USB Microcontroller An Audio Tutorial

Isochronous data transfers can be used by universal serial bus (USB) devices designed to transfer data to or from a host at a constant rate

. Systems streaming audio employ isochronous transfers to transmit audio signals in realtime.

The USB communications protocol outlines specifications for a reliable and fast system of data transfer between a peripheral and a host such as a PC. The protocol defines four different data transfer types (bulk, interrupt, control, and isochronous), each with unique characteristics and uses. USB device classes employ transfer types that suit the needs of particular applications and systems. The USB Audio class uses the isochronous transfer type to stream audio across a USB link.

USB Microcontroller Operational Overview

When a USB device connects to a USB port, the host controlling the port retrieves information about the capabilities of a device through control transfers during a process called enumeration. Devices format this information into descriptors. USB Specification documents define the structure of descriptors.

USB devices send and receive data through endpoints. All USB devices must have one control endpoint through which all enumeration data travels. Descriptors read during enumeration describe the device's other endpoints, their data flow direction, maximum packet size, transfer type, and other characteristics used after enumeration successfully completes.

After examining the device's descriptors, the host determines how to communicate with the device. If the host can allocate resources (data bandwidth, power) requested by the device, then enumeration completes successfully and the host begins operations communication with the device. During operations communications, transfer methods such as those using isochronous endpoint transfers take place.

USB Microcontroller An Audio Tutorial

By: Chris Bartik

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