PIC18F2455_07 Datasheet, PDF (185/430 Pages) Microchip Technology – 28/40/44-Pin, High Performance, Enhanced Flash, USB Microcontrollers with nanoWatt Technology

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
VBUS
~5V
VDD
VUSB
VSS
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
VBUS
~5V
VSELF
~5V
Attach Sense
100 kΩ
I/O pin
VDD
100 kΩ
VUSB
VSS
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
VBUS
~5V
100 kΩ Attach Sense
I/O pin
VDD
100 kΩ
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.
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. An 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:
1. Set BDnADRL:BDnADRH to point to FFFFh.
2. Set the KEN bit (BDnSTAT<5>) to let SIE keep
control of the buffer.
3. 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.
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.
© 2007 Microchip Technology Inc.
Preliminary
DS39632D-page 183