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DS90CR286A: Cable skew recommendation for 40-MHz operation

Ryu Yamashita
Genius 5030 points

Part Number: DS90CR286A

Hello Team,
 
I have a question regarding the cable skew recommendation in the DS90CR286A datasheet.
In Section 8.2.2.1 "Cables", the datasheet states:
"It is also recommended that cable skew remain below 150 ps (assuming 66 MHz clock rate) to maintain a sufficient data sampling window at the receiver."
Our application uses the DS90CR285A + DS90CR286A pair with a clock frequency of 40 MHz.
Questions:
  1. Is the 150-ps recommendation specifically intended for 66-MHz operation only?
  2. Can the recommended cable skew be scaled according to the clock period when operating below 66 MHz?
  3. For 40-MHz operation (25-ns clock period), would a cable skew of approximately 250 ps be considered acceptable?
  4. If not, does TI have an official maximum recommended cable skew value for 40-MHz operation?
We understand that the actual margin is related to receiver timing parameters such as RSKM. However, we would like to know whether TI has any recommended cable-skew guideline for operation at 40 MHz.
 
Thank you for your support.
Best regards,
Ryu.
  • +1
    TI__Mastermind 42790 points

    Hi Ryu,

    The 150-ps cable skew mentioned in the datasheet is a recommended guideline assuming operation at 66 MHz. It should not be directly scaled based on the ratio of the clock periods.

    The more appropriate parameter to consider is the receiver skew margin (RSKM) as you mentioned. For the DS90CR286A, the minimum specified RSKM is 400 ps at 66 MHz and increases to 490 ps at 40 MHz. This margin accounts for the total timing uncertainty at the receiver, including cable/interconnect skew, source clock jitter, and inter-symbol interference (ISI).

    Aaron