How to calculate the I2C bus capacitance

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How to calculate the I2C bus capacitance

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zhangzhiyong
Contributor III

The KEAZ128's Reference Manual description: 

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Our questions:

1.The KEAZ128 I2C baudrate,Can it reach 400K?

2.How to calculate the I2C bus capacitance?Is there a detailed calculation?

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myke_predko
Senior Contributor III

Zhiyong,

With respect to the other responders, if I go back to the subject line you're asking how to determine the capacitance of the I2C line not what pull up resistor values to use.  

Personally, I generally don't expect issues with I2C line capacitance unless I have an "extreme" number of devices on the I2C bus (for me this is more than 5 slave devices), the traces are long (more than 30cm) or I have fly wires attached to the circuit.  

BUT, this doesn't mean I don't look at the signals on an oscilloscope just to make sure that I have nice square edges on the signals.  If I see significant unexpected rounding, I will then go back to first principles (Khan Academy: RC Step Response and Wikipedia: RC time constant) calculate RC, AKA Tau, (the time for rise from 10% to 90% divided by 2.2) followed by dividing by the pull up resistor value.  I know it's not precise and doesn't include line inductance in the measurement but it gives a good rough value for the line capacitance.    

As noted by the other posters, use the recommended pull ups for the devices and the data rates that you're going to use followed by doing a 'scope check to make sure you have good signal integrity.  

myke

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bobpaddock
Senior Contributor III

See this part, and note the links to the app notes at the bottom of the page:

https://www.analog.com/en/products/ltc4311.html

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ErichStyger
Senior Contributor V
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bobpaddock
Senior Contributor III

I remember reading someone's thesis long ago that they did on how it was impossible to pick the ideal I2C pull up value across all parameters such as temperature et.al. Perhaps you can track down thesis's?


You may find this part of interest.  Note the app notes at the bottom of the page too.

LTC4311 Datasheet and Product Info | Analog Devices 

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