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USB PD negotiation during mismatch of capabilities - KBA232916

ArunKumarChoul
Employee

USB PD negotiation during mismatch of capabilities - KBA232916

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The PD negotiation occurs with the sink looking for the source PDO which matches the voltage according to its own capabilities, starting from the highest PDO. Depending on the current in the source PDO, two things can happen:

  • If the current in the source PDO is more than the operating current of the sink, it will send a request with its own operating current and draw the same current.
  • If the current in that source PDO is less than the operating current of the sink PDO, it will send a request with the operating current of the source mentioned and the "capability mismatch" bit set.

Section 6.4.2.3 of the Power Delivery specification R3 states as follows:

Capability Mismatch occurs when the sink cannot satisfy its power requirements from the capabilities offered by the source. In this case, the sink shall make a valid request from the offered capabilities and shall set the “Capability Mismatch” bit.

When a sink returns a Request Data Object in response to the 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 verify the voltage and current required by the sink for full operation.

When these operations are performed, a valid Request Message means the following:

  1. The Object position field: It contains a reference to an object in the last received Source_Capabilities message.
  1. The Operating Current/Power field: It contains a value which is less than or equal to the maximum current/power offered in the Source_Capabilities message.
  1. If the Giveback flag is set to ‘0’, which indicates that there is a Maximum Operating Current/Power field:
    • If the Capability Mismatch bit set to ‘1’:
    • The Maximum Operating Current/Power field may contain a value larger than the maximum current/power offered in the Source_Capabilities message’s PDO as referenced by the Object Position field. This enables the sink to indicate that it requires more current/power than is being offered. If the sink requires a different voltage, this will be indicated by its

                Sink_Capabilities message.

    • If the Capability Mismatch bit set to ‘0’:
      • The Maximum Operating Current/Power field contains a value less than or equal to the maximum current/power offered in the Source_Capabilities message’s PDO as referenced by the Object Position field.

4. If the GiveBack flag is set to ‘1’, i.e., there is a Minimum Operating Current/Power field:

    • The Minimum Operating Current/Power field contains a value less than the Operating Current/Power field.

    Example using BCR as a sink:

    This example requires the following:

    1) CY4532 EZ-PD CCG3PA evaluation kit

    2) CY4500 EZ-PD protocol analyzer

    3) CY4533 EZ-PD barrel connector replacement (BCR) evaluation kit

    Block diagram of the setup:

    ArunKumarChoul_0-1624384954813.png


    Source 1 PDO available: 5V@3A and 10V@ 3A

    ArunKumarChoul_1-1624385026679.png

    Figure 1. Source 1 PDO capabilities

  1. BCR sink requirement: 12V
    Because the available Source PDOs were 5V and 10V, the negotiated voltage was 10V which is according to the PD specification.

  2. ArunKumarChoul_2-1624385169193.png

    Figure 2. BCR Sink capability mismatch and PDO request


    Source 2 PDO available: 5V@ 3A and 13V@ 3A

    ArunKumarChoul_3-1624385323775.png

    Figure 3. Source 2 PDO capabilities

     

    BCR sink requirement: 15V@ 3A.
    Because the available Source PDOs were 5V and 13V, the negotiated voltage is 13V, which is according to the PD specification.

    ArunKumarChoul_4-1624385481326.png

    Figure 4. BCR Sink capability mismatch and PDO request

     

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