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Original question:

FDC1004

FDC1004: FDC1004 communication problem

ahmet bakır
Intellectual 255 points
Part Number: FDC1004
Other Parts Discussed in Thread: ISO1541

Hello,

We try to develop a system which is related to sense capacitance between a custom designed capasitance circuit and grounded plate. 

We have a servo system to set height of the system which we try to measure its height with FDC1004.

We have some connection problem between i2c and microprocessor.

We can measure the capacitance when we make run them on the bench without servo system. But when we connect the capacitive sensing system to servo system, the connection between sensor and microprocessor is broken. We tried to disconnect motor chasis and capacitive sensor ground, the problem was solved partly. Until the sensor touch the plate, everything is okay but after the sensor touch the plate, i2c connection is broken.

Do you have any idea about the problem? Do we do something wrong?  What can we do to solve this?

Thanks.

Ahmet

  • 0
    TI__Genius 13335 points
    Hello,
    Can you confirm that the microprocessor and FDC1004 share the same ground connection? and pulled up to the same supply? If they are on different grounds then the I2C communication may not work properly. Try probing the I2C connections on both the micro side and FDC side to make sure the DC levels are the same on both.
    Regards,
    Luke
  • 0 in reply to Luke LaPointe
    Intellectual 255 points
    Hello,

    Thank you for your help.

    We confirm that the microprocessor and FDC1004 share the same ground connection and pulled uo to the same supply. Everything is going well until the capacitive sensor tip touch the sheet metal on the ground.

    It is obvious that the connection between microprocessor and FDC1004 work properly when we use the system manually. As soon as we work servo system and touch the capacitive tip to sheet metal, I2C connection is always broken.

    I can explain our system connection briefly. Circuit boards and flexible cables are in a conductive housing which has nonconductive sheat. There is 15 cm distance between FDC1004 circuit board and microprocessor. We use flexible cable to connect them each other.

    Now. we will try to change flexible cable to another type of cable.

    What can we try anymore?

    Regards,
    Ahmet
  • 0 in reply to Luke LaPointe
    Prodigy 15 points
    I would like to add a I2C isolator ISO1541 to the circuit to isolate microprocessor and FDC1004.

    You adviced to connect all grounds. However if Iso1541 is used, the grounds have to be isolated.

    Do you suggest one power supply with common ground or two isolated power supply with iso1541?

    Regards
    Bahadir
  • 0 in reply to BAHADIR YILMAZ
    TI__Genius 13335 points
    Hello,
    The inputs to the FDC1004 should not be electrically connected to anything other than the sensor electrode. If you short the inputs to any other potential for example by touching the capacitive sensor tip to the sheet metal on the ground, then this may shift the local ground and VDD potentials especially if they are isolated from the microprocessor side. If a shift like this occurs then it is likely that the I2C communication may stop working. Have you tried measuring the I2C line on the MCU side during this event? If you see a spike or something else then this could certainly affect the I2C communication. Does the system recover if you clear the bus by clocking the SCL line?
    Regards,
    Luke
  • 0 in reply to Luke LaPointe
    Intellectual 255 points

    Hi,

    We connected the main ground to chassis with a capacitor. Also we connected the 3.3V to chassis by capacitor.

    The problem was solved. 

    Is it not recommended to use an isolator like ISO1541 with FDC1004 even both of them is same grounding level?

    Regards,

    Ahmet. 

  • 0 in reply to ahmet bakır
    TI__Genius 13335 points
    Hello Ahmet,
    Thanks for confirming the solution! I am not familiar with the ISO1541 so I cannot comment on its compatibility in your system. You may consider posting a new question specific to the ISO1541 and reference this thread and the appropriate expert can give their advice.
    Regards,
    Luke
  • 0 in reply to Luke LaPointe
    Intellectual 255 points

    Hello,

    Thank you for your help and advice.

    Regards,

    Ahmet