diff options
| author | Jameson Thies <jthies@google.com> | 2021-11-12 21:42:24 +0000 |
|---|---|---|
| committer | Commit Bot <commit-bot@chromium.org> | 2021-11-13 01:57:46 +0000 |
| commit | 727adfc0ef964f9acdd9793668ea19ba846443f3 (patch) | |
| tree | af2a0157162a4a7f9d1d894b72eb596422993041 /docs | |
| parent | df56c2b3e8f0e618fdf1175e851acdef4a7b296e (diff) | |
| download | chrome-ec-727adfc0ef964f9acdd9793668ea19ba846443f3.tar.gz | |
documentation: Adding background paragraph on USB PD to usb-c.md
Basic CL to ramp up on git/gerrit flow. Adding a background paragraph on
USB PD to usb-c.md to provide context for why we are using a state
machine to handle USB-PD messaging.
BUG=b:206148726
TEST=None
Signed-off-by: Jameson Thies <jthies@google.com>
BRANCH=usbc_md_update
Change-Id: I3c60669964b93ccbfa3303cd12ac54858158fb52
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3278637
Reviewed-by: Benson Leung <bleung@google.com>
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/usb-c.md | 21 |
1 files changed, 19 insertions, 2 deletions
diff --git a/docs/usb-c.md b/docs/usb-c.md index 1e009f82b2..78ca970267 100644 --- a/docs/usb-c.md +++ b/docs/usb-c.md @@ -1,7 +1,24 @@ # EC Implementation of USB-C Power Delivery and Alternate Modes -USB-C PD requires a complex state machine as USB-C PD can operate in many -different modes. This includes but isn't limited to: +USB-C is intended to be a flexible connector supporting multiple data rates, +protocols, and power in either direction. For one connector to support varying +states of power delivery, the system and what it is connected to +must decide who will act as the source (drives power) and sink (consumes power). +Additionally, they need to decide the correct voltage and current for the source +to drive by taking into account not only the source's and sink's capabilities, +but also what the cable can support. Resistance of pull-up and pull-down resistors +on the configuration channel (CC) ports of the USB-C connector are used to +negotiate who is source and who is sink when a new USB-C connection is established. +This allows for setting power characteristics to default USB2 (500mA) default +USB3 (900mA) 1.5A and 3.0A at 5V. Additional power requirements using USB-PD must +then be negotiated by the source and sink over the CC pins of the USB-C connectors. +Beyond power contract negotiations, USB PD messages can be used to enable alternate +modes (Example: DisplayPort) and send a class of messages called Structured Vendor +Defined Message (SVDMs), which are not related to power delivery. The additional +flexiblity and functionality in USB-C requires support from the OS. + +From the system, USB PD requires a complex state machine as USB PD can +operate in many different modes. This includes but isn't limited to: * Negotiated power contracts. Either side of the cable can source or sink power up to 100W (if supported by device). |