I am working with an FTDI "FT232R" IC [USB to UART conversion], and struggling to understand what the VIH spec of the IC is. I can only find reference to an 'Input switching threshold (VIN)' spec in the datasheet and that doesn't seem to change with VCCIO [IO voltage domain].

  1. What is the relation b/w the 'input switching threshold' spec and the traditional 'VIH' spec?
  2. Why wouldn't the VIN spec change with VCCIO?
  3. If the UART i/f on my MCU operates on 2.4V, does the VIN of 1.5V max mean that I might be OK if the VOH of the MCU is > (1.5V +VHys)?

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1 Answer 1


Your interpretation of the specification is correct. The input threshold is set by an internal reference that is nominally 1.2 volts, regardless of the operating voltage.

The point is to be able to interface with a wide range of logic voltage levels (like your 2.4 volts) while still powering the device from 5v or 3.3v, without having to provide a pin for you to separately provide the logic interface voltage.

  • \$\begingroup\$ Thanks! And I am guessing I will need a current limiting resistor b/w FTDI TX (3.3V domain) and MCU RX (2.4V domain) to prevent damage. Would you agree? \$\endgroup\$
    – NK2020
    Jul 28, 2016 at 10:38
  • \$\begingroup\$ Hmmm . . . maybe . . . not necessarily. I will have to examine the datasheet. Stay tuned . . . \$\endgroup\$
    – Mark
    Jul 28, 2016 at 21:04
  • \$\begingroup\$ @Mark - Hi - FYI, the additional question about current limiting resistor has now been asked as a separate question here, so perhaps best to continue that topic there, rather than duplicate effort here..? Just a thought. \$\endgroup\$
    – SamGibson
    Jul 29, 2016 at 21:07

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