I want to make a 3.3V I/O (UART) interface between a FT232RL and AVR which operate at 3.3V For this configuration responsible it's VCCIO and must be connected to 3.3V supply. But what about VCC? Can it connected at 3.3V supply line with VCCIO together? As the datasheet say for VCCIO

+1.8V to +5.25V supply for the UART Interface and CBUS group pins (2, 3, 6,7,8,9,10 11, 21, 22, 30,31,32). In USB bus powered designs connect this pin to 3V3OUT to drive out at +3.3V levels, or connect to VCC to drive out at +5V CMOS level. This pin can also be supplied with an external +1.8V to +2.8V supply in order to drive out at lower levels. It should be noted that in this case this supply should originate from the same source as the supply to VCC. This means that in bus powered designs a regulator which is supplied by the +5V on the USB bus should be used.

This which i mean that it is necessary to connect 3V3OUT to VCCIO for get 3.3V level interface? I mean: I don't want to use 3V3OUT for VCCIO supply, i want to supply VCCIO pin externaly with 3.3V line. And since i work the FTDI with internal oscillator VCC limited at 4V MINIMUM. So, VCC must be connected at > 4V supply for normal operation. Am i true?


Page 18 of the data sheet seems quite clear:

enter image description here

So yes, you need Vcc of at least 4V if you're going to use the internal oscillator.

  • \$\begingroup\$ Yes. so i am ok! and for 3.3V I/O interface (UART) it is necessary to connect 3V3OUT to VCCIO? Can i connect to VCCIO in my externaly 3.3V line? I connect the 3V3OUT with a capacitor 100n to GND \$\endgroup\$
    – MrBit
    Nov 15 '14 at 16:57
  • \$\begingroup\$ postimg.org/image/pdu2k4ml9 \$\endgroup\$
    – MrBit
    Nov 15 '14 at 17:01
  • \$\begingroup\$ Oops--sorry; misread your diagram. At least as far as VCCIO goes, it looks all right. \$\endgroup\$ Nov 15 '14 at 17:06
  • \$\begingroup\$ What will happening if in this circuit postimg.org/image/pdu2k4ml9 the 3.3V line, wich supply the VCCIO, become 0V? Whats the level of UART output then? high impedance? 0V? \$\endgroup\$
    – MrBit
    Nov 16 '14 at 18:23
  • \$\begingroup\$ @GeoG392: The data sheet specifies a minimum of 1.8 volts for VCCIO. It's hard to guess what it will do if you don't meet that requirement. I suppose the data sheet might say somewhere, but I kind of doubt it. \$\endgroup\$ Nov 16 '14 at 18:38

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