Datasheet

© 2009 Microchip Technology Inc. DS39775C-page 331
PIC18F87J50 FAMILY
22.6 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. Also provided is a means of estimating
the current consumption of the USB transceiver.
22.6.1 BUS POWER ONLY
In Bus Power Only mode, all power for the application
is drawn from the USB (Figure 22-9). 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 V
BUS and ground must be no more
than 10 µF. If not, some kind of inrush liming 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 500 μA (or 2.5 mA for high powered devices that
are remote wake-up capable) from the 5V V
BUS 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: 500 μA/2.5 mA budget.
FIGURE 22-9: BUS POWER ONLY
22.6.2 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 22-10 shows an example. Note that an
attach indication is added to indicate when the USB
has been connected and the host is actively powering
V
BUS.
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 V
BUS high. One
of the 5.5V tolerant I/O pins may be used for this
purpose.
The application should never source any current onto
the 5V V
BUS pin of the USB cable.
FIGURE 22-10: SELF-POWER ONLY
VDD
VUSB
VSS
VBUS
~5V
3.3V
Low IQ Regulator
VDD
VUSB
VSS
VSELF
~3.3V
Attach Sense
100 kΩ
100 kΩ
V
BUS
~5V
5.5V Tolerant
I/O pin