Troubleshooting EDP Display Connection Issues with modetest -M rockchip

modetest -M rockchip

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When connecting an EDP interface display to the motherboard, if the error “LT CR Failure” (Link Training & Clock Recovery failure) occurs, the following are possible causes and solutions:

1. Core Reasons for Link Training Failure

• Poor Signal Integrity
• The EDP signals (differential pairs) from the motherboard to the display may have impedance mismatches, excessive attenuation, or noise interference, preventing the receiver from correctly interpreting the signals.
• Key Indicators: Eye Closure, Jitter exceeding ( >20% UI), insufficient signal amplitude (<400mV differential).
• Parameter Negotiation Failure
• The motherboard and display may not agree on parameters such as link rate (HBR2/HBR3), lane count, pre-emphasis, and swing voltage.
• Clock Recovery Failure
• The reference clock (RefCLK) may be unstable, or the clock data recovery (CDR) circuit on the display side may be inadequate, leading to synchronization issues.

2. Specific Troubleshooting and Solutions

(1) Signal Integrity Analysis

• Tools: High-speed oscilloscope (≥6GHz bandwidth) + EDP protocol probe.
• Steps:
• Eye Height ≥120mV (HBR3 requires ≥150mV)
• Eye Width ≥0.3 UI (Unit Interval)
• No severe ringing or overshoot

1. Capture the eye diagram of the EDP differential signals (CLK+/CLK-, DATA0+/DATA0-, etc.) and check if they meet the following requirements:
2. Measure the period jitter of the reference clock (RefCLK); if it exceeds 50ps, optimize the clock source or filtering circuit.

Solutions:

• Replace with a low-jitter reference clock source (e.g., replace the crystal oscillator or clock generator chip).
• Add a π-type filter circuit (RC filter) on the clock path.

• Adjust the motherboard’s Pre-emphasis (enhanced high-frequency compensation) and Swing Voltage (increase amplitude) through BIOS or Redriver chip configuration.
• Check the EDP trace impedance on the motherboard (TDR test) to ensure the differential impedance is 90Ω±10%.
• Add series termination resistors (e.g., 10Ω) or parallel capacitors (e.g., 0.1μF) on the signal lines to suppress reflections.

• If eye closure occurs:
• If clock jitter exceeds the limit:

(2) Parameter Configuration Verification

• Troubleshooting:

1. Read the DPCD registers of the display (Address 00000h-000FFh) to confirm the supported link rate and lane count.
2. Check if the EDP configuration in the motherboard BIOS matches the display (e.g., force setting to HBR2+4 Lanes).

Solutions:

• Degrade Link Rate: Manually set the EDP rate from HBR3 to HBR2 in BIOS (even if the display supports HBR3, it may not operate stably due to signal attenuation).
• Reduce Lane Count: If designed for 4 Lanes, try using 2 Lanes (sacrificing resolution/refresh rate).
• Disable Scrambling: Some displays have poor Scrambler compatibility, which can be disabled in the DPCD registers (requires display firmware support).

(3) Power and Power-Up Timing Issues

• Troubleshooting:
• Use an oscilloscope to monitor the voltage drop of the display power supply (3.3V_AUX, 5V/12V main power) at startup (recommended ripple <5%).
• Check if the power-up timing of the motherboard PMIC complies with EDP specifications (e.g., 3.3V_AUX must stabilize before the main power).
• Solutions:
• Add a large-capacity low-ESR capacitor (e.g., 22μF tantalum capacitor) near the display power pins to suppress instantaneous voltage drops.
• Adjust the PMIC’s Power Sequencing to extend the rise time of 3.3V_AUX (e.g., from 1ms to 5ms).

(4) Firmware and Compatibility Fixes

• Steps:

1. Update the motherboard BIOS to the latest version to fix potential EDP initialization process defects.
2. Upgrade the display firmware (via I2C or dedicated tools) to correct erroneous configurations in the DPCD registers.
3. Compare the EDID/DPCD data of the normal display with the faulty one and manually modify abnormal parameters (e.g., adjust VSYNC/HSYNC polarity).

Example:

• If the display EDID claims support for HBR3, but the actual signal quality is insufficient, force the EDID to downgrade to HBR2 (modify using CRU tool).

(5) Physical Connection and Component Failure

• Troubleshooting:
• Use a multimeter to measure the continuity and contact resistance of the EDP connector pins (normal should be <0.5Ω).
• Check if the Redriver/Retimer chip on the motherboard (e.g., PS8409, PTN3624) is damaged or incorrectly configured.
• Solutions:
• Replace the EDP cable or connector (preferably use cables with better shielding performance).
• If using a Redriver chip, adjust its EQ (equalization) and gain settings (refer to the signal attenuation characteristics of the display).

3. Quick Verification Process

1. Force Downgrade Link Rate: Set EDP to HBR2+4 Lanes in BIOS and test for stability.
2. External Power Test: Provide separate power to the display (e.g., external 5V power) to rule out insufficient motherboard power supply issues.
3. Cross Testing: Connect the faulty display to other motherboards (or other model motherboards) to confirm whether the issue follows the display or the motherboard.

4. Advanced Debugging Suggestions

• Protocol Analyzer Capture: Use a DP/EDP protocol analyzer (e.g., Teledyne LeCroy, Total Phase) to capture the complete link training process and locate the specific stage of failure (e.g., Clock Recovery, Channel EQ).
• Thermal Imaging Detection: Scan the motherboard and display during startup to check for components that may be overheating and causing signal distortion.
• Design Review: Collaborate with hardware suppliers to review the layout design of the motherboard’s EDP interface (e.g., reference planes, number of vias, differential pair length matching).

If the above steps still do not resolve the issue, please provide the following information for further analysis:

• Schematic diagrams of the EDP interface for the motherboard and display (partial);
• Eye diagram/jitter test report during failure;
• Screenshots of EDP-related configurations in BIOS.