Voltage threshold chart

Many 5volt chips will working with the output from a 3.3volt part. An example is the Arduino, it sees 3volts or more as a ‘1’. A 3.3volt device with 5volt tolerant pins, such as the Bus Pirate, can interact with an Arduino despite the voltage difference.

This table shows common logic threshold voltage levels and where they’re compatible.  This is a big help for projects that need voltage translation.

The graph provides a comparison of Input and Output [I/O] logic switching levels for the CMOS, TTL, mixed CMOS/TTL, ETL, BTL, GTL, and Low voltage glue logic families. The graph above provides a comparison between the Input and Output [I/O] logic switching levels for CMOS, and TTL logic families.

The graph shows 5 volt CMOS, TTL, and mixed CMOS/TTL IC devices, and 3.3 volt LVTTL LVCMOS IC devices. BTL and GTL [Bus Driver] IC are shown for comparison. Note many Low Voltage [LV] CMOS families are 5 volt tolerant [not damaged by applying 5v to the input pins]. The output logic levels above are defined by the Terms section below. For a review of Noise Margin numbers and a short description of many of the IC logic families, refer to the Logic Family Selection page.

Thanks McZ!

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  1. This may sound like a nit-pick. . .and it is to a certain extent. . .

    Note that the chart is an average across devices in the family.

    For example, the PIC24FJ64 series MCU used in the bus pirate outputs a minimum “Voh” of around 2.8V or so with VDD at 3.3V. As Ian pointed out, an Atmega8 can take an input high voltage down to 3V (with a 5V supply).

    So, the question is, “Is a minimum output “high” voltage of ~2.8V for the PIC24Fj64 and a minimum input “high” voltage of 3V for the Atmega8 going to be a problem?”

    Most of the time, it probably won’t matter. . .but it’s certainly worth keeping in mind if you’re operating the devices at the extremes of their operating conditions (high frequencies, high/low temperatures, etc).

    To check my math, see page 235 of the Atmega8 datasheet:

    and page 239 of the PIC24FJ64 datasheet:

    If I miscalculated something, please point it out.

  2. In addition to the comment above, you must also remember the load that the source is driving. The larger the load the more likely you are to to reaching that threshold voltage.

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