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Original question:

SN75DPHY440SS: Can sn75dphy440ss support for 1 meter?

SN75DPHY440SS: Connecting multiple SN75DPHY440SS based MIPI cables in series?

Thomas Lau54
Prodigy 40 points

Part Number: SN75DPHY440SS

Hi 

We have created a DPHY MIPI CSI FPC extension cable based on the SN75DPHY440SSRHR design:

The track length between the Connector to the Camera and SN75DHY440 retimer is ~200 mm

The track length between the Connector to the Processor and SN75DHY440 retimer is ~35 mm (see below)

From Camera  ------------200 mm ---------------------   SN75DPHY440 Re-timer -------------------------- 35 mm-------------------------------  To Processor

 

We have tested images with 1280x800 (resolution) with 120 fps frame rate with only 2-lane MIPI connected, with just a single MIPI Extension cable everything works.

But as we trying to connect 2x MIPI extension cable in series together, the processor started to lose sync with the frame and in some cases not able to pick up any frame at all.

As I do not have a decent scope to measure the MIPI bus, making debugging rather difficult, wondering if someone else have tried similar design and could shed some light?

Thanks


Thomas

  • 0
    TI__Mastermind 19720 points
    Hello,
    Your issue sounds like a signal integrity issue. What is the DSI clock frequency? What EQ setting are you using? have you tuned EQ with both extension cables? What are the lengths of the 2 extension cables?
  • 0 in reply to Michael.Walker
    Prodigy 40 points

    Hi Michael

    What is the DSI clock frequency?

    The MIPI clock frequency at 400MHz, (the camera system clock is set to 24MHz)

    What EQ setting are you using? have you tuned EQ with both extension cables? 

    I have tried both 0db and 5db with different combination, but it doesnt seem to make much different.

    What are the lengths of the 2 extension cables?

    Each cable has total length of 235mm  (see below)

    From Camera  ------------200 mm ---------------------   SN75DPHY440 Re-timer -------------------------- 35 mm-------------------------------  To Processor 

    Thanks in advance


    Thomas

    Michael.Walker said:
    Hello,
    Your issue sounds like a signal integrity issue. What is the DSI clock frequency? What EQ setting are you using? have you tuned EQ with both extension cables? What are the lengths of the 2 extension cables?

  • 0 in reply to Thomas Lau54
    TI__Mastermind 19720 points
    Does the processor lose sync for all images such as just a plain white image or is it only for certain images? The inter-pair skew of the cables could be causing the problem you could test the inter-pair skew with a scope by viewing the skew between the data and the clock.
  • 0 in reply to Michael.Walker
    Prodigy 40 points

    Hi Michael


    Yes, the processor basically lost sync for all images and the driver just gave up after a few re-tires, I am also suspecting there might be some inter-pair skew issue, as in the current design, there is up to  0.3mm of different in track length between MIPI Clock and data lines at the extension cable. 

    However as we didtn have a proper scope to measure it, its hard to validate at this stage.

    Cheers,

    Thomas

  • 0 in reply to Thomas Lau54
    TI__Mastermind 19720 points
    The inter-pair skew can definitely cause what you are seeing. can you fix the intra-pair skew as best you can?
  • 0 in reply to Michael.Walker
    Prodigy 40 points

    Hi Michael

    I am updating the PCB for a better track length, shall see how its perform once I got the board.


    Cheers

    Thomas