.22 and 7.23. Customer can use these register to impement the test. Does that make sense?

Please confirm my understanding below is correct.  This procedure (as summarized below) makes sense.

• On DS280DF810, customer plans to enable PRBS generator and try different pre/main/post values
  • Note DS280DF810 requires CDR lock to enable PRBS generator.  Customer will need to provide signal to DS280DF810 for it to acquire CDR lock.
• On CPU, customer will adjust RX equalization parameters like CTLE and DFE and check FoM in order to identify best DS280DF810 TX settings.

Rannie Zhou said:

BTW, as DF810 can enable adaptive CTLE and DFE, in addition to these two parameters and pre/post/main cursor, what other parameters need to be manually adjusted by the customer? 

The key parameters that customers need to tune are adapt mode and TX settings (pre/main/post cursor).  Depending on signal amplitude, sometimes customers also need to adjust RX DC gain.  See "7.32 Set Variable Gain Amplifier (VGA) and CTLE DC Gain Mode".

FYI, there are a few other parameters that customers can typically leave at default, but sometimes we adjust these during debug:

• CDR bandwidth
• EOM timer

i think customer can work on setting adapt mode and TX settings before trying advanced adjustments.  Please let us know if customer runs into challenges with their testing.

Thanks,

Drew

Hi Rannie,

Following up on our Webex discussion, for PRBS generator to work on DS280DF810, the device requires CDR lock.

Customer can accomplish this through external loopback, as you suggested.

Thanks,

Drew

Hi Rannie,

Rannie Zhou said:

eye height was 35 and still could not meet the requirement (60

To clarify, is this CPU requirement?

From data sheet, the sum of absolute values of pre, main, and post cursor should not exceed 31.  Customer should try adding some pre and post cursor boost.  In order to do this, they should maintain constant sum of pre, main, and post.  For example, if customer starts with pre=0, main=31, post=0, customer could try pre = -1, main = 30, post = 0.

The best settings at 25G typically involve some amount of pre and post cursor.  For example, pre = -3, main = 23, post = -5.  The actual best values are dependent on the channel and the receiver equalizer.

Thanks,

Drew