DS232B Version 1.8

© Future Technology Devices Intl. Ltd. 2005

Page 16 of 25

FT232BM USB UART ( USB - Serial) I.C.

Figure 8

USB Self Powered Configuration

Figure 8 illustrates 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 power devices are as

follows –

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

powered down.

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

own POWER SUPPLY.

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

PWRCTL (pin 14) is pulled high to tell the device to use a USB Bus Power descriptor. The power descriptor in the

EEPROM should be programmed to a value of zero. The USB power descriptor option in the EEPROM should be

programmed to a value of zero (self powered).

To meet requirement a) the 1.5K pull-up resistor on USBDP is connected to RSTOUT# as per the bus-power circuit.

However, the USB Bus Power is used to control the RESET# Pin of the FT232BM device. When the USB Host or

Hub is powered up RSTOUT# will pull the 1.5K resistor on USBDP 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, RSTOUT# will also be low, so no current will be forced down USBDP via the 1.5K 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.

Note : When the FT232BM is in reset, the UART interface pins all go tri-state. These pins have internal 200K pull-up

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

VCC

27R

USB "B"

Connector

27R

USB DM

RSTOUT#

8

FT232BM

USB DP

7

5

V

C

C

V

C

C

V

C

C

I

O

3

26

13

1

2

3

4

1k5

33nF

3v3OUT

6

470R

A

V

C

C

30

0.1uF

POWERCTL

14

G

N

D

G

N

D

A

G

N

D

9

17

29

VCC

RESET#

4

10k

4k7

Decoupling Capacitors

10uF

VCC

0.1uF

0.1uF

+