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TCA9545A: VCC rising time

Yusuke Mukuno
Expert 7395 points
Part Number: TCA9545A

Hi expert,

My customer have a question about Vcc rising time. (VCCRT recommendation : 0.1 to 100ms)

Their system Vcc rising time is around 3us. If Vcc rising time will shorter than datasheet recommendation, how do you think about concerns?

Also, how should they measure rising time? (ex, the time from 10%Vcc to 90%Vcc) 

Thanks

Muk

  • 0
    TI__Guru 73271 points

    Hey Muk,

    Can the customer toggle the reset pin of the device after power up?

    10% to 90% is an okay way to measure.

    Our concern with a rise time which violates our recommendation is a possibility of a bad PoR causing the state machine of the device to start up in an unknown state. Toggling the SCL pin 8 times should resolve the issue but is not a 100% guarantee. Toggling reset should resolve the issue otherwise a proper power cycle would be required.

    Thanks,

    -Bobby

  • 0 in reply to BOBBY
    TI__Expert 7395 points

    Hi Bobby-san

    My customer want to exactly confirm about Toggling the SCL pin 8 times.

    1. How long do the need the time between Vcc power on and starting SCL toggling?

    2. Is it ok, that the toggling frequency is 400kHz?

    3.  How long do they need the time between finishing SCL toggling and device access? (tREC?)

    >Can the customer toggle the reset pin of the device after power up?

    No they don't have separated signal for Vcc and Reset pin.  (Reset pin is pulled up to Vcc)

    Can they use the delay control for reset pin signal as below?

    If yes, how long do they need the time between Vcc power up and reset threshold?

    Thanks

    Muk

  • 0 in reply to Yusuke Mukuno
    TI__Expert 7395 points

    Hi Bobby-san,

    Do you have any comment for additional questions?

    Muk

  • 0 in reply to Yusuke Mukuno
    TI__Guru 73271 points

    Yusuke Mukuno said:

    Hi Bobby-san

    My customer want to exactly confirm about Toggling the SCL pin 8 times.

    1. How long do the need the time between Vcc power on and starting SCL toggling?

    I would expect after the device has powered up, and reset is pulled high (above 70% of Vcc) then you should wait a few microseconds.

    2. Is it ok, that the toggling frequency is 400kHz?

    Yes, that should be fine.

    3.  How long do they need the time between finishing SCL toggling and device access? (tREC?)

    tREC should occur first. The reset needs to be pulled high before you begin the clock toggles.

    >Can the customer toggle the reset pin of the device after power up?

    No they don't have separated signal for Vcc and Reset pin.  (Reset pin is pulled up to Vcc)

    Can they use the delay control for reset pin signal as below?

    You can do this but it just means you have to wait longer until the reset pin has reached 70% of Vcc before you toggle the clock pin.

    If yes, how long do they need the time between Vcc power up and reset threshold?

    Thanks

    Muk