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AN249

HUMAN INTERFACE DEVICE TUTORIAL

Silicon Laboratories 

AN249

4. Enumeration and Device Detection

Before the HID device can enter its normal operating mode and transfer data with the host, the device must

properly enumerate. The enumeration process consists of a number of calls made by the host for descriptors

stored in the device that describe the device’s capabilities.

The device must respond with descriptors that follow a standard format. Descriptors contain all basic information

about a device. The USB specification defines some of the descriptors retrieved, and the HID specification defines

other required descriptors. The next section discusses the descriptor structure a host expects to receive. The two

sections after that describe the responsibilities of the device and the host during enumeration. These sections refer

to sections of the HID firmware template, which is discussed in detail later in this document.

4.1. Descriptor Structure

Descriptors begin with a byte describing the descriptor length in bytes. This length equals the total number of bytes

in the descriptor including the byte storing the length. The next byte indicates the descriptor type, which allows the

host to correctly interpret the rest of the bytes contained in the descriptor. The content and values of the rest of the

bytes are specific to the type of descriptor being transmitted. Descriptor structure must follow specifications

exactly; the host will ignore received descriptors containing errors in size or value, potentially causing enumeration

to fail and prohibiting further communication between the device and the host.

Descriptor contents are typically stored in FLASH memory space. The file named USB_Descriptor.h in the HID

firmware template declares each value of every descriptor. The file USB_Descriptor.c defines the contents for each

descriptor.

4.1.1. Descriptor Declaration Example

A declaration might look like the following.

//------------------------------------------

// Standard Device Descriptor Type Definition

//------------------------------------------

typedef struct

{

BYTE bLength;

// Size of this Descriptor in Bytes

BYTE bDescriptorType;

// Descriptor Type (=1)

WORD bcdUSB;

// USB Spec Release Number in BCD

BYTE bDeviceClass;

// Device Class Code

BYTE bDeviceSubClass;

// Device Subclass Code

BYTE bDeviceProtocol;

// Device Protocol Code

BYTE bMaxPacketSize0;

// Maximum Packet Size for EP0

WORD idVendor;

// Vendor ID

WORD idProduct;

// Product ID

WORD bcdDevice;

// Device Release Number in BCD

BYTE iManufacturer;

// Index of String Desc for Manufacturer

BYTE iProduct;

// Index of String Desc for Product

BYTE iSerialNumber;

// Index of String Desc for SerNo

BYTE bNumConfigurations;

// Number of possible Configurations

} device_descriptor;

// End of Device Descriptor Type

This declaration exactly conforms to the USB specification’s requirements for the size and content order of the

device descriptor. Some contents are stored in single bytes while others require two bytes.

Rev. 0.2

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