Silicon Laboratories Stepper Machine Specifications download pdf (Page 25)

Enumeration—Loading Descriptors

 Enumeration

 The activity that identifies

and assigns unique

addresses to devices

attached to a bus

 Makes USB devices hot-

pluggable

 The host is always checking

the bus for new devices via

Interrupt transfers

 The host cannot

communicate with a USB

device until that device has

been properly enumerated

 Invisible to user

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

// Sample Standard Device Descriptor Type

// Definition Fields

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

Length (18)

Descriptor Type (DEVICE, CONFIGURATION,

INTERFACE, ENDPOINT, HID)

USB Spec Release Number (0200h)

Device class (hub type…Human Interface defined in

other descriptor, CDC described here)

Device Sub-class (00h)

Device protocol (00h)

Maximum Packet size (64 bytes – max for the

endpoint)

Vendor ID (ID assigned by USB IF)

Product ID (ID assigned by product manufacturer)

Device release number (revision code of device)

Manufacturer (ABC Corp)

Product (string identifier)

Serial Number (1234)

Number of configurations (1 or more configurations

can follow)

VID/PID discussed next.

This is loaded in the MCU memory.

We covered a lot of detail regarding USB transfers and the means by which the host

retrieves descriptors from the device. This whole process we are describing is

called Enumeration. After the attach event the host begins by resetting the device

and then sends a series of standard request to retrieve the descriptors. If the host can

accommodate the required bandwidth request by the device as well as the power

consumption if it is bus powered, then the host will go ahead and enumerate the

device and normal operation would begin. There can be no communication on the

bus without a successful enumeration. Once again this diagram is showing the

device descriptor as part of the enumeration. This is important because when we

attach a device there is a software driver that has to be loaded on the host in order to

let an application communicate over the USB to the device. We will cover that

next.

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Silicon Laboratories Stepper Machine Specifications Page 25

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