usb in a nutshell

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USB in a Nutshell.
Making Sense of the USB Standard.


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 Terms and Abbreviations
This chapter is self-explanatory and a necessary evil to any standard.




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 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.


5 USB 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.


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.


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.


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.


9 USB 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.


10 USB Host Hardware and Software
This chapter covers issues relating to the host. This includes frame and
microframe generation, host controller requirements, software
mechanisms and the universal serial bus driver model. Unless you are
designing Hosts, you can skip this chapter.


11 Hub Specification Details
the workings of USB hubs including hub configuration, split
transactions, standard descriptors for hub class etc. Unless you are
designing Hubs, you can skip this chapter.