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DP83TG720R-Q1: RGMII clock and Crystal tolerance Requirement

Jing Liang
Prodigy 100 points
Part Number: DP83TG720R-Q1

Dear Expert,

RGMII protocol version2.0 defines the requirement of RXC and TXC clock is +-50ppm as below show, and also I found the requirement of PHY crystal is +-100ppm.

My questions are:

  1. Why the PHY crystal tolerance are larger than RGMII clock requirement?
  2. Why PHY has crystal requirement 100ppm
  3. How does the requirement(PHY crystal +-100ppm) conclusion come out: 
  4. Why RGMII requires +-50ppm on RGMII?
  5. If we use 100ppm crystal for PHY, then what is the risk between RGMII communication?

RGMII protocol:

DP83TG720R datsheet: