FT232R USB UART I.C. Datasheet Version 1.04

© Future Technology Devices International Ltd. 2005

Page 20

7.2 Self Powered Configuration

Figure 12 Self Powered Configuration

Figure 12 illustrates the FT232R in a typical USB self powered configuration. A USB Self Powered device gets its

power from its own power supply and does not draw current from the USB bus. The basic rules for USB Self powered

devices are as follows –

i) A Self Powered device should not force current down the USB bus when the USB Host or Hub Controller is

powered down.

ii) A Self Powered Device can use as much current as it likes during normal operation and USB suspend as it has its

own power supply.

iii) A Self Powered Device can be used with any USB Host and both Bus and Self Powered USB Hubs

The power descriptor in the internal EEPROM should be programmed to a value of zero (self powered).

In order to meet requirement (i) the USB Bus Power is used to control the RESET# Pin of the FT232R device. When

the USB Host or Hub is powered up the internal 1.5kΩ resistor on USBDP is pulled up to 3.3V, thus identifying the

device as a full speed device to USB. When the USB Host or Hub power is off, RESET# will go low and the device will

be held in reset. As RESET# is low, the internal 1.5kΩ resistor will not be pulled up to 3.3V, so no current will be forced

down USBDP via the 1.5kΩ pull-up resistor when the host or hub is powered down. Failure to do this may cause some

USB host or hub controllers to power up erratically.

Figure 10 illustrates a self powered design which has a 3.3V - 5V supply. A design which is interfacing to 2.8V - 1.8V

logic would have a 2.8V - 1.8V supply to VCCIO, and a 3.3V - 5V supply to VCC

Note : When the FT232R is in reset, the UART interface pins all go tri-state. These pins have internal 200kΩ pull-up

resistors to VCCIO, so they will gently pull high unless driven by some external logic.

FT232R

A

G

N

D

G

N

D

G

N

D

G

N

D

T

E

S

T

100nF

3V3OUT

VCCIO

NC

RESET#

NC

+

100nF

VCC

TXD

RXD

RTS#

CTS#

DTR#

DSR#

DCD#

RI#

CBUS0

CBUS3

CBUS2

CBUS1

USBDP

USBDM

VCC

1

2

3

4

5

OSCI

OSCO

CBUS4

4.7uF

SHIELD

GND

GND

GND

GND

VCC = 3.3V - 5V

GND

10k

4k7

100nF