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AMC7924EVM: testing issue

Tommy Tzeng
Mastermind 6330 points

Part Number: AMC7924EVM
Other Parts Discussed in Thread: TCA9548A, AMC7904

Hi Team,

This is a standalone test of TI’s AMC7949 EVM. Under normal conditions, proper I2C communication should not generate spikes. However, these spikes cause other I2C devices to malfunction, resulting in the entire I2C bus becoming non-operational. A power cycle is required to restore normal operation.

The current workaround is to adjust the pull-up resistors on the I2C bus to suppress the spike to below 530 mV. However, the trade-off is that the I2C waveform rises more slowly and cannot reach VCC, which introduces risk. (Under the same modification, at low temperatures, 3 units operated normally, while 1 unit experienced I2C communication failure.) Due to component tolerances—especially in mass production—there is no guarantee that every unit and every command will avoid I2C malfunction.

There is no crosstalk issue between I2C SCL and SDA. In the PCB design stage, we will run SI analysis to ensure signal integrity and eliminate interference.

We performed several experiments:

1.We conducted cross-testing and measured each I2C device. The probabilistic I2C communication issue only occurs when both the AMC7949 and one specific I2C device are present at the same time. 

2. When isolating only the AMC7949 control path, the I2C operates normally across all three temperatures. 

3.When isolating only the affected I2C device, the I2C also operates normally across all three temperatures. 

4.After adjusting the I2C bus pull-up resistors, the I2C shows mixed results at low temperature (3 units passed, 1 unit failed).

Would using different channels of the TCA9548A I2C switch help isolate the AMC7949’s spikes from interfering with other TCA9548A I2C channels and eliminate any potential crosstalk issues (with SI optimization applied to improve signal quality)?

Will the TCA9548A’s I2C control be affected by the spikes generated by the AMC7949? If the TCA9548A is affected, would performing a reset allow it to recover?

The I2C signals measured on the TI AMC7949 EVM:

AMC7949 EVM measure i2c signal.jpg

When the communication error occurs, the SDA spike causes SCL to be pulled low and unable to recover.:

image.png

I2C communication works normally after adjusting the pull-up resistors.:

 

  • 0
    TI__Genius 16990 points

    Hi Tommy,

    The team will take a look at this after the US holiday.

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    TI__Genius 16990 points

    Hi Tommy,

    First to confirm, are you using the AMC7904 here?

    These spikes appear to primarily be on the ACK section of the I2C cycle. The most likely situation is that the device that is ACK'ing is seeing a small delay in pulling the SDA line down, causing the spike. To clarify, neither the AMC7904 or the other device show this issue isolated? Does the AMC7904 show this issue with any other I2C device? Does the second Device show this issue with any other I2C device? Do you see these spikes when writing to a specific device, or when writing to either device?

    I'm not sure why a spike on the SDA line would cause I2C issues in other devices, I'll talk to our digital designer to see if he can come up with any solutions.

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    TI__Guru 56833 points

    Can you be very clear: what device are you using? There is no AMC7949 product.