---
title: "USB interface chips: device vs host"
subject: "Instrumentation II"
chapter: 3
key: "semester_5/instrumentation_ii:usb-interface-chips"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/usb-interface-chips
site: BCT Engineering
---

# USB interface chips: device vs host

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

- Asked in: 2071 Chaitra, 4 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/usb-interface-chips
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3

## Question

What is a USB interface chip? Why are they required? Compare and contrast USB device interface chips and USB host interface chips.

## Short answer

A USB interface chip (SIE) handles packet protocol, NRZI, and CRC to enable USB on non-USB controllers. Device chips like FT232R are slaves with pull-up resistors, responding to tokens. Host chips like SL811HS are masters that generate tokens, SOF, supply VBUS, and manage enumeration.

## Answer

A **USB interface chip** is an integrated circuit that implements the USB protocol layer (serial interface engine, SIE, packet handling, NRZI, CRC, endpoint buffers) so that a microcontroller or system with no built-in USB can communicate over USB.

#### Why required
- USB timing and protocol (differential signalling, bit stuffing, CRC, PID handling, enumeration) are complex and need hardware support.
- Reduces CPU load and software size; the microcontroller only reads and writes endpoint buffers.
- Allows a design to be USB-enabled quickly using a parallel/SPI/UART connection.

#### Device chips vs host chips
| Point | USB device interface chip | USB host interface chip |
|---|---|---|
| Role | Slave: responds to the host | Master: controls bus, enumerates devices |
| Function | Handles endpoints, descriptors, responses to tokens | Generates tokens, SOF, schedules transfers, root hub |
| Complexity | Simple, few endpoints | Complex; needs driver/stack and bandwidth management |
| Power | Draws power from bus | Supplies VBUS power to devices |
| Pull resistors | 1.5 kohm pull-up on D+ or D- | 15 kohm pull-downs on D+ and D- |
| Software | Firmware and descriptors | Host stack (class drivers, enumeration code) |
| Examples | FT232R, CP2102, PDIUSBD12, ISP1181, CY7C68013, PL2303 | SL811HS, MAX3421E, ISP1760, CY7C67300, VNC1L |
| Applications | Mouse, flash drive, adapters | Embedded system reading a USB flash drive or keyboard |

## Check yourself

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

1. Why is a USB interface chip required?
   A. It replaces the cable
   B. It supplies the 5 V
   C. It increases the speed to 480 Mbps
   D. It implements the complex USB protocol layer in hardware, reducing CPU load
   Answer: D. Chips handle SIE, NRZI, CRC and PID so the microcontroller only reads and writes endpoint buffers. (Key point: Purpose)

2. Which resistors does a host chip side use?
   A. 1.5 kohm pull-ups on D+
   B. 1.5 kohm pull-downs
   C. No resistors
   D. 15 kohm pull-downs on D+ and D-
   Answer: D. Hosts have 15 kohm pull-downs; devices have the 1.5 kohm pull-up. (Key point: Pull resistors)

3. Which statement correctly contrasts the chips?
   A. Both supply VBUS
   B. Device chip enumerates the host
   C. Host chip is master, generates tokens/SOF and supplies VBUS
   D. Device chip is master and generates SOF
   Answer: C. The host chip controls the bus; the device chip is a slave that responds. (Key point: Roles)

4. Which is a USB host interface chip?
   A. FT232R
   B. PL2303
   C. MAX3421E
   D. CP2102
   Answer: C. MAX3421E (also SL811HS, ISP1760) is a host chip; the others are device chips. (Key point: Examples)

## Related in Instrumentation II

- [USB enumeration; device vs host interface chips](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-enumeration) — Instrumentation II, chapter 3
- [USB host, hub, device and packets](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-host-hub-device-packets) — Instrumentation II, chapter 3