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7.2 Self Powered Configuration
D0
D1
VCC = 3.3V - 5V
1
2
3
4
VCC
USBDM
USBDP
D2
D3
FT245R
VCCIO
NC
D4
4k7
10k
5
D5
RESET#
NC
SHIELD
D6
GND
OSCI
D7
OSCO
GND
VCC
RXF#
TXE#
RD#
3V3OUT
100nF
100nF
4.7uF
+
A
G
N
D
T
G
N
D
G
N
D
G
N
D
E
S
T
WR#
100nF
GND
PWREN#
GND
GND
Figure 14 Self Powered Configuration
Figure 14 illustrates the FT245R 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 power
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 FT245R 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 FT245R is in reset, the FIFO interface and control 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.
FT245R USB UART I.C. Datasheet Version 1.05
© Future Technology Devices International Ltd. 2005
FTDI [ FUTURE TECHNOLOGY DEVICES INTERNATIONAL LTD. ]
