I am trying to interface a microcontroller (MSP430) with another chip through I2C. Both datasheets (MSP430 and the other chip) advice me to pull the I2C signals up to VCC. Do I need to pull SDA & SCL signals up twice, for the microcontroller and for the chip, or not. Also each one of the chips specifies the values for pull up resistors, one says 10K and the other one says 5.1K. If the case is to pull up one time, which value I'll use?


1 Answer 1


No you don't. Use only one pullup. The value of the resistor is not that critical. Use something in the range of 5k to 10k. That will be fine.

  • \$\begingroup\$ So is that's always the case, to pull the bus up one time with resistors that have average values (values between the recommendations!) \$\endgroup\$
    – A. Kier
    Commented Jun 30, 2016 at 7:30
  • 2
    \$\begingroup\$ The size of the pull up is defined by i2c and can varies with the length of your lines and the speed of the transmission. See also electronics.stackexchange.com/questions/57121/… \$\endgroup\$
    – Botnic
    Commented Jun 30, 2016 at 7:35
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    \$\begingroup\$ And how many i2c devices are on the bus. Too weak pull-ups, the line can't go high fast enough. Too strong, it can't go low fast enough. It's a capacitance issue. Typically you have 1kΩ to 10kΩ pull-up, total. 4.7kΩ standard middle ground. \$\endgroup\$
    – Passerby
    Commented Jun 30, 2016 at 7:42

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