i2c internal pull up

[Ara.1]

hi

i am using stm32mp157caa3, there is a i2c slave operating at 3v3, while design, we have not connected( "forget")pull up line on i2c clk and data line,

so i tried enabling open-drain internal pull up in dtsi , but i2cdetect is not able to find the slave device

dtsi :

            pins {

               pinmux = <STM32_PINMUX('H', 11, AF5)>, /* I2C1_SCL */

                  <STM32_PINMUX('B', 9, AF4)>; /* I2C1_SDA */

               bias-pull-up;

               drive-open-drain;

               slew-rate = <1>;

            };

let me if my configuration and understanding is right ?

[taraben]

Hi @ARa.1​ 

we had hard time not using external pull-ups.

If you try to look up for exact resistance in the datasheet then you do not find information.

I assume the "pullup" is implemented as current sink.

I recommend you to use external pullup resistor 2kOhm.

HTH, Adib.

--

[PatrickF]

Hi,

just to clarify, pull-up and pull-down values are stated in datasheet (e.g. DS12505), please look at "I/O static characteristics" table.

Value if 40kOhm typical (between 25K and 50K).

I confirm that the internal pull-up is too large for usual I2C (with 50K/30pF load, you might barely reach 100KHz).

I agree, the I2C recommended pull-up is not in STM32 datasheet, but useful and complete information on Rp could be found in I2C standard.

As stated, rule of thumb using 2.7K pull-ups work fine.

Regards.

[darla14]

hello, did it work?

[Tesla DeLorean]

Did what work? Using the internal pull-ups is unlikely to be satisfactory/reliable.

Tack on 2K7 pull-up resistors to some accessible points for the SDA and SCL signals.

[Uwe Bonnes]

For on-board low connection with capacitve load, internal pull up may work work in many case.

[taraben]

Hello Uwe,

may and should are not sufficient when things are REQUIRED.

The "pull up" is a current source and thus not guaranteed to have a specific resistance.

There is no confirmed information in the datasheet about that.

If you really depend on I2C you must add external pull up resistors.

HTH, Adib.

--

[Ozone]

With STM32 parts, internal pull-ups hardly ever worked for I2C.

They are about one order of magnitude to large (high impedance).

I don't expect it any different for the MP.