PIC18F4550ESOSQTP datasheet(188/438 Pages) MICROCHIP

188 / 438 page

PIC18F2455/2550/4455/4550

DS39632E-page 186

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.

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 VBUS 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 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 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

VBUS.

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 17-11:

SELF-POWER ONLY

17.6.3

DUAL POWER WITH SELF-POWER

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

VBUS 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 VBUS pin of the USB cable.

FIGURE 17-12:

DUAL POWER EXAMPLE

VDD

VUSB

VSS

VBUS

~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.

VDD

VUSB

VSS

VSELF

~5V

I/O pin

Attach Sense

100 k

VBUS

~5V

100 k

VDD

VUSB

I/O pin

VSS

Attach Sense

VBUS

VSELF

100 k

~5V

100 k

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