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TCA4311A: I2C CLK has abnormal waveform

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Replies: 5

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Part Number: TCA4311A

Hi Team,

We are failed to config 0xFE device, but for other I2C address is ok. And competitor-PCA9511ADP is also OK to config 0xFE.

We compare TI-TCA4311ADGKR and NXP-PCA9511ADP, the spec is all the same.

In below figure, we find CLK input (EM_I2C_SCL_U155) have clock in TCA4311ADGKR, and do not find CLK input  EM_I2C_SCL_U155 in PCA9511ADP.

It looks these clocks cause IO module confused.

Please kindly give us some suggestion about where does the clock come from? And how to delete it.

Thanks

TI-TCA4311ADGKR:

PCA9511ADP:

Schematic:

  • This looks like the problem described in section 8.4.3 of the datasheet (both ACK and clock stretching involve the slave pulling the clock low).

    The pull-up resistors are too strong; with the PCA9511A/TCA4311A's rise-time accelerators, you should use 10 kΩ. The datasheet also suggests adding capacitance.
  • In reply to daniel:

    Hey Daniel,

    As Clemens has pointed out, the device looks like it is seeing the problem described in the datasheet. This is due to the RTAs (rise time accelerators) on the clock signal activated before the low has been recognized. On the purple waveform, we see contention and on the yellow waveforms we see a rising edge and then a low after the RTAs disengage.

    We should size up the pull up resistor on the 'in' side of the device to be larger to slow down the RTA and remove any series resistance between the TCA4311A and the master/slave driving the signal low. Right now, it looks like you are using 4.7k pull ups, going to 10k or even 20k should help with this.

    Let me know if changing the pull up resistors help!
    Thanks,
    -Bobby
  • In reply to TRX Bobby:

    Hi Bobby, Clemens,

    Many thanks for your support.
    After we change the pull high resistor to 10k, the ACK become low successfully.

    Please help to confirm the current of Rise-Time Accelerators is 8mA or 2mA.
    Data sheet "6.5 Electrical Characteristics" show typical 8mA, but "8.3.1 Rise-Time Accelerators" and "8.2 Functional Block Diagram" show 2mA.

    One more question, why do we enhance the accelerators current from 2mA to 8mA?

    Thanks
  • In reply to daniel:

    Hey Daniel,

    I believe the 8.2 functional block diagram is incorrect in terms of the 2 mA current source. The TCA4311A does not use a current mirror to generate its pull up current, the current actually follows the I(t) curve of a charging capacitor. The 8mA spec is more accurate though it is dependant on the Vcc and any series impedance to the bus capacitance.

    Thanks,

    -Bobby

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