The PDO selection logic is triggered once the PD controller port is in an Attach. SNK state and the port partner (far end) has advertized its source capabilities. The PDOs received are captured in 0x30 RX Source Capabilities Register. The PDO selection algorithm iteratively processes these PDOs to identify the best fit PDO based on the criteria below:

• The Autonegotiate behavior is not applicable even if AutoNgt=1 if SNKIntrusiveMode = 1 (Bit 9, Port Control Register [0x29]).
• The autocomputed / user selected Minimum Power for the Sink port is compared with every single Source PDO to find a best fit.
• The Sink PDO voltage range must match or fall within the selected Source PDO advertized voltage.
• The Source PDO supply type (eg: Fixed / Variable / Battery) matches the supply type of the Sink PDO that closely matches the Minimum Power requirement (applicable to a perfect PDO match).
• In the process of Autonegotiation, if a perfectly matching Source PDO is found, the iteration ends here. A matching PDO would have the below characteristics:
  • Minimum Power requirement is satisfied by the selected Source PDO.
  • If there are multiple Source PDOs that can satisfy the Minimum Power requirement, the latest one from the list of PDOs that satisfies the criteria is chosen.
  • Capability mismatch bit is not set.
• If a perfectly matching PDO is not found, then the iteration resets until a Source PDO matches the setting chosen for OfferPriority. The setting of Highest Voltage / Current / Power in OfferPriority allows a Source PDO selection that meets the Highest Voltage / Current / Power parameters of the Sink PDO fully / closely matching the Minimum Power value.

The flowchart below shows the Autonegotiate Sink behavior for selecting a PDO.

Figure 1-1 Autonegotiate PDO Selection Flowchart