FT245R Future Technology Devices Intl Ltd., FT245R Datasheet - Page 19

• Manufacturer Part Number: FT245R
• Manufacturer: Future Technology Devices Intl Ltd.

Figure 12 Self Powered Configuration
Figure 12 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)
ii) A Self Powered Device can use as much current as it likes during normal operation and USB suspend as it has its
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 1.02
A Self Powered device should not force current down the USB bus when the USB Host or Hub Controller is
powered down.
own power supply.
7.2 Self Powered Configuration
SHIELD
5
100nF
1
2
3
4
GND
100nF
GND
4.7uF
+
GND
4k7
10k
VCC
VCC = 3.3V - 5V
GND
100nF
© Future Technology Devices International Ltd. 2005
VCC
USBDM
USBDP
3V3OUT
VCCIO
RESET#
OSCI
OSCO
NC
NC
A
G
N
D
FT245R
G
N
D
G
N
D
GND
G
N
D
T
E
S
T
PWREN#
RXF#
TXE#
WR#
RD#
D0
D1
D2
D3
D4
D5
D6
D7