# 5V tolerant - tolerant to 5V and how many more ? Harsh environnement

There is a lot of questions about this subject... Nevertheless this is not clear to me and clearly I m afraid of putting a signal at the input of a microcontroller which is higher than this supply voltage (ie 3V3). Nevertheless I would really happy if it works. So I have a microcontroller which is supplied with an LDO which converts 5V into 3V3. Nevertheless I need to use the 5V for driving "loads" thank to the microcontroller by using microcontroller outputs configured in open drain. Nevertheless when the transitor is open, the microcontroller sees 5V from the LDO. Nevertheless as I am working into a harsh environnement, I am afraid of seeing momentary the 5V to be higher than 5V. I have clearly a very bad knowledge about EMI, so I won't take any risks.

Apparently if I correctly understand the datasheet, a standard I/O ("TC") pin has a maximum voltage equal to Vddio (3V3) + 0.3 which gives 3V6. I know actually from previous projects into my firm that there were no failure due to broken microcontroller. So the voltage at the input pins never exceeds 3V6 due to the harsh environnement. When I look at the "5V tolerant pin", the margin is higher ! They are tolerant to 5V5 ! So I have not a margin of 0V3 but a margin of 0V5. So it should work ? What do you think about it ? Did I correctly understand the datasheet ?

Here is the datasheet if you are interested in : https://www.st.com/resource/en/datasheet/stm32f030f4.pdf

Thank you very much and have a nice day ! :D

• Probably due to the use of a current-limiting 'resistance' (and the voltage drop it contributes) on the tolerant inputs. – jonk May 31 at 18:00
• According to table 18 of the datasheet which gives abs-max ratings, "[Input voltage on FT and FTf pins] min:[VSS − 0.3] max:[VDDIOx + 4.0]" - So, the Abs Max is actually 7.3V, but I wouldn't exceed 5.5V general operating ratings. – Tom Carpenter May 31 at 18:02
• You used a lot of words in the question but forgot the most important part - what is "harsh environment" to you? Because to me it means something that requires TVS protection as well as level limiting on the inputs. Using 5V tolerant pins to interface with 5V components is not harsh environment, just taking advantage of built-in functionality – Maple May 31 at 19:01
• Thank you for your comments ! I could do what you say ! I do not know what is harsh environnement. If you have a norm which tells what is harsh environnement it will be nice ! – Jess Jun 1 at 7:58
• a jelly bean 5.1 zener on a line as a precaution wouldn't hurt against sudden voltage spikes too – Ilya Jun 9 at 14:53