iMX8M Plus PCIe Reference CLK Impedance

[Zhirong_Yang]

Hi,

  In iMX.8M Plus Hardware Developer's Guide Rev .0, the PCIe reference clock impedance is 85 ohm as below:

Zhirong_Yang_0-1748243492155.png

Rev .1 PCIe CLK impedance change to 100ohm, and add 'PCIe reference clocks must
be routed in 100 Ω differential' comment.

Zhirong_Yang_1-1748243554337.png

Rev .1.1 still remain 100 ohm, but seems remove the comment 'PCIe reference clocks must
be routed in 100 Ω differential'.

Zhirong_Yang_2-1748243667844.png

My question is:

1. Why NXP change PCIe Reference CLK to 100ohm from 85ohm?

2. Is PCIe CLK 100 ohm must or not?

3. What's the risk if we route PCIe CLK 85ohm?

[June_Lu]

This impedance of 85ohm or 100ohm does not affect much for reference CLK, on the EVK it is 85ohm.

And the PCIe CLK 100 ohm is not mandatory, either 85ohm or 100ohm is actually OK.

HDG will be updated, such as add a note either 85ohm or 100ohm is OK.

Thanks

[June_Lu]

Would you kindly share your project information?

Thanks

[Zhirong_Yang]

@June_Lu

   iMX8M Plus is our one of mainstream chips, some of projects have already been mass production.

  So we urgent need to know: 

1. Why NXP change iMX8M Plus PCIe Reference CLK to 100ohm from 85ohm?
2. Is PCIe CLK 100 ohm must or not?
3. What's the risk if we route PCIe CLK 85ohm?
4. Do we need to make the 100 ohm CLK change to our mass production projects?

[June_Lu]

Will check it, but please send the information of project name, application field.

Thanks

[Zhirong_Yang]

@June_Lu 

  The project name is iMX8M Plus SOM, application : HMI