CrOS EC (Embedded Controller)

USB Power Considerations

Users want to be able to charge external devices using their Chromebook USB ports, e.g. charge a phone from their Chromebook. We want to provide a fast charging experience to end-users, so we prefer to offer high power charging when possible.

Summary of Design Requirements

For explanations of calculations see rest of doc.

Total System Power

Total current needed for external USB devices at 5V:

((Number of Type-C Ports) * (1800mA)) + 1500mA +
((Number of Type-A Ports) * (900mA)) + 600mA§

§ The additional 600mA can be omitted if BC1.2 is not supported for Type-A

Daughter Board Considerations

If a daughter board has 1 Type-A (supporting BC 1.2) and 1 Type-C, the max potential current load at 5V is Type-A Vbus (1500mA) + Type-C Vbus (3000mA) + Type-C Vconn (300mA) = 4800mA

  • The DB ribbon cables need to be able to carry enough current to supply 24W (4.8A * 5V) of power to the DB.
    • This may be on a single or multiple power rails depending on hardware design.
  • The ground path on the ribbon cable from the DB also needs to be able to carry enough current to match the power rails.

USB Type-A Ports

For Type-A ports, the BC 1.2 Specification adds higher power modes on top of the USB 3.2 Specification. While BC 1.2 support isn't required, it is preferred, as it allows end-users to charge their devices more quickly.

BC 1.2 Specification defines multiple modes of operation including, but not limited to:

  • CDP - Charging Downstream Port
    • Allows USB Data. Provides guaranteed 1.5A @ 5V power.
    • ChromeOS device can act as a CDP.
  • SDP - Standard Downstream Port
    • Allows USB Data. Provides guaranteed current defined by USB Specifications
      • For USB3, provides guaranteed current of 0.9A @ 5V.
      • For USB2, provides guaranteed current of 0.5A @ 5V.
    • ChromeOS device can act as a SDP.
  • DCP - Dedicated Charging Port
    • No USB Data. Provides max of 1.5A @ 5V power.
    • ChromeOS device will not act as a DCP.

For detection logic of each mode (e.g. on the D+ and D- pins) and nuance of power/current power requirements, see full BC 1.2 Specification.

Without BC 1.2 support, the max power requirements match that of a Standard Downstream Port (SDP) as defined by various specification (e.g. USB 3.2 Specification).

ChromeOS as Source - Policy for Type-A

If BC 1.2 is supported on a ChromeOS device, then the first Type-A port in use will act as a CDP, providing a maximum current of 1.5A while also enabling USB data. All other Type-A ports will only be SDP, providing a maximum current of 900mA.

Note that the CDP Type-A port allocation is dynamic; the first Type-A port to draw more than 900mA gets to be the CDP, with a maximum current of 1.5A. Then all other Type-A ports get downgraded to the lower, 900mA current limit (i.e. SDP) while the first Type-A port maintains a current draw of more than 900mA. In practice, this means that the first Type-A device plugged in gets to consume 1.5A and any Type-A device inserted after that will only get 900mA.

Once the Type-A device drawing 1.5A stops pulling more than 900mA or is physically removed, then the extra 600mA (as well as CDP advertisement) becomes available to any Type-A port. In practice, Type-A devices only determine current limits when they are first inserted, so any Type-A device that is still plugged in when the 1.5A device is removed will not notice that it can pull more current. This means that the first Type-A device inserted after removing the original 1.5A device gets access to 1.5A.

The allocation of the one CDP Type-A port is unaffected by user interaction with Type-C ports. Once a Type-A port has been claimed as CDP, inserting a Type-C device will not revoke the CDP status of the Type-A port.

For example, the below sequence of events illustrates the above Type-A policy if BC 1.2 is supported:

  1. Insert Type-A phone first
    • Since no other Type-A port is currently supplying more than 900mA, this port can supply 1.5A as the CDP.
    • Phone pulls 1.5A; other Type-A ports are now marked as SDPs limiting current to 900mA, each.
    • Current state: phone @ 1.5A.
  2. Insert Type-A mouse second
    • Mouse is only allowed 900mA since port is SDP.
    • Current state: phone @ 1.5A and mouse @ 900mA.
  3. Remove phone
    • High-current port status is relinquished. Now first Type-A port to draw more than 900mA will claim the one high-current port status (as the CDP).
    • Mouse does not realize that more power is available since most Type-A devices only determine their current limits upon connection.
    • Current state: mouse @ 900ma.
  4. Insert Type-A battery pack
    • Since no other Type-A port is currently supplying more than 900mA, this port can supply 1.5A as the CDP.
    • Battery pack pulls 1.5A; other Type-A ports are now marked as SDPs limiting current to 900mA, each.
    • Current state: mouse @ 900ma and battery pack @ 1.5A.

The total current needed for all Type-A ports at 5V is:

if (BC1.2_Supported)
    (# Type-A Ports)*(900mA) + 600mA
else
    (# Type-A Ports)*(900mA)

USB Type-C Ports

USB Type-C allows for dynamic negotiation of high power contracts; this is accomplished through varying CC resistors and/or USB-C Power Delivery (PD). More in-depth information can be found in the USB Type-C Specification (section 4.5.2.3) and the USB PD Specification. CC resistor contracts can range from 500mA/5V to 3A/5V, while PD contracts can range from 0mA/3.3V to 5A/20V.

ChromeOS as Source - Policy for Type-C

ChromeOS devices currently source power to external USB devices at 5V with a typical current of 1.5A for each Type-C port. In certain scenarios, a single Type-C port can source up to 3A @ 5V.

ChromeOS prefers that the first PD-capable Type-C device that claims 3A should get 3A guaranteed at 5V. Once a PD-capable Type-C device has claimed 3A, then other PD-capable Type-C devices will only be offered a maximum of 1.5A.

If there are no PD-capable Type-C devices claiming 3A, then the first non-PD device will be given 3A until a PD-capable device that claims 3A is inserted.

The 3A is only offered after a minimum delay of 200 ms following the initial connection. One main reason for this delay is to protect against non-PD capabale devices that only sample the CC resistors once at initial connection from continuing to consume 3A after we downgrade the CC resistors to 1.5A at a later point in the future. The motivation for this is that any non-PD device that notices that it can draw more current from a CC resistor change that happens 200 ms after the initial connection will also notice a CC resistor change if we downgrade the CC resistors to a lower current advertisement. We want consistent behavior across non-PD capable devices and PD-capable devices, so we will only offer the additional 1.5A to PD ports after the same delay.

When a device that is currently claiming 3A is removed or proactively reduces its power contract to 1.5A or less, then the next oldest PD-capable device is offered 3A in order. If no PD-capable devices claims 3A, then the oldest non-PD capable device is given 3A through a CC resistor change.

Inserting a Type-A device does not affect the power assignment for Type-C ports; only Type-C devices affect the power of Type-C ports.

For example, the below sequence of events illustrates the above Type-C policy:

  1. A non-PD capable Type-C keyboard is inserted first
    • Keyboard will be offered 1.5A initially
    • Current state: keyboard @ 1.5A.
  2. More than 200ms pass.
    • Since there are no other PD-capable devices and this is the first device, offer this device 3A via CC resistor change.
    • Current state: keyboard @ 3A.
  3. A non-PD capable Type-C mouse is inserted second
    • It will be offered 1.5A since there is already another non-PD device claiming 3A.
    • Current state: keyboard @ 3A and mouse @ 1.5A.
  4. A PD-capable Type-C dock is inserted third
    • Initially negotiate for 1.5A, then wait 200ms after negotiating.
    • Since this is the first PD device, we offer it 3A after 200ms from initial power negotiation.
    • Dock does not want high power from Chromebook; dock continues to selects 1.5A.
    • Keyboard gets to maintain higher 3A current supply.
    • Current state: keyboard @ 3A and mouse @ 1.5A and dock @ 1.5A.
  5. A PD-capable Type-C phone is inserted fourth
    • Phone is initially offered 1.5A.
    • Since there isn't an existing PD-capable device claiming 3A, the phone is offered 3A after waiting the 200ms delay from initial negotiation.
    • The phone wants high power; phone selects 3A.
    • Since PD devices are preferred for 3A, the non-PD keyboard will be downgraded from 3A to 1.5A via a CC resistor change.
    • Current state: keyboard @ 1.5A and mouse @ 1.5A and dock @ 1.5A and phone @ 3A.
  6. A PD-capable Type-C tablet is inserted fifth
    • Since there is already a PD-capable device claiming 3A, the tablet is only offered 1.5A.
    • Current state: keyboard @ 1.5A and mouse @ 1.5A and dock @ 1.5A and phone @ 3A and tablet @ 1.5A.
  7. The PD-capable phone is done charging so it downgrades its power contract to 1.5A without any user interaction
    • The next oldest PD-capable device is offered 3A in order: dock then phone then tablet.
    • The dock and phone continue to select 1.5A, then the tablet takes 3A.
    • Current state: keyboard @ 1.5A and mouse @ 1.5A and dock @ 1.5A and phone @ 1.5A and tablet @ 3A.
  8. The PD-capable tablet is removed
    • The next oldest PD-capable device is offered 3A. If there are no PD-capable devices claiming 3A, then the oldest non-PD capable device is given 3A.
    • The dock and phone continue to select 1.5A, so keyboard is given 3A via CC resistor change.
    • Current state: keyboard @ 3A and mouse @ 1.5A and dock @ 1.5A and phone @ 1.5A.
  9. The non-PD capable keyboard is removed
    • The next oldest PD-capable device is offered 3A. If there are no PD-capable devices claiming 3A, then the next oldest non-PD capable device is given 3A.
    • The dock and phone continue to select 1.5A, so mouse is given 3A via CC resistor change.
    • Current state: mouse @ 3A and dock @ 1.5A and phone @ 1.5A.
  10. The non-PD capable mouse is removed
    • The dock and phone continue to select 1.5A.
    • Current state: dock @ 1.5A and phone @ 1.5A.

Note: Not all released Chromebooks implement the above policy due to pre-existing hardware design constraints.

Type-C ports also need to provide an additional 300mA @ 5V (1.5W) for Vconn on every port. Note: the 1.5W for Vconn may also be supplied at other voltages, such as 455mA @ 3.3V instead.

The total current needed for all Type-C ports at 5V is:

((Number of Type-C Ports) * (1500mA + 300mA)) + 1500mA

The total maximum current needed for a single Type-C port at 5V is (3000mA + 300mA) = 3.3A. This max current for a single port is especially relevant for sizing the daughter board ribbon cable appropriately.