• State TI Thinks Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 1 answer
  • Subscribers 33 subscribers
  • Views 1750 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TCA9517: about buffer , TCA9517... How many devices can I connect to the I2C BUS

kyeongmin choi
Intellectual 590 points
Part Number: TCA9517
Other Parts Discussed in Thread: TMP175, TCA9546A

hello

I try to measure temperature using TI device (TMP175).

There is only one bus which is communicating with I2C, but I need 26EA device(TMP175) in parallel.

I had two concern, First I need device which have many slave address . Second I need to reduce bus capacitance. (limit 400pf in I2C )

That's why I chose TMP175 and I decide to add buffer. But I don't know what i have to choose yet. 

I attached my block diagram. Is there any problem ?  Could you plz recommend any solution and you can choose buffer ( TI's device)

Board 1 is made already . I try to design board 2 now.  3.3V power is short in board 2 . So I have to use TCA9517 to shift to 5V.

Thank you.

  • 0
    TI__Guru 73221 points

    Hi kyeongmin choi,

    Just to verify, are you doing a direct board to board connection or is there any kind of long wire/cable being used to connect the boards?

    If so, how long is the cable?

    What max speed are you trying to operate at?

    Are the only I2C devices the TMP175? (Meaning there are only 26 I2C slaves?

    Maybe we can use an I2C switch to seperate slave addresses and capacitance instead of the green ''buffer'' box you have.

    Thanks,

    -Bobby

  • 0 in reply to BOBBY
    Intellectual 590 points

    Hi bobby

    Thank you for your recommendation.

    Let me reply below questions.

    Just to verify, are you doing a direct board to board connection or is there any kind of long wire/cable being used to connect the boards?

    If so, how long is the cable?

    --> no wire , just using connector.

    What max speed are you trying to operate at?

    --> Almost 100k

    Are the only I2C devices the TMP175? (Meaning there are only 26 I2C slaves?

    --> Yes , there are only tmp sesnors.

    Maybe we can use an I2C switch to seperate slave addresses and capacitance instead of the green ''buffer'' box you have.

    --> Correct!!

          I attached circuit board 2  but a little bit different compare to previous blockdiagram.  Could you please check that out?

          TCA9517#1 : For voltage level shift because of different vcc voltage between slave and  master.

          TCA9546 ( IC Switch ) : Separation 4CH.  For reducing bus capacitance and long bus which makes spike noise in data line.

          TCA9517#2,3,4,5( As buffer ) : For reducing bus capacitance  . 

          I will change resistor value after calculation.

     2063.circuit.pdf  

        Thanks,

  • 0 in reply to kyeongmin choi
    TI__Guru 73221 points

    What are the additional TCA9517s on each channel supposed to connect to on their CHx_SDA/SCl_O

    What you have in the PDF should work as long as only one channel is enabled at a time. We wouldn't want to enable multiple since B sides of the TCA9517 would connect to each other through the switch (can cause oscillation). [B side of TCA9517 are never allowed to be connected, even through a switch]

    -Bobby

  • 0 in reply to BOBBY
    Intellectual 590 points

    Hi Bobby

    What are the additional TCA9517s on each channel supposed to connect to on their CHx_SDA/SCl_O

    --> I used buffer (TCA9517) for distribute bus capacitance.  Each channel is consist of 7slaves (TMP Sensor). 

        If TCA9517s (U854 ,855,856,857) is removed, it is ok from bus capacitance.?

    What you have in the PDF should work as long as only one channel is enabled at a time. We wouldn't want to enable multiple since B sides of the TCA9517 would connect to each other through the switch (can cause oscillation). [B side of TCA9517 are never allowed to be connected, even through a switch]

    --> I don't understand your mention.  I just tried to use below way. Could you please tell me more detail. 

    Thank you

  • 0 in reply to kyeongmin choi
    TI__Guru 73221 points

    kyeongmin choi said:

    Hi Bobby

    What are the additional TCA9517s on each channel supposed to connect to on their CHx_SDA/SCl_O

    --> I used buffer (TCA9517) for distribute bus capacitance.  Each channel is consist of 7slaves (TMP Sensor). 

        If TCA9517s (U854 ,855,856,857) is removed, it is ok from bus capacitance.?

    [Bobby] It should be okay. In I2C, typically a I2C device has 10pF of capcitance max (exception are for I2C switches). If you only enable one channel at a time on the 9546, I estimate around 100pF of cap between the first TCA9517 and all 8 TMP sensors (this assumes you removed the additional 9517).

    What you have in the PDF should work as long as only one channel is enabled at a time. We wouldn't want to enable multiple since B sides of the TCA9517 would connect to each other through the switch (can cause oscillation). [B side of TCA9517 are never allowed to be connected, even through a switch]

    --> I don't understand your mention.  I just tried to use below way. Could you please tell me more detail. 

    TCA9517 has a rule which does not allow the TCA9517 to connect to other TCA9517 on the B side. B=B NOT good. This is the same for when they are connected to switches like the TCA9546A. B side = switch = B side NOT okay IF both channels are enabled.
    A side = A side okay, also A side = B side okay.

    Thank you

    [Bobby] I believe you can remove the second TCA9517s downstream from the TCA9546A secondary channels.

  • 0 in reply to BOBBY
    Intellectual 590 points

    Hi bobby

    Thank you your review. 

    As you mention, I removed second TCA9517s. I attached circuit as well.

    If there is any problem yet ? Please let me know.

    Thank you.

    5700.circuit.pdf