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AM3358: A ringing problem of eMMC interface

Anderson Hou
Prodigy 50 points

Part Number: AM3358

Hi Sir

We use your AM3358 on several project. We find sometimes the our RAM/ROM test are fail (read files from eMMC and calculate the checksum). We can fix this problem by adding a small capacitor (C232) on CLK line. 

This capacitor can improve the voltage dropping on CLK.

After measuring the data and CLK line looks like the voltage dropping come from the data line.

When data line pull up or pull low CLK line may have ringing.

 

Zoom in and check the timing.

The voltage dropping of CLK is earlier than DATA pull low. So it should not be the crosstalk from DATA line. The root cause may come from the MCU inside.

Do you have any idea that CLK have the ringing when data line change the level?

Is there any SW setting we can modify to improve the problem?

Thanks

  • 0
    TI__Guru 63605 points

    I do not think the voltage change observed on the clock signal while it is high is causing your problem. It appears you are probing the clock signal near the eMMC device based on the shape of the clock signal rise/fall edges. You need to probe the clock signal near the processor. I suspect you will see a non-monotonic transition near mid-supply on the source end of the PCB trace. This non-monotonic transition is likely causing the problem.

    The processor sources the clock out to the eMMC device and loops in back at the processor pin to improve timing margin. So there is a good chance the non-monotonic transition occurring at the source is creating a glitch on the clock that is looped back into the processor.

    The AM335x schematic checklist tells you to install a 33 Ω series resistor on MMCx_CLK (as close to the processor as possible). I do not see this resistor in your schematic. This resistor is necessary to prevent this problem.

    The capacitor is likely changing the signal distortion enough to make it work better, but it is not a good solution.

    Regards,
    Paul  

  • 0 in reply to peaves
    Prodigy 50 points

    Hi Paul

    Thanks for your reply. I check other projects that has 33 ohm series resistor. Its glitch is small when 33 ohm is placed.

    We have a lot of projects use AM335x. I measure each clock signal. All of them have glitch but level are different. Some are high but some are low. Does layout affect the clock reflection?

    Because our product is MP. Are there any register setting can fix this issue? Could we tune the eMMC driving strength?

    Thanks

  • +1 in reply to Anderson Hou
    TI__Guru 63605 points

    The resistor doesn't eliminate the non-monotonic transition. It shifts the voltage away from the input buffer switching threshold voltage, where it is less likely to create a glitch on the looped back clock going back into the processor. The only way to resolve the clock glitch issue is install a 33 ohm series termination resistor located very close to the AM335x pin. The trace length between the processor pin and the series resistor should be as short as possible.

    Regards,
    Paul