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TCA9800: I2C Bus with Unsable Supply

Part Number: TCA9800

Hi I2C Experts,

I am looking to use the TCA9800 as a repeater for my I2C communications down a long cable. The power supply downstream is prone to collapse and I want to ensure this design will work as desired

Below is a drawing of the system setup:

During the collapse of the remote 3.3V supply, the MCU will disable the I2C buffer (either by seeing the repeated I2C bus error or by using an ADC to monitor the remote 3.3V supply). Once the collapse of remote 3.3V is detected, the MCU will change the LEDs from LED Driver #2 (via I2C) to indicate a fault. Once the remote 3.3V recovers, the buffer will be re-enabled and the I2C bus should fully recover.

Are there any problems with the design as it is?

Once the TCA9800 is disabled, is it possible to communicate with LED Driver #2 without pull-up resistors?

Thank you,

Reed

  • Hi Reed,

    When the remote power supply drops out, the I2C lines on the A-side of TCA9800 will not be able to pull high. The lines will likely decay to a potential that will be considered a LOW by the buffer and this will be propagated to the MCU side. Your MCU should look to recognize this stuck bus and disable the buffer to stop the fault from affecting the B-side bus. If this would be difficult (complex for MCU programming) the pull-ups on the A-side may be sourced from the reliable Vcc (depending on concerns of LED Driver #1’s leakage).

    The B-side current sources of the TCA9800 remain active when the device is disabled (EN < Vil). You should still be able to communicate on this side of the device while it is disabled and the A-side pull-ups are unpowered (A pins will be high-Z). 

    Let me know if you have any more questions.

    Regards,

    Eric