---
title: "USB protocols followed in USB design"
subject: "Instrumentation II"
chapter: 3
key: "semester_5/instrumentation_ii:usb-design-protocols"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/usb-design-protocols
site: BCT Engineering
---

# USB protocols followed in USB design

Instrumentation II · Chapter 3: Serial Interfacing With Microprocessor Based System

- Asked in: 2071 Shrawan, 5 marks
- Video: [USB Protocol Explained: Basics, Versions, Connectors, Working, Signals, and Objectives](https://www.youtube.com/watch?v=6knlPjpa_DE) — Engineering Funda
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/usb-design-protocols
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3

## Question

Explain USB protocols which should be followed during the USB design.

## Short answer

USB design must follow 4-wire differential 90 Ω signaling, NRZI with bit stuffing, token-data-handshake packets with CRC and data toggling. Transfer types (control, bulk, interrupt, isochronous) are chosen. Enumeration uses descriptors to assign address 0 then a unique address. Power: suspend draws <2.5 mA.

## Answer

A USB design must follow the rules of the USB specification at the electrical, protocol and software levels.

1. **Physical and electrical:** four-wire cable (VBUS, D+, D-, GND); differential signalling with 90 ohm impedance; 1.5 kohm pull-up on D+ (full speed) or D- (low speed); 15 kohm pull-downs at the host; 5 V on VBUS, 100 mA default and up to 500 mA after configuration (900 mA in USB 3.0).
2. **Bus states and encoding:** J, K and SE0 states; NRZI coding with bit stuffing; LSB first.
3. **Packet protocol:** every transaction uses token, data and handshake packets with SYNC, PID, address, endpoint, CRC5/CRC16 and EOP; data toggle (DATA0/DATA1) for error recovery.
4. **Transfer protocols:** control, bulk, interrupt and isochronous, selected according to the device needs and bandwidth.
5. **Enumeration protocol:** on attach and reset, the host reads the descriptors (device, configuration, interface, endpoint, string), assigns a unique 7-bit address and selects a configuration. The device must respond at address 0 until it is addressed and must support endpoint 0.
6. **Device class protocols:** follow standard classes (HID, mass storage, CDC, audio) so that generic drivers can be used.
7. **Power management:** suspend (draw < 2.5 mA after 3 ms idle), resume/remote wake-up, bus-powered or self-powered declaration.
8. **Hub protocol:** hubs report port status, handle reset and low-speed isolation (PRE).
9. **Compliance:** pass the USB-IF compliance tests; a vendor ID (VID) and product ID (PID) must be assigned.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. During enumeration before it is addressed, a USB device must respond at which address?
   A. Address 1 on endpoint 1
   B. Any address it chooses
   C. Address 0 using endpoint 0
   D. Address 127
   Answer: C. A new device answers at default address 0 on endpoint 0 until the host assigns a unique 7-bit address. (Key point: Enumeration)

2. What is the default bus current for a USB 2.0 device before configuration, and the maximum after?
   A. 900 mA then 500 mA
   B. 100 mA then up to 500 mA
   C. 2.5 mA then 100 mA
   D. 500 mA then 100 mA
   Answer: B. A device draws 100 mA by default and up to 500 mA after configuration. (Key point: Power limits)

3. What are the packet types of every USB transaction?
   A. Start, stop and parity
   B. Token, data and handshake
   C. Setup, hold and strobe
   D. Request, grant and release
   Answer: B. A transaction uses token, data and handshake packets with SYNC, PID, CRC and EOP. (Key point: Packet protocol)

4. In power management, a device must enter suspend and draw less than 2.5 mA after how long idle?
   A. 20 ms
   B. 1 s
   C. 10 ms
   D. 3 ms
   Answer: D. The answer states suspend after 3 ms of bus idle, drawing under 2.5 mA. (Key point: Suspend)

## Related in Instrumentation II

- [USB protocols and common packet fields](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-packet-fields) — Instrumentation II, chapter 3, asked 2×
- [USB to RS-422 cable design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-rs422-cable) — Instrumentation II, chapter 3
- [USB to RS-232 adapter design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-rs232-adapter) — Instrumentation II, chapter 3