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i2c pull up calculation

Ashutosh Nahar
Intellectual 640 points
Other Parts Discussed in Thread: ASH

Hi 

I want to know that while calculating the pull up resistor value (minimum) which device vol(max) and Iol we need to consider?I2C master or slave? 

How to calculate the bus capacitance value?CB(bus capacitance) need to taken from the master or slave ?

Consider the figure 1 in the slva689 document from TI.

Thanks

ash

  • 0
    TI__Mastermind 23126 points
    Hello ASHU,
    Vol(max) refers to who ever has the highest VOL of all the devices on the bus. It could be the master or any of the devices on the bus. You are looking for worst case so evaluate it wisely. I tend to look at the VOL and IOL as a way to state the max Rds_ON of the pull down FET. That helps me understand the circuit a little better. Vol/Iol = Rds_on of pull down FET.

    Bus capacitances is a combination of the capacitance on the bus due to the devices (~5-15pF per device is an approximation, but it should be stated in the devices spec page) and the capacitance of the traces on then PCB. If you have a harness then you sum what the capacitance of the harness is from SDA/SCL to ground.

    There are lots of equations out there for PCB trace capacitance calculations. There are even some great tools that do it for you. Saturn PCB has this one:

    www.saturnpcb.com/pcb_toolkit.htm

    Let me know if this isn't clear or if you have more questions.
    -Francis Houde