PIC18F2450_08 Datasheet, PDF (151/324 Pages) Microchip Technology – 28/40/44-Pin, High-Performance, 12 MIPS, Enhanced Flash, USB Microcontrollers with nanoWatt Technology

14.6 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.
14.6.1 BUS POWER ONLY
In Bus Power Only mode, all power for the application
is drawn from the USB (Figure 14-10). 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 VBUS and ground must be no more
than 10 μF; otherwise, 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 500 A (or 2.5 mA for high-powered devices that
are capable of remote wake-up) from the 5V VBUS 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 set the IDLEIF bit in the
UIR register.
During the USB Suspend mode, the D+ or D- pull-up
resistor must remain active, which will consume some
of the allowed suspend current: 500A/2.5 mA budget.
FIGURE 14-10: BUS POWER ONLY
VBUS
~5V
VDD
VUSB
VSS
14.6.2 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 14-11 shows an example. Note that an
attach indication is added to show when the USB has
been connected and the host is actively powering
VBUS.
PIC18F2450/4450
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 VBUS high. One
of the I/O pins may be used for this purpose.
The application should never source any current onto
the 5V VBUS pin of the USB cable.
FIGURE 14-11: SELF-POWER ONLY
VBUS
~5V
VSELF
~5V
Attach Sense
100 kΩ
I/O pin
VDD
100 kΩ
VUSB
VSS
14.6.3 DUAL POWER WITH SELF-POWER
DOMINANCE
Some applications may require a dual power option.
This allows the application to use internal power prima-
rily, but switch to power from the USB when no internal
power is available. Figure 14-12 shows a simple Dual
Power with Self-Power Dominance example, which
automatically switches between Self-Power Only and
USB Bus Power Only modes.
FIGURE 14-12: DUAL POWER EXAMPLE
100 kΩ Attach Sense
I/O pin
VBUS
~5V
VDD
100 kΩ
VSELF
~5V
VUSB
VSS
Dual power devices must also meet all of the special
requirements for inrush current and Suspend mode
current, and must not enable the USB module until
VBUS is driven high. For descriptions of those require-
ments, see Section 14.6.1 “Bus Power Only” and
Section 14.6.2 “Self-Power Only”. Additionally, dual
power devices must never source current onto the 5V
VUSB pin of the USB cable.
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.
© 2008 Microchip Technology Inc.
DS39760D-page 149