Datasheet

 2012 Microchip Technology Inc. Preliminary DS41639A-page 321
PIC16(L)F1454/5/9
26.6.4 USB TRANSCEIVER CURRENT
CONSUMPTION
The USB transceiver consumes a variable amount of
current, depending on following factors:
• Impedance of USB cable
• Length of cable
•V
USB3V3 supply voltage
• Data patterns across cable
Data patterns consist of “IN” and “OUT” traffic. “IN”
traffic consumes more current and requires the
microcontroller to drive the USB cable, while “OUT”
traffic requires the host to drive the USB cable.
The data sent across the USB cable is NRZI encoded.
A ‘0’ in the NRZI encoding scheme toggles the output
state of the transceiver (from “J” state to a “K” state, or
vice versa). A ‘1’ in the NRZI does not change the
output state of the transceiver, with the exception of the
effects of bit-stuffing. Because “IN” traffic consists of
data bits of value ‘0’, the transceiver must charge/
discharge the USB cable to change states resulting in
the most current consumption.
More details about NRZI encoding and bit-stuffing can be
found in the USB 2.0 specification's section 7.1, although
knowledge of such details is not required to make USB
applications using PIC
®
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, Equation 26-1 can be used.
Example 26-2 shows how this equation can be used for
a theoretical application.
Note: Longer cables have larger capacitance
and consume more total energy when
switching output states.