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TCA9617A: Trace length

Lisa Xu
Intellectual 505 points
Part Number: TCA9617A
Other Parts Discussed in Thread: TCA9617B

Dear TI team:

We have a project design with big PCB size, Generally we calculate PCB trace length with ~3pF/inch for I2C design, 

1. But Intel layout design guide(we use Intel CPU for I2C master) shows the I2C PCB trace length should be ketp in below 30inch,

I think Intel maybe consider others except PCB cap, Do you know why?

2. Due to the trace length in our PCB is over 40inch,so we plan to use TCA9617A for I2C repeater, And could you share what PCB trace length should be kept in for I2C after TCA9617A?

Best Regards

Lisa

  • 0
    Guru 318050 points

    3 pF/inch is just an estimate. Apparently, there are boards that work with a 30 inch bus (probably at a lower speed).

    The TCA9617A allows to connect two buses of 550 pF each. So you should place the buffer in the middle.

  • 0
    TI__Guru 51328 points

    Hi Lisa,

    Lisa Xu said:

    1. But Intel layout design guide(we use Intel CPU for I2C master) shows the I2C PCB trace length should be ketp in below 30inch,

    I think Intel maybe consider others except PCB cap, Do you know why?

    3pF / inch is an estimate like Clemens has stated. 30 inches of trace also starts to look like long wiring which adds inductance into the equation as well. 

    In general, most I2C applications were meant for a bus configuration with shorter traces. Protocols like CAN/LIN, RS485/RS232 were meant for longer wired communication. 

    Lisa Xu said:
    2. Due to the trace length in our PCB is over 40inch,so we plan to use TCA9617A for I2C repeater, And could you share what PCB trace length should be kept in for I2C after TCA9617A?

    Like CLemens has stated, placing the TCA9617A (preferably TCA9617B) in the middle of the 40 inches of trace is ideal since it will split the bus cap evenly. 

    The maximum amount of trace length is based off factors such as data rate, rise-time, bus cap, etc. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Intellectual 505 points

    Hi Tyler:

    Thanks for your explanation,

    Due to we also use Switch just like TCA9548 for channel switch

    and After summary I checked the longest trace length on our board is 90 inch.

    Estimate the cap for components are totally 100pF.

    1. So could you pls estimate the safe trace length after TCA9617A for I2C Standard mode 100KHz working speed also considering inductance impact?

    2. So could you pls estimate the safe trace length after TCA9617A for I2C Fast mode 400KHz working speed also considering inductance impact?

    3. So could you pls estimate the safe trace length after TCA9617A for I2C Fast mode+ 1MHz working speed also considering inductance impact?

    Then I can check one I2C repeater is enough or not for 90inch trace length? and fix the component location in layout. and also good for further similar application.

    Best Regards

    Lisa

  • +1 in reply to Lisa Xu
    TI__Guru 51328 points

    Hi Lisa,

    Please move to TCA9617B. 

    Also, TCA9617B is capable of driving 550pF loads at 1 MHz. It is fast mode+ compliant to the i2c standard. 

    This application note should be sufficient in determining the balance between capacitance, rise-time, and pull up resistance. 

    https://www.ti.com/lit/an/slva689/slva689.pdf?ts=1716561188840&ref_url=https%253A%252F%252Fwww.google.com%252F

    To answer your question about estimating the safe trace length after the TCA9617B, this is hard to give and exact answer, as it depends on your overall implementation from each target that is connected to the bus, voltage being used, trace width, trace length, input cap of each target, PCB layout, etc. 

    In short, it is probably okay to have the TCA9617B sit about 45 inches into the 90 inch segment. This is getting quite long for a trace, but the buffer should be able to split up the capacitance with no problem. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Intellectual 505 points

    Hi Tyler:

    Give my sincerely appreciation for your reply,Thanks

    Best Regards

    Lisa

  • +1 in reply to Lisa Xu
    TI__Guru 51328 points

    Hi Lisa,

    If no further questions I will close the thread. 

    Regards,

    Tyler