UMFT231XE Datasheet, PDF (9/18 Pages) Future Technology Devices International Ltd. – UMFT231XE USB to Full-Handshake UART Development Module

UMFT231XE
Version 1.0
Document Reference No.: FT_000652 Clearance No.: FTDI# 294
5.3 Self Powered Configuration
Vcc = 1.8V – 3.3V
Vcc = 5V
VIO
MCU
TXD
RXD
RTS#
CTS#
TXD
RXD
RTS#
CTS#
GND
GND
VCCIO
3V3OUT
VCC
VBUS
Variable TTL
Mode
Jumper JP1 - Open
Figure 5.2 – Self-Powered Configuration
Self Power
Mode
Jumper JP2- Open
A self-powered configuration operates on the principle of drawing power from an external power supply,
as oppose to drawing power from the USB host. In this configuration no current is drawn from the USB
bus.
Figure 5.2 illustrates the UMFT231XE in a typical USB self-powered configuration. In this case the jumper
connection of JP1 is removed, which allows 5V power to be supplied to the module VCC pins from an
external source. VCCIO can to be powered from 3V3OUT or the VCC of an external source.
For a self-powered configuration, it is necessary to prevent current from flowing back to the USB data
lines when the connected USB host or hub has powered down. To carry out this function the UMFT231XE
uses an on-board voltage divider network connected to the USB bus and RESET# pin. This operates on
the principle that when no power is supplied to the VBUS line, the FT231X will automatically be held in
reset by a weak pull-down, when power is applied the voltage divider will apply a weak 3.3V pull-up.
Driving a level to the RESET# pin of the UMFT231XE will override the effect of this voltage divider. When
the FT231X is in reset the USB DP signal pull-up resistor connected to the data lines is disconnected and
no current can flow down the USB lines.
An example of interfacing the FT231X with a Microcontroller‟s UART interface is also illustrated in Figure
5.2. This example shows the wire configuration of the transfer and handshake lines. This example also
illustrates that a voltage other than 3.3V can be supplied to the FT231X‟s IO port, this feature is
described further and for bus powered mode in Section 5.5.
Alternatively, both the FT231X‟s IO port and MCU can be powered from the 3V3OUT pin; this approach is
described in Section 5.4.
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