PIC18F25J50 MICROCHIP [Microchip Technology], PIC18F25J50 Datasheet - Page 379

• Manufacturer Part Number: PIC18F25J50
• Description: 28/44-Pin, Low-Power, High-Performance USB Microcontrollers with nanoWatt XLP Technology
• Manufacturer: MICROCHIP [Microchip Technology]

22.6.4
The USB transceiver consumes a variable amount of
current depending on the characteristic impedance of
the USB cable, the length of the cable, the V
voltage and the actual data patterns moving across the
USB cable. Longer cables have larger capacitances
and consume more total energy when switching output
states.
Data patterns that consist of “IN” traffic consume far
more current than “OUT” traffic. IN traffic requires the
PIC
requires that the host drive the USB cable.
The data that is sent across the USB cable is NRZI
encoded. In the NRZI encoding scheme, ‘0’ bits cause
a toggling of the output state of the transceiver (either
from a “J” state to a “K” state, or vise versa). With the
exception of the effects of bit stuffing, NRZI encoded ‘1’
EQUATION 22-1:
 2011 Microchip Technology Inc.
Legend: V
®
MCU to drive the USB cable, whereas OUT traffic
USB TRANSCEIVER CURRENT
CONSUMPTION
P
P
L
applications use cables no longer than 5m.
I
cable. On the host or hub end of the USB cable, 15 k nominal resistors (14.25 k to 24.8 k) are
present which pull both the D+ and D- lines to ground. During bus Idle conditions (such as between
packets or during USB Suspend mode), this results in up to 218 A of quiescent current drawn at 3.3V.
I
is fully utilized (either IN or OUT traffic) for data that drives the lines to the “K” state, most of the time.
PULLUP
PULLUP
CABLE
USB
ZERO
IN
– Percentage (in decimal) of total bus bandwidth that is used for IN traffic.
– Voltage applied to the V
– Percentage (in decimal) of the IN traffic bits sent by the PIC
is also dependant on bus traffic conditions and can be as high as 2.2 mA when the USB bandwidth
– Length (in meters) of the USB cable. The “USB 2.0 Specification” requires that full-speed
– Current which the nominal, 1.5 k pull-up resistor (when enabled) must supply to the USB
ESTIMATING USB TRANSCEIVER CURRENT CONSUMPTION
I
XCVR
=
(40 mA • V
USB
USB
supply
pin in volts (should be 3.0V to 3.6V).
USB
(3.3V • 5m)
• P
ZERO
PIC18F46J50 FAMILY
• P
bits do not cause the output state of the transceiver to
change. Therefore, IN traffic consisting of data bits of
value, ‘0’, causes the most current consumption, as the
transceiver must charge/discharge the USB cable in
order to change states.
More details about NRZI encoding and bit stuffing can
be found in the USB specification’s Section 7.1,
although knowledge of such details is not required to
make USB applications using the PIC18F46J50 family
of microcontrollers. Among other things, the SIE handles
bit stuffing/unstuffing, NRZI encoding/decoding and
CRC generation/checking in hardware.
The total transceiver current consumption will be
application-specific. However, to help estimate how
much current actually may be required in full-speed
applications,
See
used for a theoretical application.
IN
• L
Equation 22-2
CABLE
)
Equation 22-1
+ I
PULLUP
®
to know how this equation can be
MCU that are a value of ‘0’.
can be used.
DS39931D-page 379