Best design guidance for an STM32F4 microcontroller operating in both 3.3V & 5V environments?

[MTier.1]

For long term reliability and not dealing with interface subtleties I practice using 3.3V/5V translators (e.g. SN74LV1T34, ...) rather than taking advantage of the 5 volt tolerant inputs. I see this as reducing the voltage stresses & load current drive, and heating effects to the microcontroller. In a recent design review, a colleague was critical of this approach and thought the design was actually less reliable by adding the translators. What is your opinion?

[Dvorak.Peter]

The extra translators will protect the microcontroller from death by poor IO configuration or power sequencing

[TDK]

> What is your opinion?

I'd lose the level shifter. Less parts with the same functionality is (almost?) always better from both a cost and reliability standpoint. They're designed as 5V tolerant pins, advertised as such, and can be expected to operate normally under those conditions. These are mature parts and there shouldn't be any surprises in basic functionality.

[S.Ma]

Level shifter are needed if you need 5v push pull output. Protection can be done differently like serial resistors for digital slow inputs (tv style apps), or protection diods. Besides cost, there are mono and bidir shifters. These introduce delays which limits max frequency when shifting SPI bus typically.

[prain]

I personally use level shifters when need two way connection.

[MTier.1]

Thanks for your reply. You basically used the same words as my colleague. However, the key words in your premise "the same functionality" is not true from a physics standpoint. For example, suppose the design must drive 5V TTL outputs as a PWM frequency to a fan, or that it must receive 5V inputs from a quadrature decoder for motor control. Compare the heat dissipation and temperature rise resulting from the switching frequencies for the relatively small geometry I/O buffers/receivers on the STM32Fxxx for 5V vs 3.3V. The cost penalty is less than $0.15 per translator to remove that stress from the STM32Fxxx. With regard to reliability, it too must be looked at from different standpoints. Once properly assembled and initially tested, the added translators will make the design more reliable, or at least the high valued chip should have a longer lifetime.

Also, your answer did not address potential consequential damage. For example, what happens to the STM32Fxxx if the 3V supply is lost, but the 5V supply remains on, or if the 5V supply powers on sooner than the 3.3V supply?

Just the "5V tolerant" naming is rather ambiguous to me. How does it specifically differ from components designed for 5V operation?

[S.Ma]

The original question was posted without application context.

[S.Ma]

https://www.jedec.org/standards-documents/dictionary/terms/arrhenius-equation-reliability

[TDK]

> For example, suppose the design must drive 5V TTL outputs as a PWM frequency to a fan

Well now you're changing the question. In the OP, you said it was to "[reduce] the voltage stresses & load current drive, and heating effects to the microcontroller." But be aware that most components operating with 5V logic high will happily work with 3.3V logic high as well.

Ultimately, the decision is up to the designer. There are valid reasons for a logic shifter, but I don't think "reducing the voltage stresses & load current drive, and heating effects to the microcontroller" is a valid one.

> Just the "5V tolerant" naming is rather ambiguous to me. How does it specifically differ from components designed for 5V operation?

The reference manual describes it in detail.