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MSPM0G3519: I2C Bus Cable Length and Driving Addon Card

Egemen Aksoy
Intellectual 955 points
Part Number: MSPM0G3519
Other Parts Discussed in Thread: TCA9517, P82B96, P82B715, TCA9517-Q1, TCA9617B

Tool/software:

Hello,

In our project we have a master and addon card.

In our master card we are using MSPM0G3519 MCU.

For our addon card there is 10xTMAG5173-Q1 and 1xTCAL9539-Q1 ICs.

From master card to addon card cable length shall be around maximum 80cm.

So should we use I2C buffer?

In your website I found TCA9517 (I2C buffer must be Functional Safety Capable)

https://www.ti.com/lit/ds/symlink/tca9517-q1.pdf?ts=1757181492228

If we have to use, how should be the placement of the I2C buffer IC?

Is it must be placed on master card or slave card or both of them?

Best regards,

  • 0
    Intellectual 955 points

    Hello,

    I also found P82B96 and P82B715 cable extender ICs for that purpose. Which one should be more effective and what is the difference of these ICs? Also is there functional safety capable version of this IC?

    Best regards,

  • 0 in reply to Egemen Aksoy
    TI__Mastermind 46765 points

    Hi,

    I would say you may need an I2C buffer for the cable length up to 80cm form MCU perspectival. I am going to move this post to TCA9517 for further discussion. 

    Best regards,

    Cash Hao

  • 0 in reply to Cash Hao11
    TI__Mastermind 48668 points

    Hi Egemen,

    MSPM0G351x has ODIO spec'ed at IOL,max = 4mA, 0.45V. The device has sufficient drive strength in my opinion. 

    We can estimate 1 cm / 1 pF of trace. 10pf input cap per device on the bus. Ctotal = 190 pF of load cap. The MCU should technically be able to support up to 400 pF of cap loading by itself, but if you want peace of mind and future proof by adding more re-drive capability with TCA9517-Q1, it wouldn't hurt. 

    The MCU can support up to fast mode+, so if you use TCA9517-Q1 you will be limited to only fast mode (0 to 400kHz). We do not have a Q1 grade 1 MHz I2C buffer. 

    The VOLB from TCA9517-Q1 should match up nicely with TMAG and TCAL devices. Both devices have VIL's = 0.3 x VCC. If both devices operate at 3.3V, there is sufficient margin from the large output voltage of the TCA9517-Q1 buffer (VOLB = 0.52V). 

    Connection would look like MCU <--> TCA9517-Q1 (a-side) master card side <--> slave card <--> TMAG + TCAL

    I don't think you need two buffers one for both master and slave cards. I think you are okay with just one next to the MCU. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Intellectual 955 points

    Hello Tyler,

    Thank you for your reply.

    Both supplies shall be 3V3.

    This shall be an automotive application.

    If is there functional safety capable ICs similar to P82B96 and P82B715 would not be more sufficient for noise immunity or is TCA9517-Q1 enough for our application?

    Isn't there any solution for 1MHz I2C? (Functional safety capability is mandatory or if you can provide functional safety data like pin FMEA, FIT rate etc. would be sufficient for us.)

    For the scenario that we use TCA9517-Q1 like as you recommended, can we place 100ohm serial resistors to the bus and ESD diodes for ESD protection?

    Best regards,

  • 0 in reply to Egemen Aksoy
    TI__Mastermind 48668 points

    Hi Egemen,

    Egemen Aksoy said:
    Both supplies shall be 3V3.

    TCA9517-Q1 will work here. 

    Egemen Aksoy said:
    If is there functional safety capable ICs similar to P82B96 and P82B715 would not be more sufficient for noise immunity or is TCA9517-Q1 enough for our application?

    We do not have auto grade of P82B96 and P82B715. 

    TCA9517-Q1 should be enough for buffering purposes. P82B96 might have better noise immunity from long cables, but the device must be used in a pair, is expensive, and voltage output is quite large for a 3.3V application (VOL = 1V max). 

    Egemen Aksoy said:
    Isn't there any solution for 1MHz I2C? (Functional safety capability is mandatory or if you can provide functional safety data like pin FMEA, FIT rate etc. would be sufficient for us.)

    We have TCA9617B 1 MHz fm+ I2C buffer, but not in automotive version. 

    I used the following tool from ti.com to get MTBF/FIT supporting data: 

    https://www.ti.com/quality/docs/estimator.tsp?opn=TCA9517DGKRQ1&cpn=&partNumber=TCA9517DGKRQ1#resultstable

    Egemen Aksoy said:
    For the scenario that we use TCA9517-Q1 like as you recommended, can we place 100ohm serial resistors to the bus and ESD diodes for ESD protection?

    Added external current limiting and ESD protection is allowed. 

    Adding 100 ohm series resistor will increase VOL at the downstream target. You must be sure that the VOL + Vdrop(res) <= VIL (target). This is dependent on series resistance value and PU resistors in the system. 

    Adding ESD diodes usually only add a bit of capacitance. This is usually <10 pF input cap and is small enough to not worry about for most I2C systems. 

    Regards,

    Tyler