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PCA9306: Capacitance vlaue for PCA9306 and I2C rise time

Dohan Jun11
Prodigy 40 points
Part Number: PCA9306

Dear team,

I use PCA9306 to change the level of I2C between MCU and sensor.

In the datasheet, the reference design(fig.7) use a capacitor between REF2 and GND. Why use this capacitor? and could you recommend the value?

In my circuit, the rise time of SDA is about 400~500ns. The rise time of the SDA is slower than the I2C specification, but the SDA state completely changed before the 70% of SCL. And MCU can read sensor data.

In this case, do I need to shorten the rise time? What will happen if I use the I2C in this situation(rise time is 400~500ns)? 

  • 0
    TI__Guru 73541 points

    Hello Dohan,

    "Why use this capacitor?"

    The enable voltage is used as a reference voltage for internal circuitry of this device. During operation, this reference voltage can move and become unstable. A capacitor helps to stabilize this voltage.

    "could you recommend the value?"

    Section 9.2.2.1 recommends a capacitor of 100pF.

    "In this case, do I need to shorten the rise time?"

    Because the signal integrity of your I2C bus seems to be okay, you don't have to but it means that you are not compliant with I2C standard.

    "What will happen if I use the I2C in this situation(rise time is 400~500ns)? "

    I assume you are only operating at 100kHz and in which case it seems you can still send and receive data correctly. It just means you are not I2C compliant. You may not be able to operate at faster I2C speeds with this rise time though.

    You should be able to fix this by simply changing the pull up resistors to a lower resistance to get a faster rise time.

    Thanks,

    -Bobby