Regarding voltage on I2C SCL, SDA pins dependency on VDD

[Magvel]

Hi,

 

I am using STM32U083 for one of our project. We are using the I2C peripheral to communicate with NFC dynamic tag through I2C.

In STM32U0 datasheet I found that the I2C pins SCL and SDA are 5V tolerent pins.

We need the I2C to work independently of the MCU VDD voltage ( MCU VDD 2.5v and I2C pullup resistors to 3.6v).

the slave NFC tag will work at 3.6, but I am  not sure of the MCU. Since I2C are open drain and if we meet the Vih and Vil, my thought is it should work.

If I can get some confirmation if this is okay or can have any drawbacks or problems will be very helpful.

Thanks. 

[ELABI.1]

Hi @Magvel, 

You cover VIH and VIL voltage values, and the use of a 3.6 V pull-up resistor is appropriate. This configuration will normally work without problems. Otherwise, you can use a level shifter adapter for I2C, such as TCA9406 for a more reliable solution.

Thanks.

ELABI.1

[AScha.3]

Hi,

because you have cpu VDD at 2.5V , to be on the safe side, i recommend a simple standard level shifter:

So every chip works on its perfect level. (just your pullups to 2.5v and 3.6v , not 3 + 5V )

[TDK]

Be aware that the internal pullups need to be disabled in order for the pin to be 5 V tolerant. Otherwise, no issues.

[Sco]

Connecting directly may introduce the leakage, and a risk to damage the pin.

You can choose the right MOS to do a brief level shift or use a level shift IC.

[Magvel]

Hi,

@Sco @TDK @AScha.3 @ELABI.1 ,

I found this application note regarding the I2C implantation this is the diagram given. It describes the same application how we want to use.

https://www.st.com/resource/en/application_note/an4899-stm32-microcontroller-gpio-hardware-settings-and-lowpower-consumption-stmicroelectronics.pdf

They have also suggested a zener diode in case the if I2C rail powers up before the MCU power up can cause some issue but it seems it happen only if the I2C rail is above 4v of MCU rail(ex- MCU- 0v, I2C pullup- 4+v) in our case both rail powers at same time and its at 3.6v.