Microchip Technology GPIODM-KPLCD Data Sheet
PIC18F2455/2550/4455/4550
DS39632E-page 186
© 2009 Microchip Technology Inc.
17.6
USB Power Modes
Many USB applications will likely have several different
sets of power requirements and configuration. The
most common power modes encountered are Bus
Power Only, Self-Power Only and Dual Power with
Self-Power Dominance. The most common cases are
presented here.
sets of power requirements and configuration. The
most common power modes encountered are Bus
Power Only, Self-Power Only and Dual Power with
Self-Power Dominance. The most common cases are
presented here.
17.6.1
BUS POWER ONLY
In Bus Power Only mode, all power for the application
is drawn from the USB (Figure 17-10). This is
effectively the simplest power method for the device.
In order to meet the inrush current requirements of the
USB 2.0 specifications, the total effective capacitance
appearing across V
is drawn from the USB (Figure 17-10). This is
effectively the simplest power method for the device.
In order to meet the inrush current requirements of the
USB 2.0 specifications, the total effective capacitance
appearing across V
BUS
and ground must be no more
than 10
μF. If not, some kind of inrush limiting is
required. For more details, see Section 7.2.4 of the
USB 2.0 specification.
According to the USB 2.0 specification, all USB devices
must also support a Low-Power Suspend mode. In the
USB Suspend mode, devices must consume no more
than 2.5 mA from the 5V V
USB 2.0 specification.
According to the USB 2.0 specification, all USB devices
must also support a Low-Power Suspend mode. In the
USB Suspend mode, devices must consume no more
than 2.5 mA from the 5V V
BUS
line of the USB cable.
The host signals the USB device to enter the Suspend
mode by stopping all USB traffic to that device for more
than 3 ms. This condition will cause the IDLEIF bit in
the UIR register to become set.
During the USB Suspend mode, the D+ or D- pull-up
resistor must remain active, which will consume some
of the allowed suspend current: 2.5 mA budget.
mode by stopping all USB traffic to that device for more
than 3 ms. This condition will cause the IDLEIF bit in
the UIR register to become set.
During the USB Suspend mode, the D+ or D- pull-up
resistor must remain active, which will consume some
of the allowed suspend current: 2.5 mA budget.
FIGURE 17-10:
BUS POWER ONLY
17.6.2
SELF-POWER ONLY
In Self-Power Only mode, the USB application provides
its own power, with very little power being pulled from
the USB. Figure 17-11 shows an example. Note that an
attach indication is added to indicate when the USB
has been connected and the host is actively powering
V
its own power, with very little power being pulled from
the USB. Figure 17-11 shows an example. Note that an
attach indication is added to indicate when the USB
has been connected and the host is actively powering
V
BUS
.
In order to meet compliance specifications, the USB
module (and the D+ or D- pull-up resistor) should not
be enabled until the host actively drives V
module (and the D+ or D- pull-up resistor) should not
be enabled until the host actively drives V
BUS
high. One
of the I/O pins may be used for this purpose.
The application should never source any current onto
the 5V V
the 5V V
BUS
pin of the USB cable.
FIGURE 17-11:
SELF-POWER ONLY
17.6.3
DUAL POWER WITH SELF-POWER
DOMINANCE
DOMINANCE
Some applications may require a dual power option.
This allows the application to use internal power pri-
marily, but switch to power from the USB when no inter-
nal power is available. Figure 17-12 shows a simple
Dual Power with Self-Power Dominance example,
which automatically switches between Self-Power Only
and USB Bus Power Only modes.
Dual power devices also must meet all of the special
requirements for inrush current and Suspend mode
current and must not enable the USB module until
V
This allows the application to use internal power pri-
marily, but switch to power from the USB when no inter-
nal power is available. Figure 17-12 shows a simple
Dual Power with Self-Power Dominance example,
which automatically switches between Self-Power Only
and USB Bus Power Only modes.
Dual power devices also must meet all of the special
requirements for inrush current and Suspend mode
current and must not enable the USB module until
V
BUS
is driven high. For descriptions of those require-
ments, see Section 17.6.1 “Bus Power Only” and
Section 17.6.2 “Self-Power Only”.
Additionally, dual power devices must never source
current onto the 5V V
Section 17.6.2 “Self-Power Only”.
Additionally, dual power devices must never source
current onto the 5V V
BUS
pin of the USB cable.
FIGURE 17-12:
DUAL POWER EXAMPLE
V
DD
V
USB
V
SS
V
BUS
~5V
Note:
Users should keep in mind the limits for
devices drawing power from the USB.
According to USB specification 2.0, this
cannot exceed 100 mA per low-power
device or 500 mA per high-power device.
devices drawing power from the USB.
According to USB specification 2.0, this
cannot exceed 100 mA per low-power
device or 500 mA per high-power device.
V
DD
V
USB
V
SS
V
SELF
~5V
I/O pin
Attach Sense
100 k
Ω
V
BUS
~5V
100 k
Ω
V
DD
V
USB
I/O pin
V
SS
Attach Sense
V
BUS
V
SELF
100 k
Ω
~5V
~5V
100 k
Ω