diff options
| -rw-r--r-- | docs/images/usb_power_on_attach.png | bin | 0 -> 30868 bytes | |||
| -rw-r--r-- | docs/images/usb_power_on_detach.png | bin | 0 -> 20136 bytes | |||
| -rw-r--r-- | docs/usb_power.md | 108 |
3 files changed, 54 insertions, 54 deletions
diff --git a/docs/images/usb_power_on_attach.png b/docs/images/usb_power_on_attach.png Binary files differnew file mode 100644 index 0000000000..2f2fb19743 --- /dev/null +++ b/docs/images/usb_power_on_attach.png diff --git a/docs/images/usb_power_on_detach.png b/docs/images/usb_power_on_detach.png Binary files differnew file mode 100644 index 0000000000..f0e4a5a9d4 --- /dev/null +++ b/docs/images/usb_power_on_detach.png diff --git a/docs/usb_power.md b/docs/usb_power.md index e066a7bba6..860e7fe589 100644 --- a/docs/usb_power.md +++ b/docs/usb_power.md @@ -135,43 +135,42 @@ in-depth information can be found in the [USB Type-C Specification] \(section ### ChromeOS as Source - Policy for Type-C +**Note:** Behavior outlined in this .md file reflects future-planned behavior, +and is not present in the codebase currently. + 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 +typical current of 1.5A for each Type-C port. In certain scenarios, a 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. +ChromeOS prefers that the first PD-capable Type-C device **that requires 3A** +should get 3A guaranteed at 5V. Once the maximum supported number of 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 requiring 3A, then the first non-PD +device will be given 3A until a PD-capable device **that requires 3A** is +inserted. Devices will indicate they require 3A in their sink capabilities, +and this will be used as the trigger to let the EC know to offer that port a +3A source contract. This policy is laid out in the following flow chart. + + + +When a device that is currently claiming 3A is removed, then the next oldest +PD-capable device is offered 3A. If no PD-capable devices require 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: +For example, the below sequence of events illustrates the above Type-C policy +with a board with a maximum number of 1 3A-ports supported: 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. +2. Partner is established to be non-PD through reaching PE\_SRC\_Disabled. * 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`. @@ -180,52 +179,53 @@ For example, the below sequence of events illustrates the above Type-C policy: 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 + * Initially negotiate for 1.5A. + * Since this is a PD device, query its operational current through + requesting Sink Capabilities. + * Dock does not want high power from Chromebook; dock continues to receive 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 this is a PD device, query its operational current through + requesting Sink Capabilities. + * The phone reports it wants 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. + * After tSinkAdj (60 ms), phone is offered 3A through new Source + Capabilities. * 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. + * Tablet is initially offered 1.5A. + * Since this is a PD device, query its operational current through + requesting Sink Capabilities. + * Tablet would like 3A, but the board has reached its maximum number of + supported 3A ports. Note this port's desired current for later. * 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. +7. The PD-capable phone is removed + * The next oldest PD-capable device is offered 3A: the tablet * Current state: `keyboard @ 1.5A` and `mouse @ 1.5A` and `dock @ 1.5A` - and `phone @ 1.5A` and `tablet @ 3A`. + 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`. + * The next oldest PD-capable device requiring 3A is offered 3A. If there + are no PD-capable devices requiring 3A, then the oldest non-PD capable + device is given 3A. + * The dock only requires 1.5A, so keyboard is given 3A via CC resistor + change. + * Current state: `keyboard @ 3A` and `mouse @ 1.5A` and `dock @ 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 + * The next oldest PD-capable device requiring 3A is offered 3A.. If there + are no PD-capable devices requiring 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 + * The dock only requires 1.5A, so mouse is given 3A via CC resistor change. - * Current state: `mouse @ 3A` and `dock @ 1.5A` and `phone @ 1.5A`. + * Current state: `mouse @ 3A` and `dock @ 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`. + * The dock does not require 3A. + * Current state: `dock @ 1.5A`. Note: Not all released Chromebooks implement the above policy due to pre-existing hardware design constraints. |
