A Mac and an iPad walk into a bar, Part 3

Over the last several months, I have been working on Dr. PD, an open-source USB Power Delivery analyzer and programmable sink. Diving deep into the world of USB-PD and USB-C has given me a unique appreciation for these rich technologies, and I want to share some of the things I have learned.

As we saw in the previous part, it has taken 35 messages for the Mac and the iPad to set up a basic power contract and establish USB serial communications. There still remains an important task: negotiating a long-term power arrangement.

Recall that both devices are Dual-Role Ports, meaning that they can act as either source or sink based on demand. In practice, the Mac will usually act as the source — though not always, since there are ways to charge the Mac from an iPad, which is quite cool — but that does not diminish the need for a more thorough inspection of the iPad’s power requirements.

After all, there might be multiple devices connected to the Mac’s four USB-C ports, and the Mac itself has a limited amount of power available to charge other devices, even when it is connected to mains power. Therefore, its power-management system needs to understand the needs of each connected device so it can better balance the amount of power sent to each port.

Phase 5: I've got the power

The power negotiation begins with the Mac asking the iPad about its identity and battery setup:

Get Battery Cap	Source	Sink	Battery capability request: Requested battery reference: 0
Battery Capabilities	Sink	Source	Battery capabilities: USB Vendor ID: 0x05AC Product ID: 0x0000 Design capacity: 40Wh Last full-charge capacity: 40Wh Battery reference: valid
Get Battery Status	Source	Sink	Battery status request: Requested battery reference: 0
Battery Status	Sink	Source	Battery status: Battery is present. Present capacity: 40Wh Charging state: charging.
Get Status	Source	Sink
Status	Sink	Source	Port status: Internal temperature: unsupported Present inputs: none Present battery input: none Event flags: none Temperature status: not supported Power status: not limited Power state change: not present in 6-byte SDB
Get Manufacturer Info	Source	Sink	Manufacturer information request: Target: 0 Reference: 0
Manufacturer Info	Sink	Source	Manufacturer information: USB Vendor ID: 0x05AC Product ID: 0x7109 Manufacturer string: iPad

In this exchange, the Mac first asks the iPad for information about its battery. Recall from Part 1 that the Mac learned the iPad has one battery from the Sink_Capabilities_Extended message. The iPad replies that it has a 40 Wh capacity, and that the last full charge also brought the battery to 40 Wh. This, incidentally, is a lie: this iPad’s battery is on its last legs and barely reaches 80% capacity.

Next, the Mac asks for the status of the iPad’s battery. The iPad replies that it is full and that it is charging. That is true on both counts, since the battery was at maximum charge and the iPad was topping it up with power from the Mac.

The Mac then asks for the iPad’s status so that it can determine whether there are any conditions, such as an overtemperature warning, that could affect the power profile. The iPad’s response is entirely unremarkable—to the point that it is probably perfunctory rather than a reflection of its actual status.

Finally, the Mac asks for the iPad's manufacturer information, and the iPad replies with its name, VID, and PID.

Phase 6: My turn

We're finally getting into the home stretch now.

Get Source Cap Extended	Sink	Source
Source Capabilities Extended	Source	Sink	Source capabilities extended information: USB Vendor ID: 0x05AC Product ID: 0x7308 XID value: 0x00000000 Firmware version: 0 Hardware version: 0 Voltage regulation: 150 mA/µs load step; 25% IoC load step magnitude Holdup time: 0 ms Compliance: No asserted compliance flags Touch current: No asserted touch-current flags Peak current 1: 0% overload for 0 ms at 0% duty cycle; VBUS voltage droop not allowed Peak current 2: 0% overload for 0 ms at 0% duty cycle; VBUS voltage droop not allowed Peak current 3: 0% overload for 0 ms at 0% duty cycle; VBUS voltage droop not allowed Touch temperature: IEC 60950-1 Source inputs: external supply present, external supply unconstrained, internal battery present Fixed batteries: 1 Hot-swappable battery slots: 0 Standard Power Range source power data profile rating: 7W Extended Power Range source power data profile rating: unavailable (legacy 24-byte SCEDB) Could not decode all source capability data: Legacy 24-byte Source Capabilities Extended Data Block: missing EPR Source PDP Rating byte required by USB PD 3.2.
Get Sink Cap Extended	Sink	Source
Sink Capabilities Extended	Source	Sink	Sink capabilities extended information: USB Vendor ID: 0x05AC Product ID: 0x7308 XID value: 0x00000000 Firmware version: 0 Hardware version: 0 Sink capabilities extended data block version: Version 1.0 Load step: 150 mA/µs load step Sink load characteristics: 0% overload for 0 ms at 0% duty cycle; VBUS voltage droop not tolerated Compliance: No asserted compliance requirements Touch temperature: Not applicable Fixed batteries: 1 Hot-swappable battery slots: 0 Sink modes: VBUS powered, battery powered Standard Power Range sink power data profile: minimum 5W, operational 30W, maximum 94W Extended Power Range sink power data profile: unavailable (legacy 21-byte SKEDB) Could not decode all sink capability data: Legacy 21-byte Sink Capabilities Extended Data Block: missing EPR Sink PDP bytes required by USB PD 3.2.
Get Battery Cap	Sink	Source	Battery capability request: Requested battery reference: 0
Battery Capabilities	Source	Sink	Battery capabilities: USB Vendor ID: 0x05AC Product ID: 0x0000 Design capacity: 700 Last full-charge capacity: 700 Battery reference: valid
Get Battery Status	Sink	Source	Battery status request: Requested battery reference: 0
Battery Status	Source	Sink	Battery status: Battery is present. Present capacity: 70Wh Charging state: charging.
Get Status	Sink	Source
Status	Source	Sink	Port status: Internal temperature: unsupported Present inputs: none Present battery input: none Event flags: none Temperature status: not supported Power status: not limited Power state change: not present in 6-byte SDB
Get Manufacturer Info	Sink	Source	Manufacturer information request: Target: 0 Reference: 0
Manufacturer Info	Source	Sink	Manufacturer information: USB Vendor ID: 0x05AC Product ID: 0x7308 Manufacturer string: Macintosh
Vendor Defined	Sink	Source	Structured Vendor Defined Message: DISCOVER_IDENTITY REQ Meaning: Requests identity information from the partner, including product type, USB VID/PID, certification ID, and product-type VDOs. Standard or Vendor ID: 0xFF00 Structured Vendor Defined Message version: 1.0
Vendor Defined	Source	Sink	Structured Vendor Defined Message: DISCOVER_IDENTITY ACK Meaning: Returns identity information such as product type, USB VID/PID, certification ID, and product-type VDOs. Standard or Vendor ID: 0xFF00 Structured Vendor Defined Message version: 1.0 Discover Identity data: USB Vendor ID: 0x05AC Capabilities: USB host, USB device, modal operation Certification identifier: 0x00000000 USB Product ID: 0x7308 Device release number: 0x2170 Upstream-facing port Vendor Data Object: Device capability bits: 0b1101 VCONN required: no VBUS required: yes Alternate mode bits: 0b111 Highest USB speed: USB4 Gen3 Upstream-facing port Vendor Data Object: Device capability bits: 0b0111 VCONN required: no VBUS required: yes Alternate mode bits: 0b000 Highest USB speed: USB 2.0 only

These are essentially the same messages that the Mac sent to the iPad, only in reverse. Note how the iPad asks the Mac for both its source and sink capabilities. This is necessary to determine whether it should request a role swap, which, if accepted by the Mac, would cause the source and sink roles to reverse.

In this case, no role swap takes place, because there is no reason for one. Instead, the two devices finally decide that it is time to speak in their own dialect.

Phase 7: Secret lovers

With power negotiations out of the way, the Mac and the iPad take full advantage of the Vendor_Defined message’s ability to carry arbitrary information, exchanging a flurry of opaque data:

Vendor Defined	Source	Sink	Unstructured Vendor Defined Message Standard or Vendor ID: 0x05AC Vendor payload: 0x101 Vendor payload Vendor Data Objects: 0x00000000
Vendor Defined	Sink	Source	Unstructured Vendor Defined Message Standard or Vendor ID: 0x05AC Vendor payload: 0x111 Vendor payload Vendor Data Objects: 0x20000002
Vendor Defined	Source	Sink	Unstructured Vendor Defined Message Standard or Vendor ID: 0x05AC Vendor payload: 0x101 Vendor payload Vendor Data Objects: 0x00000004
Vendor Defined	Sink	Source	Unstructured Vendor Defined Message Standard or Vendor ID: 0x05AC Vendor payload: 0x111 Vendor payload Vendor Data Objects: 0x00030029

And so on, another forty times or so. Unfortunately, there is no way to tell what the two devices are saying to each other, but it certainly looks like an interesting conversation—or, at least, a pretty detailed one.

This kind of deep exploration is just one of the many activities that a USB-PD protocol analyzer like Dr. PD enables! It also lets you troubleshoot faulty connections, characterize power supplies, and much more! If you want your own, you can join our prelaunch page on Crowd Supply for product updates, or stay tuned here for more posts on USB-C, Power Delivery, and the hardware design behind Dr. PD.

Phase 5: I've got the power

Phase 6: My turn

Phase 7: Secret lovers

A Mac and an iPad walk into a bar, Part 2

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