USB Overview
1. What USB Is
USB (Universal Serial Bus) is a hardware + electrical + protocol standard governed by the USB-IF. A USB connection = port (receptacle) + plug (connector) + cable. Two separate things define a USB product: the protocol version (how fast) and the connector shape (how it plugs).
2. Two Key Concepts: Version vs Connector
These are independent. A Type-C cable can be USB 2.0 or USB4; a Type-A port can be USB 2.0 or USB 3.x.
2.1 Protocol Versions (define speed/data capability)
| Version | Year | Data Rate | Notes |
|---|---|---|---|
| USB 1.0/1.1 | 1996 | 1.5 / 12 Mbps | Low / Full speed |
| USB 2.0 | 2000 | 480 Mbps | Hi-Speed |
| USB 3.0 | 2008 | 5 Gbps | SuperSpeed (SS) |
| USB 3.1 | 2013 | 10 Gbps | SuperSpeed+ (gen2) |
| USB 3.2 | 2017 | 20 Gbps | gen2×2 (dual-lane) |
| USB4 | 2019 | 40 Gbps | Thunderbolt-based |
2.2 Connector Shapes (define how it plugs)
| Connector | Side | Notes |
|---|---|---|
| Type-A | Host | Classic rectangle, not reversible. Blue = 3.x, black/white = 2.0 |
| Type-B | Device | Square (printers, drives). SuperSpeed variant is stacked/larger |
| Mini-USB | Device | Small trapezoid (old cameras, MP3 players, PS3) |
| Micro-USB | Device | Thin trapezoid (phones 2007–2015), USB 2.0 typically |
| Type-C | Both | Reversible, universal. Carries power + data + video + audio |
2.3 Not USB (commonly confused)
- Thunderbolt — separate standard but uses the Type-C plug; USB4 is based on TB3. Adds PCIe / 40–80 Gbps.
- Lightning — Apple proprietary, not USB at all; being phased out for Type-C.
2.4 Combining Version × Connector
| Cable | Typical version | Speed |
|---|---|---|
| Type-A → Micro-USB | USB 2.0 | 480 Mbps |
| Type-A → Type-A | USB 2.0 or 3.x | up to 20 Gbps |
| Type-C → Type-C | USB 2.0 to USB4 | up to 40 Gbps |
2.5 Key Facts
- Version = speed; connector = shape. They’re independent.
- Type-A = host, not reversible. Type-C = reversible, universal. Micro-USB = legacy phone.
- Cable rating determines the real data speed, not the port alone.
3. Core Features
USB’s two core capabilities: data transfer and power delivery.
3.1 Data Transfer
- Moves data between host and device (files, peripherals, video, audio).
- Speed set by protocol version (2.1) — but capped by the weakest link: port, cable, or device.
- Display/audio travel over the same data connection as Alternate Modes on Type-C (DisplayPort, HDMI, Thunderbolt) or as USB audio/video classes on any port.
3.2 Power Delivery
| Version | Max Power | Voltage/Current |
|---|---|---|
| USB 2.0 | 2.5 W | 5 V / 0.5 A |
| USB 3.x | 4.5 W | 5 V / 0.9 A |
| USB BC 1.2 | 7.5 W | 5 V / 1.5 A |
| USB PD | up to 240 W | 5–48 V / up to 5 A |
- Power Delivery (PD): negotiated over Type-C CC line; enables fast charging.
- High-power cables (>3 A or >60 W) need an e-marker chip.
Weak vs Strong Electricity (弱电 / 强电)
USB is extra-low voltage (ELV/SELV) — the Chinese engineering terms are 弱电 (weak) vs 强电 (strong):
| 弱电 (Weak) | 强电 (Strong) | |
|---|---|---|
| Voltage | Low, ≤ ~48 V | High, ~110–230 V AC |
| Examples | USB, Ethernet, HDMI | Mains outlets, appliances |
| Safety | Safe to touch, no arc risk | Dangerous, can kill |
- SELV (IEC 61140): ≤ 50 V AC / ≤ 120 V DC. USB max is 48 V DC → always SELV.
- Low voltage ≠ low power: 48 V × 5 A = 240 W (serious wattage, zero shock risk).
- USB danger is heat/fire from bad cables, not electrocution.
- Note: 弱电/强电 is Chinese terminology; equivalent universal terms are ELV/SELV, low-voltage, and “power vs signal” wiring.
AC vs DC
- DC (direct current): flows one direction, constant voltage. Batteries, electronics, all USB.
- AC (alternating current): reverses periodically (mains 50/60 Hz). Power plants, wall outlets.
- Wall outlet = AC; USB = DC. The charger bridges them via an AC-DC adapter (rectifier).
How a Charger (AC-DC Adapter) Works
A “charger” is a regulated power supply — it does NOT control charging.
AC 110–240 V → [filter] → [rectifier] → [switch/SMPS] → [transformer] → [output filter] → 5–48 V DC- Adapter/brick: converts AC→DC, provides stable voltage + current capacity.
- Device (phone): reads adapter capability, negotiates (PD/CC), and manages the actual charging stages (constant current → constant voltage → trickle) + protection.
- A “fast charger” isn’t just more watts — both sides must negotiate a higher voltage (PD) and accept it.
Phone Fast-Charging Protocols
Different brands use different voltages, but all build on USB-PD with a proprietary protocol on top:
| Brand | Protocol | Typical PD voltage | Max power |
|---|---|---|---|
| iPhone | USB-PD | 5 V / 9 V | 20–27 W |
| Samsung | USB-PD + PPS | 5 V / 9 V / PPS 3.3–11 V | 25–45 W |
| HONOR | SuperCharge + PD | 5/9/10/11 V | 66–100 W |
| REDMI/Xiaomi | TurboCharge + PD | 5/9/12/20 V | 33–120 W |
- Universal layer (USB-PD): any USB-C charger → 5 V/9 V, works everywhere but slower.
- Proprietary layer (SuperCharge, TurboCharge, VOOC, PPS): brand charger + brand phone → max speed.
- PPS (PD 3.0 feature): phone can request any voltage (e.g. 3.3–11 V) → enables cross-brand fast charging if both support it.
- With a non-matching charger, phones fall back to standard PD (5/9 V).
Adapter + Cable + Phone (three-way requirement)
Full-speed proprietary charging needs all three links to support the protocol:
Proprietary adapter → rated cable → phone- Adapter must speak SuperCharge/TurboCharge and provide the voltage/current.
- Cable must carry high current (≥5 A) with proper gauge + e-marker; a 3 A generic cable caps power and causes fallback.
- Phone must support the same protocol (negotiation).
- One weak link = fallback to slow standard USB-PD.
Rule of thumb: proprietary fast charging = matching adapter + rated cable + matching phone.
3.3 Cable Anatomy
- 2 power wires: VCC (+5 V), GND.
- 2 data wires: D+ / D− (differential pair).
- USB 3.x adds 4: SSRX± / SSTX± (high-speed lanes).
- Type-C adds 2 CC pins (orientation + PD negotiation).
- Shield for EMI.
3.4 Pins & Pinout
The pin is the core contact point of any wired interface — the physical contact that carries one electrical signal (power or data). A connector is just an organized arrangement of pins.
Device A chip → PCB trace → pin (plug) → pin (receptacle) → PCB trace → Device B chip| Interface | Pins | Purpose |
|---|---|---|
| USB 2.0 | 4 | VCC, GND, D+, D− |
| USB 3.x | 9 | +4 for SuperSpeed lanes |
| USB Type-C | 24 | power, data, CC, SBU, shield |
| Ethernet (RJ45) | 8 | 4 twisted pairs (Tx/Rx) |
| HDMI | 19 | video + audio + CEC |
| TRS audio | 2–3 | Left / Right / Ground |
Why pins matter:
- Pinout = which pin carries what (defined by the standard) — this is what makes interfaces interoperable.
- Pin count ≠ capability — it depends on what the standard assigns each pin to.
- Reversible connectors (USB-C, Lightning) solve orientation with mirrored pin assignments.
- A cable is ultimately wire A → pin 1 ↔ pin 1 → wire A, matching on both ends.