Ways to Use USB in Embedded Systems

by Yingbo Hu, R&D Embedded Software Engineer and Ralph Moore, President of Micro Digital

Universal Serial Bus (USB) is a connectivity specification that provides ease of use, expandability, and good performance for the end user. It is one of the most successful interconnects in computer history. Originally released in 1995 for PCs, it now is expanding into use by embedded systems and is replacing older interfaces such as serial and parallel interfaces as the preferred communication link.

This article has been written as a tutorial on the many ways that USB can be employed in embedded systems. Introduction USB is not a peer-to-peer protocol like Ethernet. One USB device, called the USB host, acts as the master. Other USB devices, called USB devices or USB peripherals, act as slaves. The host initiates all bus transfers. Up to 127 USB devices can be connected to one USB host via up to 6 layers of cascaded hubs. For embedded systems, it is very unusual to have more than one hub. In most cases, one USB device connects directly to one USB host with no hub. A USB host requires a USB host controller and USB host software. The latter is layered from the bottom up as follows: (1) USB host controller driver, (2) USB host stack, and (3) USB class driver. The first layer controls the USB host controller – i.e. it reads and writes registers in the controller and it transfers data. The second layer implements the USB protocol and thus controls connected USB devices. The third layer is device-aware and communicates with and controls the actual device (e.g. disk drive, HID human interface device, CDC communication device, etc.) One USB host stack can support multiple class drivers, simultaneously. In an embedded system there usually is only one USB host controller…

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