Datasheet

Copyright © 2009 Future Technology Devices International Limited 23
Document No.: FT_000053
FT232R USB UART IC Datasheet Version 2.02
Clearance No.: FTDI# 38
6.2 Self Powered Configuration
Figure 6.2 Self Powered Configuration
Figure 6.2 illustrates the FT232R 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) 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.
iii) 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 FT232R 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 FT232R 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 FT232R 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.
Figure 6.2 illustrates a self powered design which has a +3.3V to +5V supply. Any design which
interfaces to +3.3 V or +1.8V logic would differ from Figure 6.2 by having a +3.3V or +1.8V supply to
VCCIO. In this case the VCC operates over the range +3.3V to +5V supply to VCC
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 a
minimum of +4.0V.
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