Universal Serial Bus (USB)

is a high-speed wired communication standard used to connect peripheral devices (keyboard, mouse, printer, flash drive) to a computer or other electronic devices.

• Developed in: 1995
• By: Intel, Microsoft, IBM, and other tech companies

1. Key Features

2. USB Power Delivery (USB PD)

USB PD is a technology that allows higher power to be delivered through a USB cable. It also supports bi-directional power flow – power can go both ways between devices.

How USB Detects a Device

• A device must pull D+ or D– HIGH to 3.3V using a pull-up resistor
• This tells the host/hub that a device is connected
• No pull-up resistor → USB assumes nothing is connected

Pull up registor

┌─────────────────────────────┐
│       USB Controller Chip    │
│  ┌─────────────────────┐    │
│  │  Pull-up Resistor   │    │
│  │  (1.5 kΩ)           │    │
│  └─────────────────────┘    │
│         │                    │
│    ┌────┴────┐              │
│    │         │              │
│   D+        D-              │
└─────────────────────────────┘
         │         │
         ▼         ▼
    Connected to Host via USB Cable

User plugs in device
        ↓
Device gets +5V power
        ↓
Pull-up resistor that build in USB pulls D+ or D– HIGH (3.3V)
        ↓
Host checks D+ / D– lines
        ↓
   ┌────┴────┐
   │         │
 D+ HIGH   D– HIGH   → Low Speed Device
 (Full/Hi-Speed)
   │         │
   └────┬────┘
        ↓
Host sends Reset (D+ & D– LOW for 10ms)
        ↓
Device is ready to communicate

3 Examples of USB

3.1 Example 1: Charging Low-Power Peripherals

Simple: Laptop acts as a power source for small devices.

3.2 Example 2: Docking Station Power Delivery

Simple: One cable from docking station charges the laptop and connects all peripherals.

3.3 Example 3: Powering High-Draw External Storage

Simple: USB-C provides enough power to run external hard drives – no extra plug needed.

3.4 Example 4: Bi-Directional Monitor Charging & Display

3.5 USB On-the-Go (OTG)

Normal: Computer is the host – you connect mouse, keyboard, pen drive to it.

With OTG: Your smartphone or tablet becomes the host!

How it works:

• Use an OTG adapter to connect pen drives, keyboard, mouse, or even a 3D scanner directly to your phone
• Your phone can read files from the pen drive
• Your phone can also act like a pen drive when connected to a computer

Examples:

• Phone + USB mouse = Desktop-like experience
• Phone + Pen drive = Copy files directly

3.5 Advantages

Limitations

USB Versions & Speeds

Cable Length Limits

For longer distances, USB hubs are needed.

USB Connector Types

USB Connector Pinouts

1. USB Type A and Type B (4 Pins) USB 2.0

Type A: Flat port (computers, pen drives)
Type B: Square port (printers, scanners)

1.2. Key Points

• USB uses 4 shielded wires:

  • 2 for power (+5V and GND)
  • 2 for differential data (D+ and D–)

• D+ and D– work together (not separate connections)

• Twisted pair used to reduce noise

• Half-duplex – data flows in one direction at a time

2. Mini B Pinout (5 Pins) USB 2.0

Used in: Old phones, MP3 players, portable hard drives, digital cameras

3. Micro A & Micro B Pinout (5 Pins) USB 2.0

Micro B: Used in most Android phones before USB-C
Micro A: Completely rectangular shape

4. Type A and B USB 3.0

USB 3.0 (SuperSpeed) is the third generation of USB. It is 10 times faster than USB 2.0.

4.1 USB 2.0 vs USB 3.0 – Key Difference

4.2 Why 9 Pins?

USB 3.0 is backward compatible – it works with USB 2.0 devices too.

4.3 Type A SuperSpeed (Blue Flat Port)

Top 4 Pins (USB 2.0):

Bottom 5 Pins (SuperSpeed):

4.4 Type B SuperSpeed

Used in: External hard drives, high-speed printers, drive enclosures

Top Part (USB 2.0):

• 4 pins: Power, Data+, Data–, Ground

Bottom Extra Part (SuperSpeed):

• 5 pins: Transmit (+/–), Receive (+/–), Ground

Shape: Looks like USB 2.0 Type-B but taller (has extra bump on top)

4.4 USB 3.0 Micro-B

Why is it Wider?

USB 2.0 Micro-B had only 5 pins. For SuperSpeed, 5 extra pins were needed. So the connector was made wider to fit them.

USB 2.0 Micro-B:     ████████ (Narrow)
USB 3.0 Micro-B:     ████████████ (Wider, two-part)

Pinout (10 Pins)

4.5 USB Type-C 3.0

Flippable (Reversible) Design

Biggest advantage: You can plug it in any way – upside down or right side up – it works!

5. USB4 Architecture

Three Layers of USB4

┌─────────────────────────────────┐
│     Application Layer           │
│  (Protocol Tunneling & Routing) │
└───────────────┬─────────────────┘
                │
┌───────────────▼─────────────────┐
│     Protocol Layer              │
│  (Packet Creation & Exchange)   │
└───────────────┬─────────────────┘
                │
┌───────────────▼─────────────────┐
│     Physical Layer              │
│  (Data Transmission)            │
└───────────────┬─────────────────┘
                │
    ┌───────────┼───────────┐
    │           │           │
┌───▼───┐ ┌───▼───┐ ┌───▼───┐
│USB 3.x│ │Display│ │ PCIe  │
│ Data  │ │ Port  │ │ Data  │
└───────┘ └───────┘ └───────┘

Layer 1: Application Layer

Layer 2: Protocol Layer

Layer 3: Physical Layer

5.1 What Flows Through USB4?

USB4 can carry three types of data at the same time:

5.2 USB4 – Questions and Answers

Q1: What is Protocol Tunneling?

Answer: When devices communicate, they use a protocol. Protocol Tunneling creates a “pipe” using one protocol and sends data of another protocol through it.

USB4 Protocol Tunneling: Creates a USB-C tunnel to send DisplayPort or PCIe data. It is similar to Alt Mode but does not require a separate DP or PCIe controller.

Q2: What is USB4 Fabric?

Answer: The word “fabric” describes a network of interconnected nodes (like switches). When drawn, the crisscross pattern looks like woven cloth.

USB4 Fabric: Dynamically manages connections between USB4 routers so multiple protocols can share resources at the same time.

Q3: Will Apple Support USB4?

Answer: Yes. Apple’s new MacBooks and Mac Mini with Apple Silicon (M1, M2, M3) support both USB4 and Thunderbolt 3.

6. USB Architecture

6.1. Does USB-C Support USB 2.0?

Key Point: USB-C is just the plug shape. The speed depends on the technology inside!

6.2. USB Architecture – Tiered-Star Topology

6.3. Upstream & Downstream Connections

7. USB Host Controllers and Endpoints

7.1. Host Controllers

Host controllers are the hardware inside your computer that manage USB communication. A software layer called HCD (Host Controller Device) helps the OS talk to this hardware.

Types of Host Controllers:

1. OHCI (Open Host Controller Interface)

2. UHCI (Universal Host Controller Interface)

3. EHCI (Extended Host Controller Interface)

4. VHCI (Virtual Host Controller Interface)

5. USB4 Host Interface

7.2. Endpoints

Endpoints are small memory buffers (storage areas) inside a USB device where data is temporarily stored.

7.2.3 Key Points:

7.2.4 Endpoint 0 – The First Contact

7.2.5 Example: USB Printer

8. USB Pipes

is a logical (software) connection between the host (computer) and an endpoint (memory buffer inside a USB device).

Simple Analogy:

• Endpoint = water tank (memory in device)
• Host = main water source (computer)
• Pipe = the pipe that carries water from source to tank

Pipes are created automatically when you plug in a device and destroyed when you unplug it.

8.1 Types of Pipes

1. Message Pipes

Example: When you plug in a new device, the host uses a message pipe to ask: “Who are you? What is your name?”

2. Stream Pipes

Four Types of Transfers Supported by the USB

8.2 The Default Control Pipe

9. USB Transactions, Packets, and Handshaking

1. What is a USB Transaction?

A USB transaction is one complete communication between the host and a device. Each transaction has up to 3 packets:

2. USB Packet Fields (Structure)

Every USB packet contains these parts:

3. Handshaking – Confirming Data Transfer

Handshaking ensures data is received correctly.

IN Transaction (Host reads data from device)

Other Responses:

OUT Transaction (Host writes data to device)

Other Responses:

9. USB Device States and Classes

1. USB Device States (6 Stages)

When you plug in a USB device, it goes through 6 states:

Plug in → Attached → Powered → Default → Addressed → Configured → Working!
                                                              ↓
                                                         Suspended (idle)

2. USB Device Classes

USB uses Class Codes so the computer can use a generic driver for similar devices – no need for manufacturer-specific drivers.

Class codes are stored in the device’s Descriptor memory.

Common USB Device Classes

Device Plugged In
        ↓
Host reads Class Code from Descriptor(store in computer os)
        ↓
Host loads the matching generic driver
        ↓
Device is ready to use

10. USB Enumeration

Enumeration is the process where the computer (Host) identifies a newly connected USB device and prepares it for use. It happens automatically every time you plug in a device.

The 4 Steps of Enumeration

Step 1: Reset & Speed Detection

Step 2: Default Address & Descriptor Read

Step 3: Set Unique Address

Step 4: Driver Loading & Configuration

Complete Flowchart

Device Plugged In
        ↓
Host detects device (via Hub)
        ↓
Step 1: Host sends Reset → Speed detected
        ↓
Step 2: Device uses Address 0 → Host reads Descriptor
        ↓
Step 3: Host assigns Unique Address (1-127)
        ↓
Step 4: Host loads Driver → Device Configured → Ready!