ES: Universal Serial Bus (USB)

Introduction to USB

USB (Universal Serial Bus) was introduced to replace legacy serial and parallel ports with a unified interface. It defines:

• Physical layer (cables, connectors, signaling)
• Protocol layer (packets, transactions)
• Device classes (HID, Mass Storage, CDC, etc.)
• Power delivery rules

Unlike UART or SPI, USB is host-driven, meaning devices cannot talk unless the host initiates the communication.

	 Host (PC / MCU)
		   |
		   |  USB Cable
		   |
	-------------------
	|       HUB       |
	-------------------
	  |       |      |
   Device   Device  Device

Key architecture rule:

There is always one host controlling the bus.

USB Architecture

USB uses a tiered-star topology.

Host

• Controls bus timing
• Starts all transfers
• Performs enumeration

Device

• Responds to host requests
• Cannot initiate communication
• Has endpoints

Hub

• Expands number of ports
• Routes traffic between host and devices

USB Physical Layer

USB signaling depends on version.

USB 2.0

• Uses D+ and D− differential pair
• 4 wires:
• VBUS (5V)
• GND
• D+
• D−

   USB Cable (USB 2.0)

   ----------------------
   | VBUS | D+ | D- | G |
   ----------------------

Differential Signaling

Instead of single-ended signals:

   D+ ------
             > Differential Receiver
   D- ------

Advantages:

• High noise immunity
• Low EMI
• Higher speed

This is critical in embedded boards where routing quality matters.

USB Speed Modes

In microcontrollers (STM32, ESP32-S3, NXP etc.), most common:

• Full Speed (12 Mbps)
• High Speed (480 Mbps)

USB Logical Architecture

USB communication is based on:

• Endpoints
• Pipes
• Transfers
• Transactions
• Packets

Let’s build it step by step.

Endpoints

An endpoint is a logical channel inside a USB device.

Think of it like a mailbox.

USB Device
-----------------
| Endpoint 0    |  Control
| Endpoint 1 IN |
| Endpoint 1 OUT|
| Endpoint 2 IN |
-----------------

• Endpoint 0: Always present (Control endpoint)
• IN: Device → Host
• OUT: Host → Device

USB Transfer Types

USB defines 4 transfer types:

Control Transfer

Used for:

• Enumeration
• Configuration
• Command/Response

Example:

• Host asking device: “Who are you?”

Bulk Transfer

Used for:

• Large reliable data
• No timing guarantee

Example:

• USB flash drive

Interrupt Transfer

Used for:

• Small periodic data
• Low latency

Example:

• Keyboard

Isochronous Transfer

Used for:

• Real-time streaming
• No retransmission

Example:

• Audio streaming

USB Packet Structure

Every USB transfer is packet-based.

+--------+---------+---------+-------+
|  SYNC  |  PID    | Payload |  CRC  |
+--------+---------+---------+-------+

• SYNC: Synchronization
• PID: Packet ID
• Payload: Data
• CRC: Error check

USB Enumeration Process

Enumeration is the process where the host identifies and configures a device.

Step-by-step:

1. Device connected
2. Host detects pull-up resistor
3. Host resets device
4. Host requests descriptors
5. Host assigns address
6. Host sets configuration

Device Plugged
      |
Host Detects Pull-up
      |
Bus Reset
      |
Get Device Descriptor
      |
Assign Address
      |
Get Configuration Descriptor
      |
Set Configuration

After this → device is ready.

USB Descriptors

Descriptors describe device capabilities.

Device Descriptor

Contains:

• Vendor ID (VID)
• Product ID (PID)
• USB version
• Class type

Configuration Descriptor

• Power requirements
• Interfaces

Interface Descriptor

• Functional grouping

Endpoint Descriptor

• Transfer type
• Max packet size

Example simplified structure:

Device
 └── Configuration
      └── Interface
           ├── Endpoint IN
           └── Endpoint OUT

USB Device Classes

USB classes allow generic drivers.

HID (Human Interface Device)

• Keyboard
• Mouse

CDC (Communication Device Class)

• Virtual COM port

Mass Storage

• Flash drive

Audio

• Microphone
• Speaker

In embedded firmware:

Example (STM32 + CDC):

PC <----USB----> STM32
        |
     Virtual COM
        |
     Firmware UART bridge

Very common for debugging.

USB in Microcontrollers

Modern MCUs integrate USB peripheral.

Example MCUs:

• STM32F4
• ESP32-S3
• NXP LPC series

Internal block:

		AHB Bus
		   |
	----------------
	| USB Device   |
	| Controller   |
	----------------
		   |
	PHY Interface
		   |
		D+ / D-

Firmware must:

• Enable USB clock
• Configure GPIO pins
• Initialize stack
• Handle interrupts

USB Interrupts

USB is interrupt-driven.

Events:

• Reset
• Setup packet received
• Data IN complete
• Data OUT received

Typical ISR flow:

USB_IRQHandler()
{
   if (RESET)
      handle_reset();

   if (SETUP)
      process_setup();

   if (DATA_IN)
      transmit_next_packet();
}

USB Power Delivery

USB 2.0:

• Default: 100 mA
• After enumeration: 500 mA

USB-C:

• Can negotiate up to 100W
• Uses CC pins

In embedded design:

• Never draw >100mA before enumeration
• Implement current limiting
• Protect with TVS diodes

USB Type Connectors

USB-A

• Host side

USB-B

• Printer style

Micro USB

• Old embedded boards

USB-C

• Reversible
• High power
• High speed

Real Embedded Examples

Firmware Update via USB (DFU)

PC Tool ----USB----> MCU Bootloader
                     |
                Flash new firmware

USB CDC Debug Console

printf("Hello\n");

UART -> USB CDC -> PC Terminal

USB Mass Storage Data Logger

Device appears as flash drive:

MCU logs data
   |
FAT File System
   |
PC reads CSV file

Design Considerations

PCB Layout:

• 90Ω differential impedance
• Keep D+ / D− matched length
• Avoid stubs
• Add ESD protection

Clock Accuracy:

• USB FS requires ±0.25% accuracy
• Use crystal or internal calibration

USB Host Mode in Embedded

Some MCUs support OTG (On-The-Go).

Example:

Embedded Linux Board
        |
   USB Host Mode
        |
   USB Flash Drive

In this case MCU becomes host and enumerates device.