Document No.: FT_000052
FT245R USB FIFO IC Datasheet Version 2.10
Clearance No.: FTDI# 39
7.2 Self Powered Configuration
Figure 7.2 Self Powered Configuration
Figure 7.2 illustrates the FT245R in a typical USB self powered configuration. A USB self powered device
gets its power from its own power supply, VCC, 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)
iii)
A self powered device can use as much current as it needs during normal operation and USB
suspend as it has its own power supply.
A self powered device can be used with any USB host, a bus powered USB hub or a self
powered USB hub.
The power descriptor in the internal EEPROM of the FT245R should be programmed to a value of zero
(self powered).
In order to comply with the first requirement above, the USB bus power (pin 1) is used to control the
RESET# pin of the FT245R device. When the USB host or hub is powered up an internal 1.5kΩ resistor
on USBDP is pulled up to +3.3V (generated using the 4K7 and 10k resistor network), thus identifying the
device as a full speed device to the USB host or hub. When the USB host or hub is powered off, RESET#
will be low and the FT245R is held in reset. Since RESET# is low, the internal 1.5kΩ resistor is not pulled
up to any power supply (hub or host is powered down), so no current flows down USBDP via the 1.5kΩ
pull-up resistor. Failure to do this may cause some USB host or hub controllers to power up erratically.
Error! Reference source not found. illustrates a self powered design which has a +4V to +5.25V
supply.
Note:
1. When the FT232R is in reset, the UART interface I/O pins are tri-stated. Input pins have internal
200kΩ pull-up resistors to VCCIO, so they will gently pull high unless driven by some external
logic.
2. When using internal FT232R oscillator the VCC supply voltage range must be +4.0V to 5.25V.
3. When using external oscillator the VCC supply voltage range must be +3.3V to 5.25V
Any design which interfaces to +3.3 V or +1.8V would be having a +3.3V or +1.8V supply to
VCCIO.
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