Datasheet

© Copyright 2005-2011 Future Technology Devices International Ltd
17
Document Reference No.: FT_000051
UM232R USB - Serial UART Development Module
Datasheet Version 1.1
Clearance No.: FTDI# 125
7.2 Self Powered Configuration
J2
2 1
1 2 3
J1
UM232R © FTDI 2008
FT232RL
FTDI
1
12 13
24
MCU
Vcc = 3.3V – 5V
TXD
RXD
RTS#
CTS#
TXD
RTS# 3
RXD 5
GND 7
CTS# 10
GND
21 VCC
20 RST#
17 PU1
16 PU2
15 VCC
14 USB
Vcc = 4.0V – 5V
Figure 7.2 Self-Powered Configuration
Figure 7.2 illustrates the UM232R in a typical USB self powered configuration. In this case the link on
jumper J2 is removed, and an external supply is connected to the module VCC pins. Figure 7.2 illustrates a
self powered design which has a 4.0V – 5V supply.
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 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 us 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 Hub. In
this case 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 Power is used to control the RESET# Pin of the FT232R 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 devices 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.
To do this pin 14 (USB) is connected to PU2 and PU1 is connected to RST#. Failure to do this may cause
some USB host or hub controllers to power up erratically.
Note: When the FT232R is in reset, the UART interface 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.