PIC16F1459-ESSQTP datasheet(321/418 Pages) MICROCHIP

321 / 418 page

 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

•VUSB3V3 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.

PIC16F1459-ESSQTP Datasheet(PDF) 321 Page - Microchip Technology

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