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Thread: LVDS Signal DC offset and AC Swing

  1. #11
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    Default Re: LVDS Signal DC offset and AC Swing

    Quote Originally Posted by FvM View Post
    Why do you refer to Cyclone V GX instead of Cyclone 10 GX documents? The relevant are the transceiver PHY user guide https://www.altera.com/content/dam/a...0_xcvr_phy.pdf and the datasheet https://www.altera.com/content/dam/a...10gx-51002.pdf

    However neither C5GX nor C10GX have programmable transceiver Vocm level. Datasheet specifies 0.65 V for C5GX and 0.4/0.5 V for C10GX. Setting transceiver Vocm to 1.25 V LVDS value is simply impossible due to the low transceiver supply voltage.

    You didn't yet answer the question about AC coupling option. What's your application?

    A- This is an application of MIPI D-PHY: The physical layer is explained at the following document at page 14 under the slide title called " MIPI D-PHY at the Physical Layer "
    https://www.keysight.com/upload/cmc_...PHY-design.pdf

    This is an interface between FPGA(Cyclone 10 GX) and Video Processor ( Intel Movidius ) by using Intel/Altera IP vendor's IP ( Nortwest Logic)


    B- In order to meet the physical layer requirements. We are using the following IC from Meticom
    http://meticom.com/page2/page17/page19/AppExamp01.html


    Please see the right section of the figure: Meticom MC20902 IC is used between FPGA source and Video Processor MIPI D-PHY Sink


    C- Problem: Cyclone 10 GX devices LVDS lines are limited by 1.434 Gbps

    D- Solution: Cyclone 10 GX devices Transceiver lanes will be used to meet the data rate 2.5 Gbps ( DDR rate, it means clock speed will be half = 1.25 Ghz )

    E- Transceiver capability for DC offset (Vocm) and AC Swing ( from 0.2V to 0.4V peak to peak) is the key


    QUESTION: What is the capability of Cyclone 10 GX transceivers to adjust Vocm and AC Swing?

    MY UNDERSTANDING from the discussion above is that we don't have any control on both of Vocm and AC Swing. is this right?

  2. #12
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    Default Re: LVDS Signal DC offset and AC Swing

    Quote Originally Posted by FvM View Post
    Why do you refer to Cyclone V GX instead of Cyclone 10 GX documents? The relevant are the transceiver PHY user guide https://www.altera.com/content/dam/a...0_xcvr_phy.pdf and the datasheet https://www.altera.com/content/dam/a...10gx-51002.pdf

    However neither C5GX nor C10GX have programmable transceiver Vocm level. Datasheet specifies 0.65 V for C5GX and 0.4/0.5 V for C10GX. Setting transceiver Vocm to 1.25 V LVDS value is simply impossible due to the low transceiver supply voltage.

    You didn't yet answer the question about AC coupling option. What's your application?

    Hi, which table did you use to get Vocm 0.4/0.5V for C10GX in C10GX Datasheet? Table 17 in the transceiver PHY user guide https://www.altera.com/content/dam/a...0_xcvr_phy.pdf says that "Output Swing Level (VOD) 0-31 " based on VCCT. If VCCT is 1.0V and if I pick "Output Swing Level (VOD)" as 200 mV, is Vocm still around 0.4V/0.5V or will it be around 100 mV ( mid range of 200 mV ) ? Fifure 166 Transmit buffer in the transceiver PHY user guide https://www.altera.com/content/dam/a...0_xcvr_phy.pdf shows VCM for TX. Where is this value coming from?

  3. #13
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    Default Re: LVDS Signal DC offset and AC Swing

    Quote Originally Posted by redwood View Post
    Hi, which table did you use to get Vocm 0.4/0.5V for C10GX in C10GX Datasheet? Table 17 in the transceiver PHY user guide https://www.altera.com/content/dam/a...0_xcvr_phy.pdf says that "Output Swing Level (VOD) 0-31 " based on VCCT. If VCCT is 1.0V and if I pick "Output Swing Level (VOD)" as 200 mV, is Vocm still around 0.4V/0.5V or will it be around 100 mV ( mid range of 200 mV ) ? Fifure 166 Transmit buffer in the transceiver PHY user guide https://www.altera.com/content/dam/a...0_xcvr_phy.pdf shows VCM for TX. Where is this value coming from?
    In addition to the questions above, if Vocm is still 0.4V/0.5V regardless of "Output Swing Level (VOD)", can I change VCCT as 0.5V ( Altera documentation provides options with 0.85V, 0.95V, 1V) or VCCT as 2.V. My objective is to change Vocm into an acceptable range by changing VCCT if there is no option of programming Vocm. "Output Swing Level (VOD)" 200mV is acceptable for my application. For Vocm, I have two options: First option: If I can reduce Vocm between 100 and 200 mV by changing VCCT around 0.5V , it will be good enough. Or Second option: If I can increase Vocm around 1V by changing VCCT as 2V.


    Please see my last two posts:

    Question 1: is there any relationship between "Output Swing Level (VOD) 0-31 " and Vocm? if VOD is 200 mV or 500 mV, do I still have the same Vocm?

    Question 2: If I have the same Vocm regardless of VOD value, Can I change the Vocm by changing VCCT voltage value by decreasing or increasing? is there any limitation in terms of minimum or maximum level for VCCT?

  4. #14
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    Default Re: LVDS Signal DC offset and AC Swing

    Quite clearly the 0.4/0.5 Vocm spec is quoted from c10gx datasheet. You have been previously asking about transceiver ability to connect with LVDS IO standard. As far as I see, MIPI D isn't using LVDS levels.

    My brief reading of the transceiver spec tells that it supports two VCCT levels, resulting in the said Vocm values, but I didn't analyse it in detail. I however doubt that varying VCCT beyond the specified levels is a good idea. If you need to adjust Vocm for a DC coupled link, you better think about resistor networks. In a first place I would check if the involved peer can work with the signal level exposed by the transceiver.

  5. #15
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    Default Re: LVDS Signal DC offset and AC Swing

    Quote Originally Posted by FvM View Post
    Quite clearly the 0.4/0.5 Vocm spec is quoted from c10gx datasheet. You have been previously asking about transceiver ability to connect with LVDS IO standard. As far as I see, MIPI D isn't using LVDS levels.

    My brief reading of the transceiver spec tells that it supports two VCCT levels, resulting in the said Vocm values, but I didn't analyse it in detail. I however doubt that varying VCCT beyond the specified levels is a good idea. If you need to adjust Vocm for a DC coupled link, you better think about resistor networks. In a first place I would check if the involved peer can work with the signal level exposed by the transceiver.
    I am confused and didn't get the answer my previous two questions:

    Question 1: Vocm is 0.4/0.5V

    CONFUSED: could you give me table or figure number in Datasheet? This is not clear to me. I couldn't find any section which gives this number for Cyclone 10 GX

    Question 2: is there any relationship between "Output Swing Level (VOD) 0-31 " and Vocm? if VOD is 200 mV or 500 mV, do I still have the same Vocm?

    CONFUSED: YES??? NO????

    Question 3: If I have the same Vocm regardless of VOD value, Can I change the Vocm by changing VCCT voltage value by decreasing or increasing? is there any limitation in terms of minimum or maximum level for VCCT?

    CONFUSED: Where can I find MAXIMUM or MINUMUM values for VCCT?



  6. #16
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    Default Re: LVDS Signal DC offset and AC Swing

    1. Datasheet table 27. Vocm is 0.45/0.5 rather than 0.4/0.5, my fault.
    2. I read the datasheet so, that Vocm isn't affected by Vod setting.
    3. Datasheet table 4.

    In addition, did you notice the absolute maximum receiver input voltage specification of 1.2 V in table 25?

  7. #17
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    Default Re: LVDS Signal DC offset and AC Swing

    Are you using MC20902 to drive HS and LP signals on MIPI D interface or is it utilized as HS driver only? In the latter case, it's probably easy to divide the transceiver output down to meet MIPI D 2.0 level specifications. MC20902 LVDS input is specified with minimal Vocm of 700 mV, outside the transceiver range. The necessary level shift can be possibly achieved with a resistor network.

    I presume that you don't use the MIPI D 8b/10b coding option, so AC coupling isn't possible.

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