PCIE3.0 Differential Impedance for REFCLK_P/N

Issue Overview

The discussion focuses on the differential impedance specifications for the PCI Express (PCIE) clock signals, specifically the REFCLK_P/N lines on the Nvidia Jetson Orin Nano Dev board. Users are seeking clarification on whether these clock signals should also adhere to the 85-ohm differential impedance standard set for PCIE 3.0/4.0, as mentioned in the product design guide (Document ID: DG-10931-001_v1.2).

Symptoms and Context

• Users are uncertain about the correct impedance values for REFCLK_P/N, which can lead to potential signal integrity issues.
• The issue arises during the design phase and setup of hardware that interfaces with the Jetson Orin Nano.
• Relevant specifications include an 85-ohm differential impedance for PCIE_TX/RX_P/N, with a query on whether REFCLK_P/N can deviate to 100 ohms.
• The frequency of this issue is not explicitly stated but suggests that it is a common concern among developers working with this hardware.
• Misunderstanding of these specifications could impact system functionality, particularly in high-speed data transfer scenarios.

Possible Causes

• Hardware Specifications: Misinterpretation of the design guide could lead to incorrect hardware configurations.

• Differential Impedance Standards: Confusion about whether REFCLK_P/N should match the 85-ohm standard or if it can be 100 ohms may lead to improper circuit designs.

• Documentation Clarity: Lack of explicit guidance in the product design documentation could contribute to user uncertainty.

Troubleshooting Steps, Solutions & Fixes

Step-by-Step Instructions

1. Review Documentation:

   • Revisit the Nvidia Jetson Orin Nano Series Product Design Guide (DG-10931-001_v1.2) to confirm specifications regarding differential impedance.

2. Check Circuit Design:

   • Ensure that PCB designs conform to the specified impedance values. Use simulation tools to verify signal integrity.

3. Consult with Nvidia Support:

   • If uncertainties persist, reach out to Nvidia support or community forums for clarification on impedance specifications.

Commands and Procedures

• To gather relevant system information regarding your current design, you can utilize tools like:

  # For checking PCIe configuration
  lspci -vvv
  

Isolation Methods

• Test with different PCB designs that utilize both 85-ohm and 100-ohm configurations for REFCLK_P/N to observe any differences in performance.

Potential Fixes or Workarounds

• Based on user replies, it is confirmed that REFCLK_P/N should maintain an 85-ohm differential impedance with a tolerance of +/-15%. Stick to this standard in your designs to ensure compatibility and signal integrity.

Best Practices

• Always double-check impedance values against official documentation before finalizing hardware designs.

• Engage with community discussions or technical support when in doubt about specifications.

Unresolved Aspects

While users have confirmed that REFCLK_P/N should be set at 85 ohms, further clarification from Nvidia regarding any exceptions or specific use cases where different values might be acceptable would be beneficial.