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TUSB213: About distortion of first Sync signal at circuit with CMF

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Guru 21045 points
Part Number: TUSB213

Hi Team,

 

Our customer are evaluating TUSB213.

Then, "Distortion of first Sync signal" is occurring(It is the red circle part) at circuit with CMF(Common mode filter).

So, this eye pattern can’t pass the compliance test because the waveform goes into the eye mask.

However, this distortion isn’t occurring at circuit without CMF.

 

Is CMF connection recommended with TUSB213?

They use DLW21SN900HQ2L.

Could you please let us know if you have any concern and your recommend parts number of CMF?

 

Regards,

Hide

  • +1
    TI__Guru 60870 points

    Hi Hide,

    Yes, this is a known impact on USB 2.0 High Speed eyes from a common mode choke.

    Can you take the eye diagram from the scope and run it through the official USB-IF tool (below)?  It may mark the eye as pass since it provides automatic waivers for first transition / last transition excursions.

    https://usb.org/document-library/usbet20

    The highest boost levels of the redriver are typically not needed for most applications, we recommend that the customers use the lowest setting that passes the eye mask to prevent overdriving.

    Regards,

    JMMN

  • 0 in reply to JMMN
    Guru 21045 points

    Hi JMMN-san,

    Thank you for the information and advice.

    Regards,

    Hide

  • 0 in reply to Hide
    Guru 21045 points

    Hi JMMN-san,

     

    I have three questions.

     

    ---------

    [Q1]

    According to our customer, USBET2.0 is an inrush current measurement tool for Full Speed / Low Speed.

    Therefore, they understand that USBET2.0 is irrelevant for High speed measurements.

    Is their understanding correct?

    ---------

     

    [Q2]

    The occurrence status of "Distortion of first Sync signal" is as follows.

    Case1: CPU -> CMF -> USB BUS SW -> USB connector ->Test fixture: if they can’t use TUSB213, " Distortion of first Sync signal " don’t occur but eye pattern is crushed

    Case2: CPU -> CMF -> TUSB213 -> USB BUS SW -> USB connector->Test fixture : " Distortion of first Sync signal " don’t occur but this eye pattern can’t pass the compliance test

    Case3: CPU -> TUSB213 -> CMF -> USB BUS SW -> USB connector-> Test fixture: " Distortion of first Sync signal " occur but this eye pattern can pass the compliance test

     

    You said that “this is a known impact on USB 2.0 High Speed eyes from a common mode choke.”

    Does this happen with a combination of TUSB213 and CMF?

    So, is it prohibited to use TUSB213 and CMF together?

     

    And, "Distortion of first Sync signal" is different depending on the position of CMF.

    Could you please let us know if you have information of this cause?

    ---------

     

    [Q3]

    Case3 is good result.

    Therefore, they are evaluating the reverse USB signal at case3 but "Distortion of first Sync signal" is occurring.

    Case3’: CPU <- TUSB213 <- CMF <- USB BUS SW <- USB connector <- PC

     

    Following the above results, they designed the case 4 circuit(with CMF x2cps).

    Case4: CPU <- CMF <- TUSB213 <- CMF <-USB BUS SW <-USB connector<-PC

     

    However, "Distortion of first Sync signal" is occurring.

    Is there any improvement for "Distortion of first Sync signal"?

    They want to eliminate the distortion of this signal.

     

    ---------

     

    Regards,

    Hide

  • +1 in reply to Hide
    TI__Guru 60870 points

    Hi Hide,

    A1. No, USBET 2.0 is required by USB-IF for all signal quality tests:

    USBET20 is a stand-alone electrical signal analysis tool for USB Compliance testing. USBET20 is the official compliance electrical analysis tool that performs pass/fail assessments on signal quality and inrush current data captured from an oscilloscope. NOTE: Some of the approved oscilloscope test solutions use proprietary software to assess signal quality and inrush current events. The only official analysis tool for certifying signal quality and inrush current is USBET20 published by the USB-IF. Please be sure to run captured signal quality and inrush current test data through USBET20 for an official assessment of the measurement.

    A2. Common mode chokes are known to impact the first bits of a USB HS test packet due to coupling (TDK has a good article here: https://product.tdk.com/system/files/dam/doc/content/emc-guidebook/en/eemc_practice_04.pdf).  If the eye is closed, this impact will be negligible.  Since the first bit is common mode shifted the impact of the TUSB213 may be slightly different as well.   A distorted first sync bit should have no impact on data quality, I would recommend running the USB-IF post processing tool on the waveform to confirm if it is a valid failure or not.

    A3. Have they tried reducing the BOOST / EQ settings on TUSB213 to reduce any impact of the redriver?

    Regards,

    JMMN

  • 0 in reply to JMMN
    Guru 21045 points

    Hi JMMN-san,

    Thank you for the detail information and comment.

    Regards,

    Hide

  • 0 in reply to Hide
    TI__Guru 60870 points

    Let us know if there are any additional questions.

    Regards,

    JMMN