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DM8168 eye diagram issue

Chiajung Hsu
Intellectual 280 points


Our DM8168 design can't totally pass the eye diagram test even though we  have populated  the 44.2 ohm at pin USB0_R1 and USB1_R1. The eye opening is marginal to eye mask. This small eye opening cause specific USB device is unstable. After changing USB0_R1 and USB1_R1 to 33 ohm, the issue is resolved and the eye opening looks good. I am not sure if there is any side effect after change this resistor 33 ohm. someone can comment it ?

Our USB trace length is less than 1800 mil, the impedance is 90 ohm. Full GND plane for return path. No stub.

  • 0
    TI__Guru**** 170625 points

    Hello,

    The value of 44.2 Ohms resistors with 1% tolerance comes from the Synopsys USB PHY documentation.


    See also the below e2e thread:

    http://e2e.ti.com/support/dsp/davinci_digital_media_processors/f/717/t/165644.aspx

    BR
    Pavel

  • 0 in reply to Pavel Botev
    Intellectual 280 points

    Hello,

    I understood that 44.2 ohm is recommend by  Synopsys USB PHY documentation. But I'd like to know if there is any side effect if we change this resistor a little bit. If  Synopsys USB PHY documentation can't explain it, is it possible to expect the side effect from other USB IP documentation since many SOC also have this pull down resistor to adjust the USB eye opening. I think TI should have the expert from other product line can explain it .

    Some USB devices connected to DM8168 is unstable. The cause is eye opening from Netra, So, we need to improve the eye opening to resolve this issue. Unfortunately, DM8168 has no register to adjust the eye opening. Therefore, I'd like to know what is the expected side effect if this resistor is changed.