PIC18F2450/4450
14.6.3
DUAL POWER WITH SELF-POWER
DOMINANCE
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.
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.
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.
FIGURE 14-12:
DUAL POWER EXAMPLE
100 kΩ
Attach Sense
I/O pin
FIGURE 14-10:
BUS POWER ONLY
VBUS
~5V
VDD
VBUS
~5V
VDD
100 kΩ
VUSB
VSS
VSELF
~5V
VUSB
VSS
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.
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 indicate when the USB
has been connected.
14.7 Oscillator
The USB module has specific clock requirements. For
full-speed operation, the clock source must be 48 MHz.
Even so, the microcontroller core and other peripherals
are not required to run at that clock speed or even from
the same clock source. Available clocking options are
described in detail in Section 2.3 “Oscillator Settings
for USB”.
FIGURE 14-11:
SELF-POWER ONLY
Attach Sense
VBUS
~5V
I/O pin
VDD
100 kΩ
VSELF
~5V
14.8 USB Firmware and Drivers
VUSB
VSS
100 kΩ
Microchip provides a number of application-specific
resources, such as USB firmware and driver support.
Refer to www.microchip.com for the latest firmware and
driver support.
DS39760A-page 148
Advance Information
© 2006 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
