Designing Embedded Hardware, 2nd Edition

Implementing a USB Interface

One possible solution to implementing USB in your embedded system is to use a USB-to-SPI bridge, such as the Atmel AT76C711. This chip is an AVR processor with a USB subsystem, designed to act as a slave USB controller to a host processor. It has 2K of data RAM, 2K of dual-port RAM for packet processing, 16K of program RAM (organized as 8K × 16 words), an inbuilt DMA controller, an upstream USB port (with one control and five data end points), a separate IrDA-compatible UART, and SPI. The processor may be run at up to 24 MHz and operates off a 3.3 V supply. At reset, the AT76C711 automatically loads its software from an external AT45DBxxx data flash (Chapter 7) to the program RAM. Since the AT76C711's program space is small, one of the smaller AT45DBxxx data flashes will be sufficient. Alternatively, a host processor may load the AT76C711's code directly into its program RAM while it is held in reset.

The AT76C711 may act as a standalone processor, performing USB bridging functions to RS-232C, RS-422/RS-485, IrDA, or other protocols. Alternatively, it may be incorporated as a slave processor in a larger embedded system. The host processor may communicate with the AT76C711 either via SPI or by a standard serial interface through one of the AT76C711's UARTs. The AT76C711 also has general-purpose I/O lines and a UART module that supports RZ encoding for IrDA.

If the processor you are using has a bus interface, then you can add USB using a chip ...