Cypress Semiconductor FX2LP Technical Information PDF herunterladen (Seite 34)

EZ-USB FX2 Technical Reference Manual

Page 1-8 EZ-USB FX2 Technical Reference Manual v2.1

Control transfers configure and send commands to a device. Because they’re so important, they

employ the most extensive USB error checking. The host reserves a portion of each USB frame

for Control transfers.

Control transfers consist of two or three stages. The SETUP stage contains eight bytes of USB

CONTROL data. An optional DATA stage contains more data, if required. The STATUS (or “hand-

shake”) stage allows the device to indicate successful completion of a CONTROL operation.

1.9 Enumeration

Your computer is ON. You plug in a USB device, and the Windows™ cursor switches to an hour-

glass and then back to a cursor. Magically, your device is connected and its Windows™ driver is

loaded! Anyone who has installed a sound card into a PC and has had to configure countless

jumpers, drivers, and IO/Interrupt/DMA settings knows that a USB connection is miraculous.

We’ve all heard about Plug and Play, but USB delivers the real thing.

How does all this happen automatically? Inside every USB device is a table of descriptors. This

table is the sum total of the device’s requirements and capabilities. When you plug into USB, the

host goes through a sign-on sequence:

1. The host sends a Get Descriptor-Device request to address zero (all USB devices must

respond to address zero when first attached).

2. The device responds to the request by sending ID data back to the host to identify itself.

3. The host sends a Set Address request, which assigns a unique address to the just-attached

device so it may be distinguished from the other devices connected to the bus.

4. The host sends more Get Descriptor requests, asking for additional device information. From

this, it learns everything else about the device: number of endpoints, power requirements,

required bus bandwidth, what driver to load, etc.

This sign-on process is called

Enumeration

1.9.1 Full-Speed / High-Speed Detection

The USB 2.0 Specification requires that high-speed (480 Mbit/sec) devices must also be capable

of enumerating at full-speed (12 Mbit/s). In fact, all high-speed devices begin the enumeration pro-

cess in full-speed mode; devices switch to high-speed operation only after the host and device

have

agreed

to operate at high speed. The high-speed negotiation process occurs during USB

reset, via the “Chirp” protocol described in Chapter 7 of the USB 2.0 Specification.

When connected to a full-speed host, the FX2 will enumerate as a full-speed device. When con-

nected to a high-speed host, the FX2 automatically switches to high-speed mode.

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Cypress Semiconductor FX2LP Technical Information Seite 34