MM232R Mini USB-Serial UART Development Module Datasheet Version 1.00

© Future Technology Devices International Ltd. 2005

Page 1

6.2 Self Powered Configuration

Figure 6 - Self Powered Configuration

Figure 6 illustrates the MM232R in a USB self powered configuration. In this case an external supply is connected to

the VCC50 pin. In this example pin 3 (VCCIO), the supply to the FT232RQ’s UART and CBUS IO pins, is connected to

pin 2 (VCC). Doing this will make the UART and CBUS IO pins drive out at the same voltage level as VCC50.

The configuration shown in Figure 5 can have a 3.3V - 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 is

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 Hubs

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 Bus Power is used to control the RESET# input 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 device 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.

MM232R

© FTDI 2005

1

2

3

4

16 15 14 13

5

6

7

8

10

9

12

11

TXD

RXD

VCC

VCCIO

VCC50

RTS#

CTS#

CLK12M

GND

RTS#

TXD

CTS#

RXD

CLK_IN

I/O

MCU

3.3V – 5V VCC

3.3V – 5V VCC

PWREN#

RESET#

4k7

10k