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diff --git a/docs/usb_power.md b/docs/usb_power.md index 860e7fe589..cabbe1e264 100644 --- a/docs/usb_power.md +++ b/docs/usb_power.md @@ -1,250 +1,5 @@ -# USB Power Considerations +*** note +**Warning: This document is old & has moved. Please update any links:**<br> +https://chromium.googlesource.com/chromiumos/platform/ec/+/HEAD/docs/usb_power.md +*** -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. - -[TOC] - -## 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 - -**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 -Type-C port can source up to 3A @ 5V. - -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 -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. 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`. -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. - * 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 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 - * 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 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 `tablet @ 3A`. -8. The PD-capable tablet is removed - * 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 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 only requires 1.5A, so mouse is given 3A via CC - resistor change. - * Current state: `mouse @ 3A` and `dock @ 1.5A`. -10. The non-PD capable mouse is removed - * 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. - -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. - -[BC 1.2 Specification]: <https://www.usb.org/document-library/battery-charging-v12-spec-and-adopters-agreement> -[USB 3.2 Specification]: <https://www.usb.org/document-library/usb-32-specification-released-september-22-2017-and-ecns> -[USB PD Specification]: https://www.usb.org/document-library/usb-power-delivery -[USB Type-C Specification]: https://www.usb.org/document-library/usb-type-cr-cable-and-connector-specification-revision-14-march-29-2019 |