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P82B715: Chip application problem

Tonyx.wu
Genius 3145 points
Part Number: P82B715

Tool/software:

There is a problem with the waveform of I2C CLK

If the 5V power supply of the chip is removed, the falling edge cannot be pulled down.i2c pulls up 1.8V

The 1.8V I2C pull-up was working fine when the normal 5V was present.

If the 5V power supply to the chip is removed, the falling edge cannot be pulled low. When the 5V power supply to the chip is removed, there is also voltage regurgitation, and after power-on, the 1.8V I2C becomes 2.2V, which should be 1.8V.

There are two questions that need to be clarified:
1. Why does the chip pull up to 2.2V internally, and the 5V power supply is disconnected, only the pull-up resistor has power, which affects the chip?
2. Why does the voltage drop to 1.8V during communication when it is 2V?
Please help me respond to the above questions on how they arose and how they can be resolved.

It works like this

Problematic waveform

  • 0
    TI__Mastermind 35255 points

    Hi Tony,

    Tonyx.wu said:

    If the 5V power supply of the chip is removed, the falling edge cannot be pulled down.i2c pulls up 1.8V

    Without schematic shown, I presume 5 V removed means Vcc floating or at GND (note that floating means unknown)? If so, I2C pulled up to 1.8 V using a pull-up. With Vcc removed, it is not expected for device to be functioning. Hence, device is not driving I2C low.

    1. The chip is most likely disconnected if Vcc is disconnected and not driving I2C. Pull-ups connected are keeping the line high and the power supply used is fluctuating. If Vcc is disconnected, device should not be operational as recommended min for Vcc is 4.5 V.

    2. The voltage will be similar to the power supply voltage current sinking/sourcing capabilities with pull-up resistor values used.

    Unsure why Vcc is disconnected as the solution is to use the device with the recommended voltage range, thanks.

    Best Regards,

    Michael