PIC18F4550ESOSQTP datasheet(185/430 Pages) MICROCHIP

185 / 430 page

Preliminary

DS39632D-page 183

PIC18F2455/2550/4455/4550

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.

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.

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 prima-

rily, but switch to power from the USB when no internal

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.

FIGURE 17-12:

DUAL POWER EXAMPLE

17.7

Streaming Parallel Port

The Streaming Parallel Port (SPP) is an alternate route

option for data besides USB RAM. Using the SPP, an

endpoint can be configured to send data to or receive

data directly from external hardware.

This methodology presents design possibilities where

the microcontroller acts as a data manager, allowing

the SPP to pass large blocks of data without the micro-

controller

actually

processing

it.

application

example might include a data acquisition system,

where data is streamed from an external FIFO through

USB to the host computer. In this case, endpoint

control is managed by the microcontroller and raw data

movement is processed externally.

The SPP is enabled as a USB endpoint port through

the associated endpoint buffer descriptor. The endpoint

must be enabled as follows:

Set BDnADRL:BDnADRH to point to FFFFh.

Set the KEN bit (BDnSTAT<5>) to let SIE keep

control of the buffer.

Set the INCDIS bit (BDnSTAT<4>) to disable

automatic address increment.

Refer to Section 18.0 “Streaming Parallel Port” for

more information about the SPP.

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.

Note 1: If an endpoint is configured to use the

SPP, the SPP module must also be

configured to use the USB module.

Otherwise, unexpected operation may

occur.

2: In addition, if an endpoint is configured to

use the SPP, the data transfer type of that

endpoint must be isochronous only.

VDD

VUSB

I/O pin

VSS

Attach Sense

VBUS

VSELF

100 k

~5V

100 k

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