I2C with ST7 SUPPLYED WITH 5V

jps · ‎2010-01-07

Posted on January 07, 2010 at 09:53

jps · ‎2009-12-18

Posted on December 18, 2009 at 11:36

As the data and clock pins of ST7FOX are true open drain, is it possible to connect the I2C bus pull-up resistor to a voltage different than the ST7FOX VDD voltage? For exemple, as the ST7FOX VDD is +5V, is it possible to connect the I2C pull-up resistors to a +3,3V that is the VDD of an other controler?

Regards,

If someone can give me a little help, thank you.

wolfgang2399 · ‎2009-12-22

Posted on December 23, 2009 at 06:28

Hi Witebull,

it will not certainly work, if you use a 3.3V logical level at the input of a 5V-ÂµC (your ST7Fox).

For further information please follow the link to the post

http://www.st.com/mcu/forums-cat-4752-1.html

of this forum.

Regards,

WoRo

jps · ‎2010-01-06

Posted on January 07, 2010 at 05:02

Hi Woro,

Thank you for your answer!(they are not so much on this forum!)

As the IÂ²C ports are true open drain on the ST7FOX I guess if it can works whith pull-up to the STM32 3V3? As IÂ²C works with TTL levels 0 and around 3V seems to be efficient levels?

What do you think about that?

Regards

whitebull

wolfgang2399 · ‎2010-01-07

Posted on January 07, 2010 at 09:53

Hi Witebull,

anyhow you have to meet the I/O input specifications of your ÂµC as described in Table 95 in Chap. 12.9.1 of the manual.

- Input high level voltage min: 0.7VDD -

I read this as follows: for save recognition of a high level you have to ensure min 3.5V on the input port of your Vdd=5.0V supplied ÂµC. I'm sure, that the ÂµC input doesn't match with TTL-level - not even at an IÂ²C port.

Regards,

WoRo

P.S.: BTW - (they are not so much on this forum!) where have all the moderators gone...?