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PIC18F2450/4450

DS39760A-page 148

Advance Information

2006 Microchip Technology Inc.

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.

FIGURE 14-10:

BUS POWER ONLY

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.

FIGURE 14-11:

SELF-POWER ONLY

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

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”.

14.8

USB Firmware and Drivers

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.

VDD

VUSB

VSS

VBUS

~5V

VDD

VUSB

VSS

VSELF

~5V

I/O pin

Attach Sense

100 k

VBUS

~5V

100 k

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.

VDD

VUSB

I/O pin

VSS

Attach Sense

VBUS

VSELF

100 k

~5V

100 k

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