Details

The 4-L SerDes PCIe Reference Clock Output will be temporarily disabled when changing Data Rates to or from 8.0 GT/s in Derived Refclk mode (as opposed to Received Refclk mode) and using a single SerDes PLL to generate the PCIe TX and RX clocks. This is due to the PLL reprogramming which must be performed when changing the data rate from 2.5 GT/s or 5.0 GT/s to 8.0 GT/s in this mode.

Some external PCIe components that are using the PCIe Reference Clock may not tolerate the disabling of the clock when changing data rates. However, the 2-L and 4-L SerDes in this Device family does not have an issue accepting this Reference Clock behavior. This means that a link that connects the 2-L or 4-L SerDes in one Device to the 2-L or 4-L SerDes in a second Device will not have an issue when one Device generates the Reference Clock and the other Device receives the Reference Clock.

Workaround

Option 1:

Configure the 4-L SerDes to use one PLL to generate the clocks for 2.5 GT/s and 5.0 GT/s data rates, and a second PLL to generate the clocks for 8.0 GT/s data rate. This option imposes some limitations:

A) If Internal SSC mode is used, the two PLLs will not spread in sync with each other. This could result in up to 5000ppm difference between frequency of the two PLLs, and therefore between the TX and RX of the link partners. Because of this, Internal SSC mode is not recommended.

B) Protocols used simultaneously with PCIe on different Lanes of the SerDes must be compatible with sharing the PLL configuration of at least one of the two PLLs used for PCIe.

Option 2:

Use Received Refclk mode. Note that this mode is impacted by the separate Output Refclk jitter errata advisory (i2241)

Option 3:

Do not operate the PCIe interface at the 8.0 GT/s Data Rate

Option 4:

Use an external clock source to supply the PCIe Reference Clock to both the Root Complex and End Point Devices of the Link.