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P82B96: P82B96: Adding multiplexer

Krunal K
Intellectual 280 points
Part Number: P82B96


Hi,
I want to make communication between MCU and four I2C devices (accelerometer) without changing the I2C address.

In my current design I am using multiplexer in between MCU and Four P82B96 buffers and it is working fine.

i.e.
MCU(Master)->MUX(74HC4052D)->P82B96->Connector_one-->long wire-->Connector->P82B96->Accelerometer_one

Now I want to make cost effective solution where I want to use only one P82B96 buffer at master side and want to move multiplexer at output of the P82B96.
i.e.

MCU(Master)->P82B96->MUX(74HC4052D)->Connector_one. Here, MUX will choose output connector(i.e one of the four slaves) as needed.

As per my current understanding it should work but I have few questions
1. Will there any capacitance impact due to MUX? Communication speed=100Khz, distance between I2C master and slave is around 15 to 20 Meters.
2. Do you have any better suggestion?

Thanks,
Krunal

  • 0
    TI__Guru* 78111 points

    Hey Krunal,

    Just to verify you mean this is your current implementation:

    And then your idea is to do this:

    "1. Will there any capacitance impact due to MUX? Communication speed=100Khz, distance between I2C master and slave is around 15 to 20 Meters."

    The MUX should keep each cable's capacitance separated from each other as long as the MUX is truly a MUX (can only select one channel). I don't think the MUX's capacitance will affect the system compared to the capacitance of a 20 meter cable. My caution would be to make sure the MUX can handle the current that is being driven through the cable since the communication is based on a open drain architecture still and to support 20 meters you would need to provide a fairly strong pull up resistor to support that capacitance. (I'm saying to check and make sure the IoL  current of the cable can be supported by the MUX.) Second thing is to make sure the MUX does not saturate it's pass FET since IoL may be large, you could saturate the MUX causing the VOL of the transmitter of the P82B96 to be larger than the ViL of the receiver P82B96.

    "2. Do you have any better suggestion?"

    This looks like a suitable solution to me if the two 'gotchas' pointed out above are addressed. Aside from that, I would make sure the MUX has suitable ESD on each channel since the connector ends may be in contact with people/equipment/ect.

    -Bobby