USB in a Nutshell Page 2 www.beyondlogic.org
Starting out new with USB can be quite daunting. With the USB 2.0 specification at 650 pages one could easily be
put off just by the sheer size of the standard. This is only the beginning of a long list of associated standards for
USB. There are USB Class Standards such as the HID Class Specification which details the common operation of
devices (keyboards, mice etc) falling under the HID (Human Interface Devices) Class - only another 97 pages. If
you are designing a USB Host, then you have three Host Controller Interface Standards to choose from. None of
these are detailed in the USB 2.0 Spec.
The good news is you don’t even need to bother reading the entire USB standard. Some chapters were churned
out by marketing, others aimed at the lower link layer normally taken care off by your USB controller IC and a
couple aimed at host and hub developers. Lets take a little journey through the various chapters of the USB 2.0
specification and briefly introduce the key points.
Chapter Name Description Pages
1 Introduction Includes the motivation and scope for USB. The most important piece
of information in this chapter is to make reference to the Universal
Serial Bus Device Class Specifications. No need reading this chapter.
2
2 Terms and
Abbreviations
This chapter is self-explanatory and a necessary evil to any standard. 8
3 Background Specifies the goals of USB which are Plug’n’Play and simplicity to the
end user (not developer). Introduces Low, Full and High Speed ranges
with a feature list straight from marketing. No need reading this chapter
either.
4
4 Architectural
Overview
This is where you can start reading. This chapter provides a basic
overview of a USB system including topology, data rates, data flow
types, basic electrical specs etc.
10
5USB Data Flow
Model
This chapter starts to talk about how data flows on a Universal Serial
Bus. It introduces terms such as endpoints and pipes then spends
most of the chapter on each of the data flow types (Control, Interrupt,
Isochronous and Bulk). While it’s important to know each transfer type
and its properties it is a little heavy on for a first reader.
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6 Mechanical
This chapter details the USB’s two standard connectors. The important
information here is that a type A connector is oriented facing
downstream and a type B connector upstream. Therefore it should be
impossible to plug a cable into two upstream ports. All detachable
cables must be full/high speed, while any low speed cable must be
hardwired to the appliance. Other than a quick look at the connectors,
you can skip this chapter unless you intend to manufacture USB
connectors and/or cables. PCB designers can find standard footprints
in this chapter.
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7 Electrical This chapter looks at low level electrical signalling including line
impedance, rise/fall times, driver/receiver specifications and bit level
encoding, bit stuffing etc. The more important parts of this chapter are
the device speed identification by using a resistor to bias either data
line and bus powered devices vs self powered devices. Unless you are
designing USB transceivers at a silicon level you can flip through this
chapter. Good USB device datasheets will detail what value bus
termination resistors you will need for bus impedance matching.
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8 Protocol Layer Now we start to get into the protocol layers. This chapter describes the
USB packets at a byte level including the sync, pid, address, endpoint,
CRC fields. Once this has been grasped it moves on to the next
protocol layer, USB packets. Most developers still don’t see these
lower protocol layers as their USB device IC’s take care of this.
However an understanding of the status reporting and handshaking is
worthwhile.
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9USB Device
Frame Work
This is the most frequently used chapter in the entire specification and
the only one I ever bothered printing and binding. This details the bus
enumeration and request codes (set address, get descriptor etc) which
make up the most common protocol layer USB programmers and
designers will ever see. This chapter is a must read in detail.
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