PIC18F2450/4450
2006 Microchip Technology Inc.
Advance Information
DS39760A-page 151
The USB specification limits the power taken from the
bus. Each device is ensured 100 mA at approximately 5V
(one-unit load). Additional power may be requested, up
to a maximum of 500 mA. Note that power above a one-
unit load is a request and the host or hub is not obligated
to provide the extra current. Thus, a device capable of
consuming more than a one-unit load must be able to
maintain a low-power configuration of a one-unit load or
less, if necessary.
The USB specification also defines a Suspend mode.
In this situation, current must be limited to 500
μA,
averaged over 1 second. A device must enter a
Suspend state after 3 ms of inactivity (i.e., no SOF
tokens for 3 ms). A device entering Suspend mode
must drop current consumption within 10 ms after
Suspend. Likewise, when signaling a wake-up, the
device must signal a wake-up within 10 ms of drawing
current above the Suspend limit.
14.9.5
ENUMERATION
When the device is initially attached to the bus, the host
enters an enumeration process in an attempt to identify
the device. Essentially, the host interrogates the device,
gathering information such as power consumption, data
rates
and
sizes,
protocol
and
other
descriptive
information; descriptors contain this information. A
typical enumeration process would be as follows:
1.
USB Reset: Reset the device. Thus, the device
is not configured and does not have an address
(address 0).
2.
Get Device Descriptor: The host requests a
small portion of the device descriptor.
3.
USB Reset: Reset the device again.
4.
Set Address: The host assigns an address to the
device.
5.
Get Device Descriptor: The host retrieves the
device descriptor, gathering info such as
manufacturer, type of device, maximum control
packet size.
6.
Get configuration descriptors.
7.
Get any other descriptors.
8.
Set a configuration.
The exact enumeration process depends on the host.
14.9.6
DESCRIPTORS
There are eight different standard descriptor types of
which five are most important for this device.
14.9.6.1
Device Descriptor
The device descriptor provides general information,
such as manufacturer, product number, serial number,
the class of the device and the number of configurations.
There is only one device descriptor.
14.9.6.2
Configuration Descriptor
The configuration descriptor provides information on
the power requirements of the device and how many
different interfaces are supported when in this
configuration. There may be more than one configura-
tion for a device (i.e., low-power and high-power
configurations).
14.9.6.3
Interface Descriptor
The
interface
descriptor
details
the
number of
endpoints used in this interface, as well as the class of
the interface. There may be more than one interface for
a configuration.
14.9.6.4
Endpoint Descriptor
The endpoint descriptor identifies the transfer type
some other specifics for the endpoint. There may be
many endpoints in a device and endpoints may be
shared in different configurations.
14.9.6.5
String Descriptor
Many of the previous descriptors reference one or
more string descriptors. String descriptors provide
human
readable
information
about
the
layer
they
describe. Often these strings show up in the host to
help the user identify the device. String descriptors are
generally optional to save memory and are encoded in
a unicode format.
14.9.7
BUS SPEED
Each USB device must indicate its bus presence and
speed to the host. This is accomplished through a
1.5 k
Ω resistor which is connected to the bus at the
time of the attachment event.
Depending on the speed of the device, the resistor
either pulls up the D+ or D- line to 3.3V. For a low-
speed device, the pull-up resistor is connected to the
D- line. For a full-speed device, the pull-up resistor is
connected to the D+ line.
14.9.8
CLASS SPECIFICATIONS AND
DRIVERS
USB specifications include class specifications which
operating
system
vendors
optionally
support.
Examples of classes include Audio, Mass Storage,
Communications and Human Interface (HID). In most
cases, a driver is required at the host side to ‘talk’ to the
USB device. In custom applications, a driver may need
to be developed. Fortunately, drivers are available for
most common host systems for the most common
classes of devices. Thus, these drivers can be reused.
