Datasheet

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Debugging Serial Buses in Embedded System Designs
Electrical Configuration
The host uses an upstream “A” connector and devices use
a downstream “B” connector, as shown in Figure 16. Each
connector has three versions: standard, mini and micro.
The USB 2.0 cable has four wires as shown in Figure 16. Two
wires are used to provide power from the host: 5 V power (red
wire) and ground (black wire). The connectors are designed
so that the power and ground pins are connected before the
data pins. The host provides current from 100 mA to 500 mA
with intelligent power management. For example, power to a
device can be monitored by the host or hub and switched off
if an over-current condition occurs.
Device Endpoints
Device endpoints are data sources and sinks in the device.
Each device can have up to 16 data endpoints (Figure
15). Endpoint 0 is mandatory and is used by the host to
communicate to the device. A pipe is the logical connection
between the application software in the host and device
endpoint.
Enumeration
Enumeration is the configuration process that occurs at
power-on or when a device is hot plugged. The host detects
the presence of the device on the USB bus. Next, the host
polls the device with the SETUP token using address 0 and
endpoint 0. Then, the host assigns a unique address to the
device in the range of 1 to 127. Also, the host identifies the
device speed and data transfer type. During enumeration
a device’s class is determined. The device class defines a
device’s functionality such as printer, mass storage, video,
audio, human interface, etc.
Figure 16. A USB four-wire cable uses the “A” connector at the upstream port and
the “B” connector at the downstream port.
Figure 15. USB endpoints are the device’s data sources and sinks that have logical
pipes to the application software.
Host Controller
A Connector
Device
B Connector
Upstream
Port
USB
Cable
Downstream
Port
5V
D -
GND
D +
Source
Pipe
Device
Address 0 to 127
Sink
Pipe
Endpoint 0
Data Sink
Endpoint 15
Data Sink
Endpoint 0
Data Source
Endpoint 15
Data Source
Host Controller