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PCA9517: Problem when two repeaters are connected in parallel to single I2C bus

Hafiz Haja
Intellectual 470 points
Part Number: PCA9517
Other Parts Discussed in Thread: TCA9517

Hi,

I have designed a board which contains micro-controller and FPGA. Micro-controller has two I2C channels and FPGA has one. First channel of micro-controller is shared between a  temperature sensor, a voltage sequencer and an EEPROM. The input capacitive load of these chips are maximum 7 pF. The I2C repeater PCA9517 is used between micro-controller I2C bus and these devices. I2C channel of FPGA is connected to the B side of the I2C repeater mentioned above. FPGA I2C channel is also interfaced with I2C repeater PCA9517. So in effect B side of both the PCA9517 chip will be shorted. The pull up resistors used are 10 K ohm for micro-controller I2C channel and 4.7K ohm for FPGA I2C channel. So in effect 10K // 4K will be equivalent resistance. It seems sufficient pull up is there. Without shorting the lines of two I2C repeaters the signals SDA and SCL are coming correctly for both the channels. But after shorting both repeater B sides the signals are not pulling high. The high level of the signals are reaching around 0.8V only. Is there any issue when connecting multiple I2C repeaters in parallel to a single I2C bus.

Regards

Hafiz Haja

  • 0
    TI__Mastermind 49110 points

    Hey Hafiz,

    I am having a bit of a hard time following your description so a block diagram may give me a better idea.

    From my understanding, you are seeing the B outputs latch to 0.8V without rising back up to their pull ups. This device has a lot of Errata associated with it beginning on page 10 of the datasheet so I believe you are seeing one of these cases. I believe it to be the load susceptibility Errata (figure 6).

    I would recommend you move from this device which is now "Not Recommended for New Designs" to our Newest device, TCA9517,  which correct these issues.

    Another (from how you described it), is you have two buffered outputs tied together, this can present problems as low from A side will not propagate to the other A side. This is something we point out in our datasheets.

    Essentially, you cannot connect two buffered outputs with static offsets to each other. If you can give me a block diagram with what you are interfacing with (I2C devices) I can help try to find a better solution/alternative.

    Thanks,

    -Bobby