PIC16F1459-ESSQTP datasheet(320/418 Pages) MICROCHIP

320 / 418 page

PIC16(L)F1454/5/9

DS41639A-page 320

Preliminary

 2012 Microchip Technology Inc.

26.6

USB Power Modes

The USB peripheral often has different power

requirements and configurations depending on the

application.

The most common cases are presented here:

•Bus Power Only

•Self-Power Only

• Dual Power with Self-Power Dominance

Means of estimating the current consumption of the

USB transceiver are also provided.

26.6.1

BUS POWER ONLY

In Bus Power Only mode, all power for the application

is drawn from the USB (Figure 26-8). 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

F. Circuitry is required to limit inrush current,

see section 7.2.4 of the USB specification for more

detail.

All USB devices must support a Low-Power Suspend

mode which meets the current limits from the 5V VBUS

line of the USB cable according to the USB 2.0 specifi-

cation. For high-powered devices that are remote

wake-up capable, a higher limit is allocated. Refer to

USB Specification 2.0, 7.2.3 for information.

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 budget.

FIGURE 26-8:

BUS POWER ONLY

26.6.2

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 26-9 shows an example.

In order to meet compliance specifications, the USB

module (and the D+ or D- internal pull-ups) should not

be enabled until the host actively drives VBUS high.

The application should never source any current onto

the 5V VBUS pin of the USB cable.

FIGURE 26-9:

SELF-POWER ONLY

26.6.3

DUAL POWER WITH SELF-POWER

DOMINANCE

In Dual Power with Self-Power Dominance mode, the

application uses internal power as the primary source,

but can switch power from the USB when no internal

power is available. Figure 26-10 shows a simple Dual

Power with Self-Power Dominance mode example,

which automatically switches between Self-Power Only

and USB Bus Power Only modes.

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 (or the

D+/D- internal pull-ups) until VBUS is driven high. See

Section 26.6.1

“Bus

Power

Only”

and

Section 26.6.2 “Self-Power Only”

for descriptions of

those requirements. Additionally, dual power devices

must never source current onto the 5V VBUS pin of the

USB cable.

FIGURE 26-10:

DUAL POWER EXAMPLE

VDD

VUSB3V3

VSS

VBUS

Note:

Users should keep in mind the limits for

devices drawing power from the USB.

Refer to USB Specification 2.0, 7.2.3 for

more information.

VDD

VUSB3V3

VSS

VSELF

VDD

VUSB3V3

VSS

VBUS

VSELF

~5V

100 k



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