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TCA9617B: I2 bus the rising will take very long time

David lin
Prodigy 135 points
Part Number: TCA9617B

Hi 

our customer highlight I2C read/write error randomly
as our measure waveform, we find out
the rising will take very long time ,i2c may capture worng data
do you have any suggestion to improve ??

[schematic connection]
MCU(3.3V) > PU 1K-ohm to 3.3V -> LAN switch and TCA9617 A side(3.3V)
TCA9617 B side(5V) -> Pull high 1K-ohm to 5V -> supercap controller(5V)

[i2c speed] 400K

[waveform as attach]
GREEN waveform is 3.3V_I2C_SDA)
YELLOW waveform is 3.3V_I2C_SCL

  • 0
    TI__Mastermind 38373 points

    Hi David,

    This seems like a rise-time issue. Try changing the 1k pull-ups on the MCU side to something larger such as 2.7kohm. 

    Regards,

    Tyler

  • 0
    TI__Mastermind 38373 points

    David,

    Sorry for double reply, but looking at the scope it looks like the device experiences a negative voltage around -2V. Is this the case, or am I reading the reference on the o-scope incorrectly? 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 135 points

    Hi Tyler
    thanks for your reply, please refer as below

    This seems like a rise-time issue. Try changing the 1k pull-ups on the MCU side to something larger such as 2.7kohm.
    -->we have place 1K pull-ups near on MCU side, please refer attached file, do you suggest increae Res value??
    if increase from 1K to 2.7K, the rise time will change to fast  ?

    looking at the scope it looks like the device experiences a negative voltage around -2V. Is this the case, or am I reading the reference on the o-scope incorrectl
    -->i think you are right, we need to double confirm , the voltage of  waveform is weird

  • 0 in reply to David lin
    TI__Mastermind 38373 points

    David,

    Sorry I was backwards in my explanation. 1k pull-up resistors would make for a faster rise-time, but increase the VOL. Larger pull-up resistors (i.e. larger resistance) would increase rise-time and decrease overall VOL. 

    David lin said:
    if increase from 1K to 2.7K, the rise time will change to fast  ?

    Rise-time will become slower. 

    David lin said:
    -->i think you are right, we need to double confirm , the voltage of  waveform is weird

    Please let me know what you find out about this. 

    The voltage on the SDA waveform looks to have larger pull-up resistors and/or a combination of weak pull-down strength by the driver. Can we confirm that the 1kohm pull-up resistors are the only pull-up resistors on SDA? 

    Regards,

    Tyler