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Copyright © 2012 Future Technology Devices International Limited

UMFT221XE

Version 1.0

Document Reference No.: FT_000651

Clearance No.: FTDI#

293

5.3

Self Powered Configuration

MCU

Vcc = 1.8V

– 3.3V

IO0

SCLK

SS#

MISO

GND

Vcc = 5V

IO0

Jumper JP2 - Open

Jumper JP1 - Open

GND

Self Power

Mode

Variable TTL

Mode

VIO

VCCIO

3V3OUT

VBUS

VCC

SCLK

SS#

MISO

Optionial

IO1

Optionial

IO2

Optionial

IO3

Optionial

IO4

Optionial

IO5

Optionial

IO6

Optionial

IO7

Figure 5.2 – Self-Powered Configuration

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 UMFT221XE 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 UMFT221XE

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 FT221X 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 UMFT221XE will override the effect of this voltage

divider. When the FT221X 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 FT221X in FT1248 1-bit mode is 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 FT221X‟s IO port, this feature is described further and for

bus powered mode in Section 5.5.

Alternatively both the FT221X‟s IO port and MCU can be powered from the 3V3OUT pin; this approach is

described in Section 5.4.