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Document Reference No.: FT_000214
MM232R USB - Serial UART Development Module Incorporating Clock Generator
Datasheet Version 1.1
Clearance No.: FTDI# 132
6.2 Self Powered Configuration
Figure 6.2 Self-Powered Configuration
Figure 7.2 illustrates the MM232R in a typical USB self powered configuration. In this case the link on
jumper J2 is removed, and an external supply is connected to the module VCC pins. Figure 7.2 illustrates a
self powered design which has a 3.5V – 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)
ii)
iii)
A Self Powered device should not force current down the USB bus when the USB Host or Hub
Controller us powered down.
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.
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 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 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 pin 15 (USBPWR) is connected to 4K7 resistor and Reset# is connected between
4K7 and 10K. Failure to do this may cause some USB host or hub controllers to power up erratically.
Note: When the FT232Ris 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 7.2 is
also an example of interfacing the FT232R 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. One of the CBUS pins could be configured as a 6/12/24/48 MHz clock
output which can be used to clock the MCU. If the MCU is handling power management functions, then a
CBUS pin can be configured as PWREN# and should also be connected to an I/O pin of the MCU.
? Copyright 2010 Future Technology Devices International Ltd
14
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