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Deepak1602
Associate
March 28, 2026
Solved

STM32H745 I2C Not true open drain - OK to pull up to 5V ?

Hi ST Family,

We are using STM32H745BIT6 MCU in our design and interfaced the same to multiple I2C slaves. For I2C lines, we have connected pull up of 2.2K to 5V power rail. In STM32CubeMX, we are enabling "No pullup or No pull down" for I2C lines.

Since this MCU family is "not true open drain" ( Page 202 of datasheet), we need to know if we can continue with this configuration.
Over the long run, do we see any issues of I2C engine going bad because of this configuration.?

Regards..

Andrew Neil
Andrew NeilBest answer
Super User
March 28, 2026

See: Previous discussion on STM32 I2C pullups to 5V.

A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
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3 replies

AScha.3
AScha.3
Super User
March 28, 2026

Hi,

as the cpu is working at 3v3 , have the signals at 3v3 -> pullups to VDD , 3v3 . (not 5V ! )

If connecting to 5V chips and need 5V I2C signal level , use a I2C level shifter 3v3 -- 5V .

Then you are on the safe side.

shifter..standard like this :

Bildschirmfoto_2026-03-28_10-09-52.png

or in a chip :

Bildschirmfoto_2026-03-28_10-12-57.png

(1v2  here --->  5v )

 

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    Andrew Neil
    Andrew NeilBest answer
    Super User
    March 28, 2026

    See: Previous discussion on STM32 I2C pullups to 5V.

    A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
      Deepak1602Author
      Associate
      March 28, 2026

      So, if we ensure that Controller is powered up before 5V, then the design is fine. 

      Thanks for the direction @Andrew Neil 

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