The Problem: Random Crashes Without Pattern

Our custom embedded system based on the i.MX6 processor was experiencing mysterious, unpredictable crashes. The system would lock up or reboot at random intervals with no discernible pattern. We couldn’t reproduce the issue on demand, making it nearly impossible to debug using traditional methods.

The symptoms suggested hardware instability, but we had no clear indication of what was failing. It could be:

  • The CPU?
  • The bootloader?
  • The application?
  • The kernel?

Weeks of investigation went nowhere.

The Breakthrough: A Simple MD5 Hash Test

The breakthrough came from a simple observation. I decided to test file integrity by calculating MD5 hashes of a large binary file in RAM:

md5sum large_file.bin
# Result: abc123def456...

md5sum large_file.bin
# Result: 789ghi012jkl...

The hashes were different every time, even though the file hadn’t changed.

This was the smoking gun: data corruption in RAM. Every time the file was loaded into memory for hashing, the bits were getting corrupted differently due to the underlying memory hardware issue.

Root Cause: Missing DDR Calibration

Our custom board was based on an open-source reference design from NXP, but we had made PCB layout changes and updated some components without updating the DDR calibration registers in the device tree.

The problem: different PCB layouts affect signal timing. DDR memory requires precise calibration of write leveling, DQS (Data Strobe) gating, and read/write delays to function reliably. These values are board-specific and must be recalibrated whenever the hardware layout changes.

The Solution: NXP DDR Stress Test Tool

NXP provides the i.MX 6/7 DDR Stress Test Tool specifically for this purpose:

What the Tool Does

According to NXP’s documentation, the DDR Stress Test Tool is a PC-based software that performs:

  • Write Leveling Calibration - aligns the DQS (Data Strobe) signal with the SDCLK
  • DQS Gating Calibration - captures valid data within a specific window
  • Read/Write Delay Calibration - fine-tunes timing parameters

The tool generates calibrated register values that must be manually updated in your device tree DDR initialization script.

Supported Platforms

The tool supports all major i.MX 6 and 7 variants:

  • i.MX 6DQ / 6DQP (Dual/Quad Plus)
  • i.MX 6DL / 6S (Dual Lite / Solo)
  • i.MX 6SL / 6SoloX
  • i.MX 6UL / 6ULL / 6ULZ
  • i.MX 7D / 7S / 7ULP

Three Usage Options

Option 1: GUI (Recommended for development)

  • Download ddr_stress_tester_vX.XX.zip
  • Connect your board via USB
  • Load your DDR initialization script
  • Run calibration from the GUI
  • Results show on screen

Option 2: JTAG Interface

  • Use with hardware debugger (JTAG)
  • Download ddr_stress_tester_jtag_vX.XX.zip
  • Useful when USB isn’t available
  • Results via UART serial port (115200-8-n-1)

Option 3: U-Boot

  • Download ddr_stress_tester_uboot_vX.XX.zip
  • Load test binary via U-Boot
  • Results via UART serial
  • Note: NXP recommends GUI for best results; U-Boot is a “last resort” option

My Implementation Steps

Step 1: Prepare the DDR Initialization Script

Our device tree contained a basic DDR initialization. I extracted this as a compatible script for the tool.

Step 2: Run the Calibration

Using the GUI version:

  1. Selected i.MX6 Dual/Quad as the target
  2. Set ARM Clock: 800MHz, DDR Frequency: 396MHz
  3. Loaded our custom DDR initialization script
  4. Ran the Write Leveling Calibration
  5. Ran the DQS Gating Calibration
  6. Ran the Read/Write Delay Calibration

The tool performed hardware-level testing and returned new calibration values:

MMDC_MPWLDECTRL0: 0x004D005C
MMDC_MPWLDECTRL1: 0x00420045
MMDC_MPRDDLCTL: 0x42464A42
MMDC_MPWRDLCTL: 0x3F3F3F3F
(and many more registers...)

Step 3: Update Device Tree

I manually updated the DDR initialization registers in the device tree with the newly calibrated values. These values are specific to:

  • Our board layout
  • Our memory chips
  • Our operating frequency
  • Our component placement

Step 4: Flash and Test

After rebuilding the kernel image and flashing the new device tree:

The system became 100% stable.

No more random crashes, no more data corruption, no more mysterious hangs.

Key Lessons Learned

1. Custom PCB? Calibrate Your DDR

This is not optional for i.MX6/7 products:

Recommendation from NXP: When you design a new PCB based on an i.MX processor reference design, you must run DDR calibration with the stress test tool if you make any layout changes to the memory bus.

2. Evidence-Based Diagnosis

Instead of guessing, I used a simple test (MD5 hashing) to prove the problem was RAM. This guided me toward the correct solution rather than chasing unrelated issues.

3. The Tool Removes Guesswork

The DDR Stress Test Tool doesn’t just give you one set of values—it:

  • Tests multiple frequencies
  • Tests read and write paths separately
  • Validates timing margins
  • Reports confidence levels
  • Catches edge cases

Manual calibration or copy-pasting values from reference boards would have failed.

  1. Create your custom board based on NXP reference design
  2. Generate initial DDR script using NXP’s DDR Register Programming Aid (RPA)
  3. Run Stress Test Tool calibration on your actual hardware
  4. Update device tree with calibrated values
  5. Test thoroughly - the tool includes stress test patterns
  6. Further validate with OS-based tests like memtester on Linux

Error to Avoid

If you’re using an older device tree from a reference board without updating for your custom layout, you’re running on borrowed time. Your system might work occasionally—especially under light load—but will fail catastrophically when actual memory stress occurs (large file transfers, video streaming, etc.).

Resources

Conclusion

What looked like a catastrophic system design flaw turned out to be a missing calibration step—one that NXP explicitly recommends for custom board designs. By running the DDR Stress Test Tool and updating the calibration registers, we transformed an unstable system into a rock-solid platform.

The lesson: Hardware instability is often memory-related, and memory issues can appear random and untraceable. Always calibrate your DDR memory when designing custom boards.