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PCA9545A: Maximum bus capacity

Antony Lin
Genius 13480 points
Part Number: PCA9545A
Other Parts Discussed in Thread: TCA9545A

Hi,

Under table1 in datasheet as below, we specify "Care should be taken not to exceed the maximum bus capacity.".  What does "maximum bus capacity" mean?  Where is the constraint from?

Additional, when the specific channel(s) is(are) chosen by writing corresponding bit(s) in control register, is(are) the channel(s) always be active before control register was written in a different value?  Or the channel(s) is (are) just valid for one I2C-access after the control register is configured?

THanks!

Antony

  • 0
    TI__Guru 73991 points

    Hello Anthony,

    "What does "maximum bus capacity" mean?  Where is the constraint from?"

    -Maximum capacitance is the constraint defined by the I2C standard and is dependent on the speed being used. Table 10 of the standard specifies this.

    "Additional, when the specific channel(s) is(are) chosen by writing corresponding bit(s) in control register, is(are) the channel(s) always be active before control register was written in a different value?  Or the channel(s) is (are) just valid for one I2C-access after the control register is configured?"

    Once you write to the register to enable the channel(s), the channel(s) will permanently stay enabled until you either rewrite to the register, if the part is power cycled (turned off and turned back on), or reset.

    For example: Let's say you enable ch 1 and ch 2 then send data through these channels. After the data transfer, the switches still remain enabled at ch 1 and ch 2. You can access these channels an infinity number of times as long as you don't rewrite the switch register or POR.

    Lets say you write back to the switch to enable just ch 0. When you send data through the I2C bus, the data will now only go through ch 0.

    Lastly, I would recommend the TCA9545A. This device is a revision of the PCA9545A and fixes some of the bugs that existed and typically our TCA family provides a wider voltage range.

    Thanks,

    -Bobby

  • 0 in reply to BOBBY
    TI__Genius 13480 points

    Hi Bobby,

    THansk for your comment.  ONe more questin here.  

    Customer's SW engineer would like to know how long of the delay should be inserted in his code to access the I2C device after configuration register is changed to a new value.  

    May I have your comments?

    Thanks!

    Antony

  • 0 in reply to Antony Lin
    TI__Guru 73991 points

    Hey Anthony,

    The device will keep the last byte it was sent and will change to its selected channels after a stop condition is initiated on the I2C bus. The minimum amount of time between a stop and start condition is defined by the I2C standard and depends on the frequency. For 100kHz the minimum time is 4.7uS and for 400kHz is 1.3uS. This can be found on table 10 of the I2C standard document.

    Thanks,

    -Bobby