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USB_PD

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USB PD

Category: Power Management

Overview

USB Power Delivery (USB PD) is a protocol that allows USB ports to negotiate higher voltages and currents for faster charging and powering of devices. It enables up to 100W (20V @ 5A) delivery over a USB-C cable using the CC (Configuration Channel) lines.

1. Theory & Fundamentals

  • Negotiates power profiles over CC1/CC2 pins (not VBUS/D+/D-)
  • Standard voltages: 5V, 9V, 15V, 20V (fixed PDOs)
  • Standard currents: 0.9A, 1.5A, 2A, 3A, 5A
  • Power: Up to 100W (USB PD 2.0), 240W (USB PD 3.1 with EPR)
  • BMC encoding: Bi-phase Mark Coding at 300 kbps on CC line
  • Roles: Source (provides power), Sink (consumes), DRP (dual-role)
  • USB PD 3.0: Adds PPS (Programmable Power Supply) for variable V/I

2. Frame / Packet Structure

USB PD message over CC line:
  Preamble(64b) | SOP(20b) | Header(16b) | Data Objects(0-7×32b) | CRC(32b) | EOP(5b)

Message Header:
  NumDataObjects(3b) | MsgID(3b) | PowerRole(1b) | SpecRev(2b) | DataRole(1b) | MsgType(4b)

Control Messages: GoodCRC, GotoMin, Accept, Reject, Ping, PS_RDY,
  Get_Source_Cap, Get_Sink_Cap, DR_Swap, PR_Swap, VCONN_Swap, Wait, Soft_Reset

Data Messages:
  Source_Capabilities (PDO list)
  Request (RDO - Request Data Object)
  BIST, Sink_Capabilities, Vendor_Defined, Battery_Status

PDO (Power Data Object):
  Type(2b) | bits depend on type (Fixed, Variable, Battery, PPS)
  Fixed PDO: Voltage(10b)×50mV | Current(10b)×10mA

3. Protocol Mechanics

  • Negotiation: Source sends Capabilities → Sink sends Request → Source sends Accept → PS_RDY
  • PD state machine: Unattached → Attached → SourceReady/SinkReady
  • GoodCRC: Every message requires GoodCRC acknowledgment
  • Hard Reset: Resets PD state machine; returns to 5V
  • Soft Reset: Resets message IDs without VBUS change
  • Cable communication: SOP' messages to active cable plug EMCA

4. Hardware Implementation

  • PD controller ICs: FUSB302 (onsemi), STUSB4500, IP2721, Renesas ISL95338
  • FUSB302: Most popular open-source PD controller; I2C to MCU
  • USB-C connector: CC1/CC2 pins; orientation detection
  • Rd/Rp resistors: 5.1kΩ Rd on CC pins of sink device
  • CC line: Not connected to USB data; dedicated for PD
  • VCONN: 5V power for active cable; provided by source

5. Register-Level / Configuration

// FUSB302 I2C control (popular PD controller)
#include "fusb302.h"
fusb302_init(); // Initialize FUSB302 via I2C
// State machine: runs in interrupt handler
void fusb302_irq_handler(void) {
    fusb302_handle_irq();
}
// Called when source capabilities received:
void pd_received_capabilities(PDO *pdos, int n) {
    // Find 9V PDO
    for(int i=0;i<n;i++) {
        if(PDO_VOLTAGE(pdos[i]) == 9000) { // 9V in mV
            pd_send_request(i, 2000); // Request 9V @ 2A
            return;
        }
    }
}

6. Driver / Software Development

// Complete PD negotiation flow
void USB_PD_Task(void) {
    switch(pd_state) {
    case PD_STATE_RECV_CAP:
        // Parse source PDOs
        best_pdo = find_best_pdo(src_pdos, num_pdos);
        pd_send_request_rdo(best_pdo, 3000); // 3A
        pd_state = PD_STATE_WAIT_ACCEPT;
        break;
    case PD_STATE_RECV_PS_RDY:
        // VBUS now at negotiated voltage
        start_application(); // e.g. charge at 15V
        break;
    }
}

7. Debugging & Testing

  • USB PD Analyzer: Ellisys USB Explorer, Teledyne LeCroy Voyager
  • CC line scope: Verify BMC encoding at 300 kbps
  • FUSB302 register dump via I2C
  • Common issues: Missing Rd resistors; no GoodCRC response; wrong message ID
  • USB Type-C cable tester: Verify CC pin continuity and E-marker chip

8. Real-World Applications

  1. Laptop charging (65W–100W fast charge)
  2. Smartphone fast charging (USB PD + PPS)
  3. Embedded single-board computers (Raspberry Pi 5 uses USB PD)
  4. Display monitors with USB-C power delivery
  5. Industrial power supplies with dynamic voltage control

9. Advanced Topics & Edge Cases

  • USB PD 3.1 EPR: Extended Power Range up to 240W (48V @ 5A)
  • PPS (Programmable Power Supply): Variable V/I in 20mV/50mA steps
  • Fast Role Swap (FRS): Ultra-fast power role change
  • Alternate Modes: HDMI, DisplayPort, Thunderbolt negotiated via PD
  • Source-only: Simple chargers only send Capabilities, no negotiation complexity

10. Standards & Variants

Version Max Power Features
USB PD 1.0 60W Basic V/I profiles
USB PD 2.0 100W Most common
USB PD 3.0 100W PPS, extended msgs
USB PD 3.1 240W EPR (48V)

💡 Practical Examples

Ex 1: Negotiate 9V: Receive source caps → find 9V PDO → send RDO requesting 9V/2A → wait Accept → PS_RDY.

Ex 2: STUSB4500 standalone: Configure NVM to auto-request 20V @ 5A without MCU firmware.

Ex 3: PPS charging: Request 5V-21V programmable supply; adjust voltage in 20mV steps for optimal charging.

🧪 Practice Questions

Beginner: 1) USB PD max power? 2) Which pins carry PD? 3) What is a PDO? 4) What is PS_RDY? 5) What is PPS?

Intermediate: 1) PD negotiation sequence. 2) Fixed PDO encoding. 3) RDO construction. 4) Hard reset behavior. 5) FUSB302 initialization.

Advanced: 1) Implement full PD sink state machine. 2) Source capabilities design. 3) PPS charger implementation. 4) USB PD 3.1 EPR 240W design. 5) Alternate mode negotiation.

Checklist

  • [ ] Understand USB PD negotiation flow
  • [ ] Initialize FUSB302 via I2C
  • [ ] Parse source PDOs
  • [ ] Send Request Data Object (RDO)
  • [ ] Handle Accept and PS_RDY
  • [ ] Implement GoodCRC ACK
  • [ ] Handle Hard Reset
  • [ ] Implement PPS variable voltage
  • [ ] Debug with CC line oscilloscope
  • [ ] Design complete sink device
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