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MSPM0C1103: Reset capacitor at I2C_SDA pin

Expert 4021 points
Part Number: MSPM0C1103

Tool/software:

Hi Champs,

I am using M0C1103 as an I2C I/O expander. The package is SOT-16, so the PA1/SDA is merged with NRST. Thus I guess the RC reset circuit may affect the I2C waveform.

May I confirm if the pull up resistor of 47K and capacitor of 10nF (recommended RC values for reset circuit: https://www.ti.com/lit/an/slaaeg4/slaaeg4.pdf) work well with I2C normal mode? How about Fast mode?

Regards,

Kien Nguyen

  • Hello Kien,

    On this device, the package pin is double bonded to the NRST signal and PA1.  In your application code, you must disable the NRST functionality. Based on the note in the TRM below, I would focus on sizing the pull-up for your I2C speed.  NRST must be kept high from power-up until it's disabled which the I2C pull-up should ensure but no I2C communication must come in during this window or it will still reset the device.    

    Does this answer your question?  

    Thanks,

    JD

  • Hi JD,

    Thanks for your reply. Actually, my question was about the HW, not the SW. Normally for I2C people only need pull up resistors. However, in this case we have a capacitor (10nF) connected to the pin. That's why I wanted to confirm if the that R&C circuit won't affect the I2C waveform (especially in fast mode). What do you think?

    Regards,

    Kien Nguyen

  • Hello Kien,

    You should just design the pin to use for I2C.  I2C standards do provide guidelines for max bux capacitance for different speeds.which I think is generally <500pF total.   So I would leave the capacitor off for the sake of I2C communication.  

    Thanks,

    JD

  • JD,

    Thanks for your support. If I can't change to one of the 20-pin packages that allows me to separate the NRST and I2C_SDA, I will follow your suggestion.

    Regards,

    Kien Nguyen