Document Reference No.: FT_000367
UM232H Single Channel USB Hi-Speed FT232H Development Module
Datasheet Version 1.2
Clearance No.: FTDI# 198
7.3 Self Powered Configuration
7.3.1 Self-Powered Configuration with 3V3 I/O and running on +5V external supply:
5V or 3V3
5V
J1-1
J2-1
VIO
3V3
5V0
USB
RST#
PU2
PU1
MCU
RXD
TXD
TXD
RXD
CTS#
RTS#
RTS#
CTS#
J2-14
J1-14
Figure 7.3 : Self-Powered Configuration – 5V0 External Supply
Figure 7.3 illustrates the UM232H in a typical USB Self-Powered configuration. An external supply +5.0V is
connected to the module’s 5V0 pin. J2-2 (VIO) is also connected to J2-3 (3V3) to supply the VCCIO supply
from the on board regulator but a separate supply could have been used.
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 use 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 FT232H 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 J1-3 (USB) is connected to PU2 and PU1 is connected to J2-4 (RST#). Failure to do this may
cause some USB host or hub controllers to power up erratically.
Note: When the FT232H 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.
Figure 7.3 is also an example of interfacing the FT232H to a Microcontroller (MCU) UART interface. This
example uses TXD and RXD for transmission and reception of data and RTS# / CTS# hardware
handshaking.
Optionally, RI# can be connected to another I/O pin on the MCU and could be used to wake up the USB
host controller from suspend mode. If the MCU is handling power management functions, then an ACBUS
pin can be configured as PWREN# and should be connected to an I/O pin of the MCU.
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