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FT231XQ-T Datasheet(PDF) 23 Page - Future Technology Devices International Ltd. |
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FT231XQ-T Datasheet(HTML) 23 Page - Future Technology Devices International Ltd. |
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23 / 44 page  Copyright © 2013 Future Technology Devices International Limited 23 FT231X USB TO FULL HANDSHAKE UART IC Datasheet Version 1.2 Document No.: FT_000565 Clearance No.: FTDI# 261 6.2 Self Powered Configuration FT231X 1 2 3 4 5 SHIELD GND GND VCC GND VCC 3V3OUT USBDM USBDP VCCIO VCC(3.3-5.25V) VBUS_SENSE 100nF 100nF 100nF 4.7uF + GND 4k7 10k 47pF GND 47pF 27R 27R RESET# Figure 6.2 Self Powered Configuration Figure 6.2 illustrates the FT231X in a typical USB self powered configuration. A USB self powered device gets its power from its own power supply, VCC, 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 needs 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, a bus powered USB hub or a self powered USB hub. The power descriptor in the internal MTP memory of the FT231X should be programmed to a value of zero (self powered). In order to comply with the first requirement above, the USB bus power (pin 1) is used to control the VBUS_Sense pin of the FT220X device. When the USB host or hub is powered up an internal 1.5kΩ resistor on USBDP is pulled up to +3.3V, thus identifying the device as a full speed device to the USB host or hub. When the USB host or hub is powered off, VBUS_Sense pin will be low and the FT220X is held in a suspend state. In this state the internal 1.5kΩ resistor is not pulled up to any power supply (hub or host is powered down), so no current flows down USBDP via the 1.5kΩ pull-up resistor. Failure to do this may cause some USB host or hub controllers to power up erratically. Figure 6.2 illustrates a self powered design which has a +3.3V to +5.25V supply. Note: 1. When the FT231X is in reset, the UART interface I/O pins are tri-stated. Input pins have internal 200kΩ pull-up resistors to VCCIO, so they will gently pull high unless driven by some external logic. |
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