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Original question:

P82B715: Please help to review Schematics

P82B715: support both long wire and hot swap

CJ CJ
Prodigy 10 points
Part Number: P82B715
Other Parts Discussed in Thread: TCA4307,

Hi I have a design that require the sub-board to be hot swappable, plugin/unplugged while the main-board will operating.

The sub-board is a I2C sensor, the long wire is 2m soldered with sub-board, at the other side is the connector to the main board. 

To illustrate relation:

main-board + connector ----i2c---- connector + long wire + sub-board

I see that to support the hot swap, I need to use TCA4307. And from the P82B715 datasheet, I need to have it at both sides.

My question is how the setup should be:

P82B715(main) <----> TCA4307(sub)
or
P82B715(main) <----> P82B715(sub) + TCA4307(sub)
or
P82B715(main) <----> TCA4307(sub) + P82B715(sub)
Thanks
/CJ

 

  • 0
    Guru 260890 points

    The P82B715 creates a bus segment that uses more pull-up current than normally allowed by the I²C specification. This implies that that segment bus be separated from the normal segments with P82B715s. This also means that it is not possible to use the TCA4307 on the hot-plugged segment.

    But 2 m might be short enough that the TCA4307 alone, without any P82B715s, can handle it.

  • 0
    TI__Mastermind 27443 points

    Hi CJ,

    As Clemens stated in the first half of his response, the P82B715 bus extender is meant to be used in pairs. P82B715 <-> P82B715 connection is required due to the high amount of current on the driving side. This allows the P82B715 to drive a higher load up to 3 nF (3000 pF) when using I2C protocol. The load could result from long wired communication during I2C which the P82B715 would be used to solve. 

    Therefore, the connections of 

    CJ CJ said:
    P82B715(main) <----> TCA4307(sub)

    and

    CJ CJ said:
    P82B715(main) <----> TCA4307(sub) + P82B715(sub)

    Are not recommended. 

    The connection 

    CJ CJ said:
    P82B715(main) <----> P82B715(sub) + TCA4307(sub)

    Might work, but I have never dealt with a customer using both the bus extender and a hot-swap buffer such as the TCA4307 together. So I am unsure of the possibility of this application not working. 

    As Clemens stated however, 2m of cabling is not the longest cable in the world. If we estimate that the cable parasitic capacitance could be around 100pF / meter, then the cable alone takes ups 200pF. Adding in 10pF / connector, as well as some PCB trace cap (10pF), and 10pF / input cap / slave device, we could estimate that we are dealing with something like 200pF ~ 300pF of capacitance in total, which the TCA4307 might work standalone without the need for excessive load driving capabilities of the P82B715. 

    Please let me know if you have any comments or concerns. 

    Regards,

    Tyler

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thanks Clemens for the reply, my case is a bit complex, the cables will be sewed into textile, but right now we have to make assumption before we can actually test the cables being sewed. So we need to assume the worst case, what if this 2m cables behaves like a 20m cables(or long enough cable)? Hot swap is a must to have feature. What will be your solution then? Thanks.

  • 0 in reply to Tyler Townsend
    Prodigy 10 points

    Thanks Tyler for the reply as well, as said to Clemens, my case is a bit complex, the cables will be sewed into textile, but right now we have to make assumption before we can actually test the cables being sewed. So we need to assume the worst case, what if this 2m cables behaves like a 20m cables(or long enough cable)? Hot swap is a must to have feature. What will be your solution then? Good to hear more ideas. Thanks.

  • 0 in reply to CJ CJ
    Guru 260890 points

    What matters is the cable's capacitance. It should be specified in the datasheet.

    The total capacitance (cable+traces+devices) must be less than 400 pF per the I²C specification, but you might be able to tolerate more by reducing the speed.

  • 0 in reply to CJ CJ
    TI__Mastermind 27443 points

    Hi CJCJ,

    Like Clemens stated, the capacitance for the cabling should be present in the datasheet. I would trust the numbers in the datasheet since comparing a 2m vs. 20m cable will yield much different results especially in I2C. If you intend to design with 10x tolerances, then I think you will find that almost nothing will be within spec for I2C. Spec the system for 2meters of cabling, but maybe add +/- 20 pF on the tolerance, I don't think you have to go too crazy here when estimating. 

    If during your testing the standalone TCA4307 waveforms look good on a scope, and you don't see much shark finning (slow rise-times due to large RC constants), then I think you can avoid the bus extender all together. 

    Now if you plan to communicate across 20meters of cabling, then P82 bus extender might need to be an absolute in your system, and the TCA4307 would become secondary. Otherwise, you might be better suited with a different protocol such as CAN or RS-485 using differential pair signaling. 

    Regards,

    Tyler