UMFT240XE Datasheet, PDF (9/18 Pages) Future Technology Devices International Ltd. – UMFT240XE USB to 8-bit 245 FIFO Development Module

UMFT240XE
Version 1.0
Document Reference No.: FT_000653 Clearance No.: FTDI# 295
5.3 Self Powered Configuration
Vcc = 1.8V – 3.3V
Vcc = 5V
VIO
MCU
DATA0-7
RXF#
TXE#
RD#
WR#
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 UMFT240XE 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. (Note that
Figure 5.2 is for illustration only and that the pins do not actually go all the way through the PCB)
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 UMFT240XE
uses an on-board voltage divider network connected to the USB power bus and RESET# pin. This
operates on the principle that when no power is supplied to the VBUS line, the FT240X 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 UMFT240XE will override the effect of this voltage
divider. When the FT240X 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 FT240X 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 FT240X‟s IO port, this feature is
described further and for bus powered mode in Section 5.5.
Alternatively both the FT240X‟s IO port and MCU can be powered from the 3V3OUT pin; this approach is
described in Section 5.4.
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