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LMX2572: Category 3 SYNC and register R49

Part Number: LMX2572

Hello TI Team,

We have been struggling with inconsistent Category 3 SYNC results for the past several weeks.  We have resolved a number clock issues and timing problems, but one issue remains. 

We discovered, completely by accident, that register R49 has a major impact on the category 3 SYNC performance.  We inadvertently overwrote the default value of 0x4180  with 0x0401; and the new value of 0x0401 yielded a very stable and repeatable phase relationship between the two LM2572 devices.  We tried writing other values to R49, and found these results:

R49 Values

Comment

 

0x4180

Data Sheet Default

Final Phase relationship after SYNC highly dependent on 100MHz OSCin input level, and the programmed output frequency.  (Up to three final phase states are observed)

0x4280

 

stable and repeatable phase relationship after SYNC operation. All frequencies work with no sensitivity to 100 MHz OSCin 

0x4380

 

stable and repeatable phase relationship after SYNC operation. All frequencies work with no sensitivity to 100 MHz OSCin 

0x0000

 

stable and repeatable phase relationship after SYNC operation. All frequencies work with no sensitivity to 100 MHz OSCin 

0xFFFF

 

stable and repeatable phase relationship after SYNC operation. All frequencies work with no sensitivity to 100 MHz OSCin 

0x0401

 

stable and repeatable phase relationship after SYNC operation. All frequencies work with no sensitivity to 100 MHz OSCin  (original accidental write to R49 that corrected phase sync operation)

Questions:

1) Could you give us any insight into why this works? 

2) Is this a legitimate solution? 

3) Are we doing something that should be corrected by another more appropriate register write?  

Thanks in advance,

Chuck Beam