USB-C Power Negotiation Link to heading

1. 🔌 Power Role Determination (CC Line) Link to heading

USB‑C uses two Configuration Channel (CC) pins to negotiate roles:

  • Pull‑up (Rp) → indicates a power Source (provides VBUS)
  • Pull‑down (Rd) → indicates a power Sink (consumes VBUS)
  • Resistor magnitude on Rp/Rd also signals default current capability (up to 3 A)

Dual‑Role devices (DRP) randomly pick Rp or Rd during attach, then can swap roles dynamically

2. ⚡ Establishing Standard Power (≤15 W) Link to heading

  • Initial VBUS is set to 5 V after role detection.
  • Passive current advertisement (via CC resistor) allows up to 1.5 A or 3.0 A at 5 V (7.5–15 W)

3. 📘 USB Power Delivery (PD) Negotiation Link to heading

To exceed 15 W (i.e., up to 100 W), the PD protocol kicks in over CC:

  1. Source → Sink: Source_Capabilities (list of PDOs: voltages, currents)
  2. Sink → Source: Request (selects one PDO, specifying current)
  3. Source → Sink: Accept, then adjusts VBUS
  4. Source → Sink: PS_RDY once voltage stabilizes
  5. Each message is followed by GoodCRC.

Negotiated ranges: 5–20 V up to 5 A (100 W). Newer Extended Power Range (EPR) supports up to 48 V/240 W.

4. 🔄 Data Role & Alternate Mode Negotiation Link to heading

USB‑C also supports data role switching and Alternate Modes:

  • Data roles: DFP = host; UFP = device; DRD = dual‑role
  • Data role is auto-assigned via CC similar to power roles.
  • Alternate Modes (e.g., DisplayPort, Thunderbolt): negotiated via vendor-specific messages over CC after PD setup.

Wide reuse of CC for power, data, and orientation detection.

5. 🛠️ Why Crossing USB‑C Is (Usually) Safe Link to heading

  • Standardized roles & limits: Devices won’t exceed safe default scenarios.
  • PD negotiation ensures voltage/current are mutually agreed before ramping VBUS :contentReference[oaicite:8]{index=8}.
  • Safe to plug mismatched USB‑C: If the other end can’t supply/accept power, VBUS stays off or remains at safe 0 V/5 V :contentReference[oaicite:9]{index=9}.
  • Data security: “Juice jacking” (malicious/data-stealing via public USB ports) is a remote theoretic risk – mitigated by modern OS prompts or using charge-only cables/adapters :contentReference[oaicite:10]{index=10}.

6. ⚠️ Why It Can Be Unsafe Link to heading

  1. Poor-quality cables:

    • Non-compliant resistor values (e.g., 10 kΩ instead of 56 kΩ)
    • Fake e‑marker chips can misreport capabilities → device damage.
    • Overvoltage incidents where VBUS pin contacts CC, damaging devices.

  2. Counterfeit chargers:

    • Can overheat, catch fire (NY Post reported generic charger burning blanket).
    • Cyber‑hacked chargers can install malware (“juice‑jacking”).

  3. Alternate Mode failures:

    • If Alternate Mode (e.g. DP) fails, device may display billboard or fall back to USB‑2.0 data only.

✅ Key Takeaways Link to heading

  • Power negotiation: CC lines define roles; PD protocol negotiates higher voltage/current.
  • Data negotiation: Roles and Alternate Modes negotiated via CC/PD after power roles.
  • Safety: Generally safe when using compliant cables/devices—negotiation prevents unsafe power delivery.
  • Risks: Cheap/counterfeit cables or chargers bypass specs, risking overheating, damaging devices, or data theft.
  • Best practices:
    • Use certified USB‑C PD cables with proper e‑markers.
    • Buy reputable chargers.
    • Avoid public “free” USB ports or use data‑blockers.
    • Watch for OS prompts before enabling data transfers.