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PCA9306: SDA1 Abnormal voltage

Part Number: PCA9306
Other Parts Discussed in Thread: , TXS0104E

Hi :

We used TI's PCA9306, but in actual use, we found that SDA1 was connected with our module's I2C_SDA, and the SDA level was 0.5-0.6v.

problem description:

Lift the SDA1 pin of the chip to separate it from the PCB pad, and measure the voltage of 3V3 and 1V8 with the multimeter. The voltage values are all normal. The VFEF2 and EN pins of the chip use the voltage multimeter to measure the voltage of about 0.5v.R13 resistance was measured normally, and SDA1 was separately pulled up from 2.2k to 1V8. The voltage of SDA1 was measured to be about 0.6v.

1.In PCA9306EVM, VREF1 is connected with 0.01uf capacitor and VREF2 is connected with 100pF capacitor,My schematic USES a capacitance of 100nF. Does this affect it?

2.The voltage measurement of SDA2/SCL2 is 3.3v,SCL1 is 1.8v and SDA1 is 0.5v. What is the problem?

The schematic diagram is as follows:

  • Hi Ren,

    The purpose of the capacitors at the reference pins is to keep supply voltage stable. Section 9.2.2.1 suggests a 100-pF filter capacitor at Vref2. A lager value here will give you less high-frequency filtering to the supply pin, but will otherwise serve the same purpose in noise reduction. This should not impact how the device operates with DC inputs.

    The PCA9306 acts to connect SDA1 and SDA2 when one side is pulled bellow the device's threshold voltage. With a voltage of 0.5v present on SDA1, the device should be allowing SDA2 to discharge to SDA1 through its pins. This seems like a problem with how the device is interpreting the reference voltage at Vref2 and EN pins. Could you clarify the voltage present at these pins relative to GND when the device is connected? (NetC1 in the layout). With the reference voltages of 1.8v and 3.3v, it should be near 2.4v (1.8v + 0.6v across the FET).

    There is no internal mechanism in the PCA9306 designed to pull SCL or SDA pins low. Is there something else on the I2C bus that could be pulling SDA1 down to 0.5v?

    Regards,
    Eric

  • Hi Eric:

    Reply is as follows, please help guide, thank you。

    1.Could you clarify the voltage present at these pins relative to GND when the device is connected?

    Reply:

    Measure the voltage:

    VREF2&EN PIN  voltage : 0.4v   SCL2&SDA2 Voltage:3.3V  SCL1 Voltage:1.8V VREF1 Voltage:1.8V  SDA1 Voltage:0.5V

    2.There is no internal mechanism in the PCA9306 designed to pull SCL or SDA pins low. Is there something else on the I2C bus that could be pulling SDA1 down to 0.5v?

    Reply:We were directly connected to the I2C pin of MSM8909. At that time, the peripheral TP was also removed during the test, so there were no other factors causing the abnormal voltage。

              In addition, we directly pulled up the chip SDA1 pin 2.2k to 1.8v, and measured that SDA1 was 0.6v

    3.I use TI TXS0104E for flight line test, I2C is OK, the function is normal, what is the reason?

  • Hi Ren,

    1. Vref2/EN in the configuration in the diagram should be much higher than 0.4V. As shown in the figure, the 200k-Ohm resistor works with the internal FET to create a bias voltage of Vcc1+Vth. Could you check that the voltage on the other side of R13? Is the drop across this resistor consistent with the 2.7v we should expect?

    2. If PCA9306 is indeed the only other device on SDA1 other than a pullup reisistor, it seems that the device is shorting SDA1 to GND internally. A low voltage at Vref2/EN does not explain this behavior. As I said, this is not intended behavior and may be signs of damage. Could you try using a new part after verifying R13?

    3. Could you specify in what test case TXS0104E worked in your application? Was it on SCL1/SDA1 side while PCA9306 was active? Could you also confirm that MSM8909 was configured for I2C communication in your testing? (SPI config uses i2c_data pin as spi_cs_n which may be pulling low).

    Regards,
    Eric

  • Hi Ren,

    I just wanted to check in on this - are you still seeing issues?  If so, have you had a chance to review Eric's comments above?  Your responses to these should help us to debug this further.

    Regards,
    Max