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I'm a bit confused about input pin voltage tolerances and output pin voltage values of ICs.

I would think that datasheets for ICs that involve signal inputs/outputs would list two values:

  1. The maximum voltage for a signal HIGH that it can tolerate at an input pin.
  2. The minimum voltage for a signal HIGH that it will set at an output pin .

But many datasheets don't appear to list them for all pins. For example, I'm experimenting with STM6600, and its datasheet doesn't provide them; see below:

STM6600

Above, "PB" is an input; "EN" and "INT" are outputs. Notice that only EN's minimum HIGH output is provided, but neither INT's minimum HIGH output nor PB's maximum HIGH input is provided.

Thus, I'm unsure about what to do in this situation.

Let's say I set VCC = 3.0V on this IC.

Then how do I determine:

  • Will this IC (STM6600) tolerate a given input signal HIGH from a different IC running at, e.g., a higher VCC = 3.3V?

  • And correspondingly, will a given output signal HIGH from this IC (STM6600) cause damage to a different IC running at, e.g., a lower VCC = 2.7V?

Even if the answer to these questions is No, I would still be interested in knowing the actual numerical values that I should expect, hence my problem with the datasheet.

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On page 38 of the datasheet you'll see for instance that the voltage on the PB should never exceed Vcc + 0.3 V. You'll find that this is true for almost all inputs on ICs: the input levels should remain between the rails. An exception is when you have for instance 5 V tolerant inputs on a 3.3 V powered microcontroller, but then that will be clearly indicated in the datasheet. (The manufacturer will consider this as an important feature, so he will make sure you notice that.)

To know if the high output will damage the input of another IC with a Vcc of 2.7 V you'll have to consult the datasheet of that IC. Probably the same limits will apply: input voltage should be no higher than the 2.7 V supply.

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  • \$\begingroup\$ That clarifies. So absolute maximum ratings can be used as a guide for tolerances when designing a circuit? (I always thought of them as very worst-case specs) As in, it's acceptable practice to design a circuit with the assumption that, e.g., here I can safely send in an input signal that is Vcc + 0.2 V without concern? ( I ask because you mentioned, as I too thought till now, that "the input levels should remain between the rails", instead of the positive rail plus X tolerance ) \$\endgroup\$ – Thomas E Oct 11 '12 at 11:09
  • \$\begingroup\$ @ThomasE - The section Absolute Maximum Ratings (AMR) in datasheets will almost always have a notice which always ends with the phrase "Exposure to absolute maximum rating conditions for extended periods may affect device reliability." So you're not supposed to operate your device at these conditions. But where the limit lies is up to you to decide. If Vcc = 3 V then 3 V is safe, and 3.3 V isn't. I would stay away from AMR as far away as possible. \$\endgroup\$ – stevenvh Oct 11 '12 at 11:16

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