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TPS65982: Autonegotiation logic

Alexander Guschin
Intellectual 340 points
Part Number: TPS65982


Hello!

I am trying to understand how autonegiation works. Are there any documents that describes autonegotiation in details? How do TPS65982 use values from TxSinkCap(0x33), RxSourceCap(0x30) and AutonegotiateSink(0x37) registers.

In example,
I have a PowerSource device that declares 5V/3A, 9V/3A, 12V/3A, 15V/3A and 20V/3.0A power capabilities, both PDO are Fixed Type. It is connected to Sink (based on TPS65982). Sink declares 3 PDO in the TxSinkCaps: Fixed 5V/0.5A, Battery 1V/20V/5W and Variable 1V/20V/3A. AutonegotiateSink (0x37) register configured as follow:

In this case TPS65982 selects for contract PDO number five (20V/3.0A), but requests 1.5A Max/1.5A Operative currents. How this 1.5A is calculated? Why Battery Sink PDO is selected? Why not Variable Sink PDO?

Regards,
Alexander

  • 0
    TI__Genius 12990 points

    Hi Alexander,

    Can I see this PDO/RDO message in a PD sniffer? (like the Lecroy) or active RDO and act PDO register 0x34 and 0x35.

    I attached a great document that will explain all you need to know about PD contracts and using the tool. Let me know if anything is unclear.

    http://www.tij.co.jp/jp/lit/an/slva842/slva842.pdf

    Thanks

  • 0 in reply to Jeffrey Mueller
    Intellectual 340 points
    Hi Jeff,

    Thanks for document, I will learn it.
    Is TotalPhase PD log suitable?

    Regards,
    Alexander
  • 0 in reply to Alexander Guschin
    TI__Genius 12990 points
    Hi Alexander,

    It sure is, its the cheapest currently on the market and very suitable - I like the Lecroy for is user friendly GUI but the TotalPhase does a good job as well. I posted the link for TotalPhase below:)

    www.totalphase.com/.../

    Thanks
  • 0 in reply to Alexander Guschin
    Intellectual 340 points

    I have this PD log.

    power contract.zip

    Regards,
    Alexander

  • 0 in reply to Alexander Guschin
    Intellectual 340 points
    BTW. I noticed that TPS65982 limits advertised current to 3A for 20V PDO if cable has not 5A rating. Is there any way to avoid such limitation?
  • 0 in reply to Alexander Guschin
    TI__Genius 12990 points
    Hi Alexander,

    Ok Thanks, I will take a look at this log and see if I can see what your talking about. I will get back to you asap.

    Thanks
  • 0 in reply to Alexander Guschin
    TI__Genius 12990 points
    If you are wanting 5A with PD commucication the cable has to be e-marked and communicate back to the controller (SOP'/SOP'') to allow this 5A.
  • 0 in reply to Alexander Guschin
    TI__Genius 12990 points
    Hi Alexander,

    You are correct, the sink is requesting 5V@1.5A which I assume is due to the 'Ask for max' bit not being checked in the sink capabilities register. In this case the 82-sink will request only its minimum operating current and depending on the autonegotiate register the sink will choose the source's highest current, voltage or power PDO.

    Let me know if you have any more questions.

    Thanks,
  • 0 in reply to Jeffrey Mueller
    Intellectual 340 points

    Hi Jeff,

    I have read the document you attached. In the paragraph 6.1 there is a phrase: "Sometimes the accepted PDO is Object 1, meaning that the source provides only 5 V. If the sink has higher voltage capabilities (9, 12, 15, or 20 V) then none of the source PDOs 2 through X matched any of the sink PDOs 2 through Y."

    Bolded text is what I am really looking for: How does TPS65982 check that Source PDO matches to Sink PDO?

    Regards,
    Alexander

  • 0 in reply to Alexander Guschin
    TI__Genius 12990 points

    Hi Alexander,

    - the fixed sink PDO is a match if the voltage matches the source PDO and is less current than the source PDO

    - the variable sink PDO is a match if the upper limit of the sink PDO is higher than the source PDO
    and the lower limit of the sink PDO is below the source PDO (so the sink PDO expands a greater range than the source PDO)

    note: variable source PDO is not applicable.

    -you will only achieve a capabilites mismatch bit if you mismatch on the current. If you mismatch on voltage then this voltage mismatched PDO will not be selected. this is because all sources need to be able to source 5V and all sinks need to sink 5V

    - typically the first thing to do is match the highest voltage you can match.Then if the sink wants say 3A and the source can provide only 1.5A then the request will be 'highest_matched_voltage'@1.5A with the capabilities mismatch checked. (which basically means Ill take what you give me but ill check this bit becuase im not happy and its not enough)

    Thanks

  • 0 in reply to Jeffrey Mueller
    Intellectual 340 points

    Hi Jeff,

    Thanks for explaination. It almost clear. Few questoins still opened.

    What is about Battery Sink PDO? Do they follow the same rules?
    What is the PDO matching order? Fixed first?
    If multiple PDO matches, which one is preferred?
    Is it possible cross-type matching? I.e. Fixed Sink PDO to Variable Source PDO and vise versa.

    >> note: variable source PDO is not applicable.
    What does it mean? 

    Regards,
    Alexander.

  • 0 in reply to Alexander Guschin
    TI__Genius 12990 points

    Hi Alexander,

    What I mean is a variable source PDO comes from a AC supply and would basically be a PD contract where the source voltage will be changing within the variable range in real time. An AC supply can be used to supply charging power and whatever is necessary for normal operation
      
    For battery sink PDO and matching preferred order and cross matching, I will update you tomorrow with stuff from the spec and how this matches the 82.

    Thanks

  • 0 in reply to Jeffrey Mueller
    TI__Genius 12990 points
    Hi Alexander,

    This is taken directly from the PD spec:

    A Request Message Shall return one and only one Sink Request Data Object that Shall identify the Power Data Object being requested.
    The Request Message includes the requested power level. For example, if the Source_Capabilities Message includes a Fixed Supply PDO that offers 12V @ 1.5A and if the Sink only wants 12V @ 0.5A, it will set the Operating Current field to 50 (i.e. 10mA * 50 = 0.5A). The Request Message requests the highest current the Sink will ever require in the Maximum Operating Current Field (in this example it would be 100 (100 * 10mA = 1.0A)). The request takes one of two forms depending on the kind of power requested. The Fixed Power Data Object and Variable Power Data Object share a common format (see Table 6-13 and Table 6-14). The Battery Power Data Object uses a different format.

    When a Sink returns a Request Data Object in response to advertised capabilities with this bit set, it indicates that the Sink wants power that the Source cannot provide. This can be due to either a voltage that is not available or the amount of available current. At this point the Source can use the information in the Request Message combined with the contents of the Sink_Capabilities Message to ascertain the Voltage and Current required by the Sink for full operation.
    In this context a Valid Request Message means the following:
     The Object position field Shall contain a reference to an object in the last received Source_Capabilities Message.
     The Operating Current/Power field Shall contain a value which is less than or equal to the maximum current/power offered in the Source_Capabilities Message.
     If the GiveBack flag is set to zero i.e. there is a Maximum Operating Current/Power field:
    _________
    For the 82, battery PDOs are similar to variable PDOs but they operate under power requirements. The preferred PDO is chosen by 0x37 the autonegotiate regsiter and will tell you which one of the source caps the sink will request. You are able to mix and match PDO types.

    Thanks