How Capacitance Decides No. Of Devices on I2C, CAN

[lokeshbhatt]

Hi,

Since Last few months I m looking for how this capacitance decides no. of devices to be connect. as we heard on I2C Bus we can connect maximum 128 devices. this is why so?
somewhere I read that capacitance decides no. of devices same way in CAN also.
so plzz give me any idea about that and any pdf , link

Thanx in advance
Lokesh Curious for knowledge.............

 

[IanP]

Refer to the I2C bus specification (see link below) for details on max capacitance for each pin (SCA or SDA), max bus capacitance, min width of clock pulse etc. etc ..
Take into account that I2C bus has specified max rise and fall times of ANY transition that have to be executed by ANY device connected to the bus ..
In other words, any driver has to discharge bus equivalent capacitor from H to L (and pullups do it from L to H) in order to achieve bits transmission over the bus, so the more devices are added the higher the capacitance and the more difficult for a driver is to discharge it ..

Here is a quotation on pullups:

The external pull-up devices connected to the bus lines must be adapted to accommodate the shorter maximum permissible rise time for the fast-mode I2C-bus.
For bus loads up to 200pF, the pull-up device for each bus line can be a resistor; for bus loads between 200pF and 400pF, the pull-up device can be a current source (3mA max.) or a switched resistor circuit as shown in Figure 37.

Give it a read ..

https://www.aurel32.net/elec/i2c.pdf

IanP