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PCA9306: Pull up resistor calculation

HARI T O1
Expert 1645 points
Part Number: PCA9306

Dear Team,

I am using PC9306 for one of my project with Vrfe2=5v and Vref1=1.8V.Below is the block diagram.

My CLK is running at 100Khz

From your application note SLVA689 below equations are used for Calculations.

My question is when using level shifter we have two Vcc's for me here 5V and 1V .

For finding Rp(min) which VCC I need to use 5V or 1.8V.

For rise time and Vol I need to take the rise time and Vol of My IC?Please correct me if I am wrong.

For bus capacitance do I need to consider the bus capacitance of Arduino,level shifter and My IC.

Please confirm.

Regards

Hari

  • 0
    Guru 316690 points
    1. A signal line has one pull-up resistor on each side. You use both 1.8 V and 5 V.
    2. tr indeed is the maximum rise time that your IC allows. The I²C specification says that this is 1 µs.
    3. Yes; during a rising edge, the total bus capacitance is charged only through the pull-up resistors. Keep in mind that traces, connectors, and cables also have a capacitance.

    Please note that you have two resistors in parallel. But in practice, the computed limits result in a very wide range of possible values, and most people simply use 10 kΩ.

  • 0 in reply to Clemens Ladisch
    Expert 1645 points

    Hi Clemens,

    Thank you very much for your time and concern.

    "A signal line has one pull-up resistor on each side. You use both 1.8 V and 5 V.",

    if I use both 1.8V and 5V then I will have two Rp(min) values.Will that be a problem?

    "Yes; during a rising edge, the total bus capacitance is charged only through the pull-up resistors. Keep in mind that traces, connectors, and cables also have a capacitance."

    so I need to add each ICs pin capacitace to 400pf.(according to I2C spec for standard mode bus capacitance is 400pf,really  from where did they get the value of 400pf)

    Regards

    HARI

  • +1 in reply to HARI T O1
    Guru 316690 points

    The limits will be so far away from 10 kΩ that it does not matter.

    400 pF is the limit on the total bus capacitance. Your circuit is unlikely to come near this limit, unless you are using long cables.

  • 0 in reply to Clemens Ladisch
    Expert 1645 points

    Hi Clemens,

    Thank you.

    Now I got it.

    Regards

    HARI