Microchip Technology DM240015 Data Sheet

 2012-2013 Microchip Technology Inc.

DS30009312B-page 269

19.1.3

USING AN EXTERNAL INTERFACE

Some applications may require the USB interface to
be isolated from the rest of the system.
PIC24FJ128GC010 family devices include a complete
interface to communicate with and control an external
USB transceiver, including the control of data line
pull-ups and pull-downs. The V

BUS

 voltage generation

control circuit can also be configured for different V

BUS

generation topologies. 
Refer to the “PIC24F Family Reference Manual”,
Section 27. “USB On-The-Go (OTG)”

 for information

on using the external interface.

19.1.4

CALCULATING TRANSCEIVER 
POWER REQUIREMENTS

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

USB

 supply

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. The total transceiver current consumption will
be application-specific. 

 can help

estimate how much current actually may be required in
full-speed applications.
Refer to the “PIC24F Family Reference Manual”,
Section 27. “USB On-The-Go (OTG)”

 for a complete

discussion on transceiver power consumption.

Legend: V

USB

 – Voltage applied to the V

USB3V3

 pin in volts (3.0V to 3.6V).

P

ZERO

 – Percentage (in decimal) of the IN traffic bits sent by the PIC

®

 microcontroller that are a value

of ‘0’.

P

IN

 – Percentage (in decimal) of total bus bandwidth that is used for IN traffic.

L

CABLE

 – Length (in meters) of the USB cable. The “USB 2.0 OTG Specification” requires that

full-speed applications use cables no longer than 5m.

I

PULLUP

 – Current which the nominal, 1.5 k

 pull-up resistor (when enabled) must supply to the USB

cable. 

40 mA • V

USB

 • P

ZERO

 • P

IN

 • L

CABLE

IXCVR =

3.3V • 5m

+ I

PULLUP