topic Re: iMX8M Plus PCIe Reference CLK Impedance in i.MX Processors

iMX8M Plus PCIe Reference CLK Impedance

Zhirong_Yang — Mon, 26 May 2025 07:18:06 GMT

Hi,

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

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

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

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 — Tue, 27 May 2025 03:30:23 GMT

Would you kindly share your project information?

Thanks

Zhirong_Yang — Tue, 27 May 2025 04:10:23 GMT

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

So we urgent need to know:

June_Lu — Tue, 27 May 2025 04:15:18 GMT

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

Thanks

Zhirong_Yang — Tue, 27 May 2025 06:37:32 GMT

The project name is iMX8M Plus SOM, application : HMI

June_Lu — Wed, 28 May 2025 01:46:26 GMT

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