USB is tough to wrap your head around. It’s ubiquitous, and by far the most painless way for people to connect to your widget of the week, but while the HW is relatively simple (on the USB device side anyway), the firmware gets messy really quick and it’s very hard to debug USB since the usual tools in your arsenal like breakpoints and HW debuggers often don’t apply (setting a breakpoint inside the USB interrupt handler will probably break the connection). There are a number of open source USB stacks that make the process of getting USB working less painful (LUFA for AVR, LPCUSB for ARM, etc.), but if you ever want to do anything that varies slightly from the beaten path of USB CDC (Serial over USB), or USB HID, you’re going to at least need to understand the main classes, and how USB communication is structured and passes between the host; the hubs and the end devices.
Jan Axelson’s book is a huge help in trying to get over the initial hurdles and get a bigger picture of what USB is on a protocol and stack level. It’s accessible, clear and you don’t need a Ph.D. in computer science to benefit from reading it. It won’t replace working through some of the key parts of the official USB specs, but it can be a kinder, gentler bridge between relative ignorance and USB enlightenment. It’s a bit light on details in some places, but for a solid overview of how USB works on a low-level messaging level or if you just want to know how to customize a USB descriptor, this is probably the easiest way to get started.
Author: Jan Axelson
Publisher: Lakeview Research