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MIPI DPHY Impedance

David Harkness
Intellectual 385 points


We are using OMAP4430 with an Aptina MT9M114 camera sensor.  Camera has MIPI CSI2 interface with SE output impedance of 40 - 62.5 ohms (diff of 80 - 125 ohms) and a drive level of 140 - 270 mV differential.  Our PCB differential impedance is low at 87 ohms +/- 10% due to design constraints.  Will this be a problematic design.  Do we need to change our PCB Z closer to 100 ohms.  My experience says no.  Please advise.

  • 0
    TI__Mastermind 29520 points

    Hello David,

    #Q: Our PCB differential impedance is low at 87 ohms +/- 10% due to design constraints.  Will this be a problematic design.  Do we need to change our PCB Z closer to 100 ohms

    - For the design of the PCB differential lines on the OMAP board, the PCB designers need to keep in mind the requirements:

    The general guidelines for the PCB differential lines of CSI2 are given as:
    - Single-ended Z0 = 50 Ω
    - Total conductor length on OMAP board < 100 mm.


    In the step, the general rule of thumb for the space S = 2 × W is not designated.

    Although the S = 2 × W rule is a good rule of thumb, it is not always the best solution. The electrical performance will be checked with the frequency-domain specification in Step 3. Even though the designers does not follow theS = 2 × W rule, the differential lines are ok if the lines satisfy the frequency-domain specification.

    Value when driving into differential load impedance anywhere in the range of 80 to 125 Ω. See Chapter 8 of the MIPI D-PHY standard v1.0 for complete specification on the electrical characteristics.

    I assume that this is not problematic design. It might be necessary to apply some types of compensations in case of problems. However, it would be fine if you make PCB Z0 closer to 100ohms. 

    If the lines can have poor electrical performance due to many reasons. The vertical connections such as vias and nonuniform line connection can degrade the electrical performance of the differential lines. And the ground design around the lines can also affect the electrical performance. So, to ensure that the differential lines are well designed, the frequency-domain behaviour must be compared to the frequency-domain specification

    See section A.4.2.4 Step 3: Frequency-domain Specification Guidelines for OMAP Boards in OMAP4430 Data Manual.

    Best regards,

    Yanko

  • 0 in reply to Yanko Todorov-XID
    Intellectual 385 points

    Thank you for your prompt reply.  I understand these requirements and general rute of thumb geometrical requirements of the traces (we use this as a guide but use for true data from the manufacturer calculated parameters from the final design).

    Can you please confirm my understanding regarding requirements for CSI2 (I do not have a copy of MIPI D-PHY V1.00 available).

    1) SE 50 ohms - needed to support MIPI low power single ended signalling.  What tolerance is acceptable?  +/- 10%

    2) Trace length < 10 cm - needed to reduce ISI.  Recommended.

    3) Intralane length match < UI/300 - this is < 0.6 mm at 1 GHz and PCB prop speed of C/1.5.  Your spec is tighter at 0.5 mm.

    4) Interlance length match < UI/100 - this is < 2 mm at 1 GHz and PCB prop speed of C/1.5.  Your spec is tighter at 1.5 mm.

    5) Differential impedance DE 100 ohms - range of 80 - 125 ohms