USB-CC

[elagil]

See #47 (comment)

[Finomnis]

Here a high level AI summary as base for discussions.

This is only meant as a quick overview, we should definitely use the official USB spec to implement this crate instead of AI summaries.

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A usb-cc crate would implement the non-PD USB Type-C CC state machines: attach/detach detection, role detection, orientation detection, VBUS/VCONN decisions, and current advertisement/detection for 5 V Type-C operation.

It would not implement USB-PD, PDO negotiation, alt modes, e-marker discovery, or PD role swaps.

Scope

The crate should cover:

• source-only ports
• sink-only ports
• dual-role-power ports
• source-preferred DRP behavior
• sink-preferred DRP behavior
• accessory/debug detection where relevant
• debounce, timing, detach, and error recovery behavior

Shared logic

All modes need common CC-pin classification logic. Given the state of CC1 and CC2, the crate should decide whether the port sees:

• no connection
• a source
• a sink
• plug orientation
• required VCONN
• advertised source current
• audio/debug accessory patterns
• or an invalid/unsafe state

This shared classifier is the core primitive used by all state machines.

Source state machine

A source presents Rp and waits for a sink presenting Rd.

Abstract flow:

Unattached.SRC
  -> AttachWait.SRC
  -> Attached.SRC
  -> Unattached.SRC / ErrorRecovery

Behavior:

• advertise source capability on CC
• debounce sink detection
• determine orientation
• optionally enable VCONN
• enable VBUS once attach is valid
• monitor for detach or invalid CC states
• disable VBUS/VCONN on detach

Sink state machine

A sink presents Rd and waits for a source presenting Rp.

Abstract flow:

Unattached.SNK
  -> AttachWait.SNK
  -> Attached.SNK
  -> Unattached.SNK / ErrorRecovery

Behavior:

• advertise sink presence on CC
• debounce source detection
• determine orientation
• wait for valid VBUS
• detect advertised source current
• enable sink path once attach is valid
• monitor for VBUS loss, detach, or current changes

Dual-role state machine

A DRP alternates between source and sink presentation until a partner is found.

Abstract flow:

Unattached.DRP
  -> present Rp
      -> if Rd detected: Source attach path
  -> present Rd
      -> if Rp detected: Sink attach path

Once attached, it behaves exactly like the corresponding source or sink state machine.

Try.SRC / Try.SNK

For preferred-role devices, the crate should also model biased DRP behavior.

Try.SRC:

Prefer source
  -> try source attach first
  -> if that fails, allow sink attach

Try.SNK:

Prefer sink
  -> try sink attach first
  -> if that fails, allow source attach

These are still CC-only behaviors; they should not imply USB-PD role swaps.

Error recovery

All modes should share an error recovery path for impossible, unsafe, or failed states.

Examples:

• invalid CC combinations
• unexpected VBUS
• missing VBUS after attach
• source power fault
• failed detach cleanup

Error recovery should return the port to a safe unattached state.

Summary

At a high level, the crate is a timer-driven Type-C CC connection manager containing:

CC classification
+ Source attach state machine
+ Sink attach state machine
+ DRP toggle state machine
+ Try.SRC / Try.SNK preference handling
+ VBUS/VCONN decision logic
+ debounce, detach, and error recovery

The output of the crate should be abstract attach-state decisions such as role, orientation, current limit, VCONN requirement, attached/detached state, and error state.

[Finomnis]

@Finomnis Interesting suggestion! I didn't think about generalizing that piece of the stack yet. For existing solutions like embassy, its UCPD driver would probably have to change and then implement some trait of that new usb-cc crate in order to be compatible. I'm not sure what to put in such a crate yet, I need to do some research on that.

@elagil I don't think it would have to change, I think its existing driver is fully compatible with what we need. So we can implement a cc driver just like usb-pd.

So the flow will probably be: initial cc handshake and role discovery using usb-cc, then once connected handover to usb-pd. Once disconnected, back to usb-cc.