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[SOLVED] What is Bus capacitance?

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snaku

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HI All,
What is Bus capacitance?
In I2C, what are the advantages and disadvantages of bus capacitance?

Also, suggest some links or documents related to this.

Thanks,
Naveen
 

ckshivaram

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The maximum bus capacitance is 400pF, which sets the maximum number of devices on the I2C bus and the maximum line length...
I2C bus is defined as having a maximum capacitance of 400pF, that defines the maximum number of I2C components on the I2C bus and the I2C length [in combination]. So the I2C Bus length is never defined in the I2C Bus specification.

http://www.i2c-bus.org/fileadmin/ftp/i2c_bus_specification_1995.pdf

---------- Post added at 11:14 ---------- Previous post was at 11:06 ----------

http://www.datsi.fi.upm.es/docencia/Micro_C/i2c.pdf
 

snaku

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Please let me know what exactly is bus capacitance and how it decides the number of devices that can be connected to I2C bus?

Naveen
 

ckshivaram

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i2c pull up resistor
As the number of devices connected to the bus increases, the bus capacitance also increases (I think the maximum allowable capacitance is 400pf).

All the ICs connected to the I2C bus can drive either drive it to logic 0 or Z.
To bring the bus to a logic 1, a pull up resistor is connected on SDA and SCL lines.
If the value of resistance is less, more current flows from the supply to quickly charge the bus capacitance. i.e., the bus reaches logic 1 very quickly.
But if the pull up resistor value is too low, it will act like a short circuit when the device connected to it is trying to drive the bus to logic 0.

So, if the bus capacitance is between 200pf to 400pf, we go for a current source instead of a low resistance pull-up. If the bus capacitance is below 200 pf, we can use pull-up of 1kohm to 10 kohm depending on the load.

the pdf i gave in the link explains everything....
 

FvM

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Your question is exactly answered in the linked documents:
The bus capacitance is the total capacitance of wire, connections and pins.
 
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snaku

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Hi Sivaram

thanks for the clarification

Naveen
 

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