FX3 Programmers Manual, Doc. # 001-64707 Rev. *C 17
Introduction to USB
Figure 2-5. Control Transfer
2.1.3.5 Link/Physical Layer
The link layer performs additional tasks to increase the reliability of the data transfer. This includes
byte ordering, line level framing, and so on.
More commonly known as the electrical interface of USB 2.0, this layer consists of circuits to
serialize and de-serialize data, pre and post equalization circuits and circuits to drive and detect
differential signals on the D+ and D– lines. All error handling is done at the protocol layer and there
is no discernible low level link layer to manage errors.
2.1.4 Device Detection and Enumeration
One of the most important advantages of USB over other contemporary communication system is its
plug-and-play capability. A change in termination at the USB port indicates that a USB device is
connected.
When a USB device is first connected to a USB host, the host tries to learn about the device from its
descriptors; this process is called enumeration. The host goes through the following 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 every-thing else about the device such as number of endpoints, power
requirements, required bus bandwidth, and what driver to load.
All high-speed devices begin the enumeration process 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.
Because the FX2 configuration is 'soft', a single chip can take on the identities of multiple distinct
USB devices. When first plugged into USB, the FX2 enumerates automatically and downloads
firmware and USB descriptor tables over the USB cable. A soft disconnect is triggered following
which, the FX2 enumerates again, this time as a device defined by the downloaded information. This
patented two-step process, called ReNumeration™, happens instantly when the device is plugged
in, with no hint that the initial download step had occurred.
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