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USB Protocol

Overview

USB (Universal Serial Bus) is a widely adopted serial bus standard for connecting peripherals to host computers and embedded systems. It supports multiple speeds from 1.5Mbps to 40Gbps, hot-plugging, power delivery, and a rich class driver ecosystem. Embedded USB stacks (device mode) are common in STM32, PIC32, and similar MCUs.

1. Theory & Fundamentals

  • Solves: Universal peripheral connectivity replacing RS-232, PS/2, parallel ports
  • Physical layer: Differential pair (D+ and D−), 5V VBUS power
  • Speeds: Low Speed 1.5Mbps, Full Speed 12Mbps, High Speed 480Mbps, SuperSpeed 5Gbps+
  • Topology: Tree — 1 host, up to 127 devices via hubs
  • Connector types: Type-A, Type-B, Mini-B, Micro-B, Type-C
  • Power delivery: 5V@500mA (USB 2.0), up to 100W (USB PD)

2. Frame / Packet Structure

Token Packet:  SYNC | PID | ADDR(7) | ENDP(4) | CRC5
Data Packet:   SYNC | PID | DATA(0–1023 bytes) | CRC16
Handshake:     SYNC | PID (ACK / NAK / STALL)

PID values: OUT=0xE1, IN=0x69, SETUP=0xB4, DATA0=0xC3, DATA1=0x4B, ACK=0xD2, NAK=0x5A
  • Frames: 1ms (Full Speed), 125µs microframes (High Speed)
  • SOF (Start of Frame) token sent every frame to keep sync

3. Protocol Mechanics

  • Host-centric: Host initiates all transfers; devices respond
  • Transfer types: Control, Bulk, Interrupt, Isochronous
  • Endpoint: Logical data pipe; EP0 reserved for control (enumeration)
  • Enumeration: Host assigns address, reads descriptors, loads driver
  • Data toggle: DATA0/DATA1 alternates to detect lost packets
  • NRZI encoding: Data encoded as No-Return-to-Zero-Inverted + bit stuffing

4. Hardware Implementation

  • USB transceiver: Built into most MCUs (STM32F1, STM32F4, etc.)
  • Pull-up resistors: 1.5kΩ on D+ (Full Speed) or D− (Low Speed) signals device presence
  • ESD protection: Essential — PRTR5V0U2X or similar on D+/D−
  • Crystal: USB requires accurate clock (8MHz or 12MHz with PLL to 48MHz)
  • Cable: 90Ω differential impedance; keep D+/D− matched and paired

5. Register-Level / Configuration

USB at bare-metal level is complex. Most use USB middleware:

// Using STM32 HAL USB Device library (CDC class)
extern USBD_HandleTypeDef hUsbDeviceFS;
USBD_Init(&hUsbDeviceFS, &FS_Desc, DEVICE_FS);
USBD_RegisterClass(&hUsbDeviceFS, &USBD_CDC);
USBD_CDC_RegisterInterface(&hUsbDeviceFS, &USBD_Interface_fops_FS);
USBD_Start(&hUsbDeviceFS);

// Send data via CDC (Virtual COM Port)
uint8_t msg[] = "Hello USB\r\n";
CDC_Transmit_FS(msg, sizeof(msg)-1);

6. Driver / Software Development

// USB CDC receive callback
static int8_t CDC_Receive_FS(uint8_t *Buf, uint32_t *Len) {
    // Process received data
    for (uint32_t i = 0; i < *Len; i++) {
        process_byte(Buf[i]);
    }
    USBD_CDC_SetRxBuffer(&hUsbDeviceFS, &Buf[0]);
    USBD_CDC_ReceivePacket(&hUsbDeviceFS);
    return USBD_OK;
}

7. Debugging & Testing

  • Use USB protocol analyzer (e.g., Total Phase Beagle, Wireshark USBPcap)
  • Device Manager / lsusb: Verify enumeration
  • Common issues: Clock not 48MHz → enumeration fails; missing pull-up → not detected; stall on EP0 → descriptor error
  • Test with USB HID class first (simplest class)

8. Real-World Applications

  1. USB CDC (Virtual COM Port): Replace RS-232 with USB for PC communication
  2. USB HID: Custom joystick, keyboard, mouse implementation
  3. USB MSC (Mass Storage): Expose SD card as USB drive
  4. USB DFU: Firmware update over USB
  5. USB Audio: Custom audio interface for DAW applications

9. Advanced Topics & Edge Cases

  • USB OTG: Device can be both host and device (USB On-The-Go)
  • USB 3.x: SuperSpeed uses additional SS TX/RX pairs alongside USB 2 D+/D−
  • USB PD: Power delivery negotiation up to 100W via CC pins
  • Composite device: Appear as multiple classes simultaneously
  • USB bootloader: DFU mode for field firmware updates

10. Standards & Variants

Version Speed Notes
USB 1.0/1.1 1.5/12 Mbps Low/Full speed
USB 2.0 480 Mbps High speed, most common
USB 3.2 Gen 1 5 Gbps SuperSpeed
USB 3.2 Gen 2 10 Gbps SuperSpeed+
USB 4 40 Gbps Thunderbolt compatible

💡 Practical Examples

Example 1: Virtual COM Port (CDC)

Configure STM32 as CDC device; PC sees it as a serial port. Send debug output at full USB 2.0 speed.

Example 2: USB HID Joystick

// Define HID report: X, Y axes + 4 buttons
uint8_t report[3] = {x_axis, y_axis, buttons};
USBD_HID_SendReport(&hUsbDeviceFS, report, 3);

Example 3: USB DFU Bootloader

Implement DFU class; PC uses dfu-util to flash new firmware over USB.

🧪 Practice Questions

Beginner

  1. What is the maximum number of USB devices on one host?
  2. Name the 4 USB transfer types.
  3. What voltage does USB 2.0 provide on VBUS?
  4. What is EP0 used for?
  5. What does USB enumeration mean?

Intermediate

  1. Explain USB NRZI encoding and bit stuffing.
  2. How does a USB device signal its speed to the host?
  3. Describe the USB enumeration sequence step by step.
  4. What is the difference between USB HID and CDC class?
  5. How does USB PD negotiate charging voltage?

Advanced

  1. Write a USB HID descriptor for a custom 6-axis controller.
  2. Implement a USB composite device with CDC + MSC simultaneously.
  3. How would you debug enumeration failure on a custom USB PCB?
  4. Explain USB isochronous transfer and why it's used for audio.
  5. Design a USB 2.0 full speed PHY circuit for a custom MCU board.

Hands-on Projects

  1. USB Serial Logger: STM32 CDC device — receive UART data, forward to PC over USB.
  2. USB HID Macro Keyboard: Custom keyboard with programmable macro keys.
  3. USB MSC Reader: Expose SPI flash chip as USB mass storage device.

Checklist

  • [ ] Explain USB topology and enumeration
  • [ ] Implement USB CDC (Virtual COM Port) on STM32
  • [ ] Implement USB HID device
  • [ ] Write USB descriptors (device, configuration, interface, endpoint)
  • [ ] Use USB protocol analyzer to debug enumeration
  • [ ] Implement USB DFU bootloader
  • [ ] Create composite USB device
  • [ ] Handle USB suspend/resume power states
  • [ ] Test USB at FS and HS speeds
  • [ ] Implement USB OTG host mode
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