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Mux with PCA9515B

Stefano Cumia
Prodigy 10 points
Other Parts Discussed in Thread: PCA9515B

HI,

we are using the chip PCA9515B (as you can see on the enclosed file).

We've to do a mux for I2C.

U13 and U8 (AND ports) are useful to disable and/or enable the right i2c path (when I say path I mean comunication from gpioxx_x to M24C04 memory).

Considering that every SCL1 and SDA1 is shorted with M24C04 lines the question are:

1) Is the setup corrected? 

2) Do I have to enable just one PCA9515B at time or do I have to keep every PCA9515B enabled?

I found that in case we keep enabled just one of the eight PCA9515B the gpioxx_x  signal for SCL seems to experiment an high capacitance load (high rise time) while gpioxx_x signal for SDA seems to be stuck at one

Thanks and best regards.

Stefano Cumia

i2c.pdf

  • 0
    TI__Genius 16430 points

    Hello Stefano,

    Looking at your schematic, I believe your pull up resistors might be too high in resistance since it appears that you have several of these buffers on the master I2C line.

    Please take a look at our App Note: I2C Bus Pullup Resistor Calculation

    Also, when you toggle these enables, please make sure you do not enter a condition as described in the datasheet under the description:

    The PCA9515B has an active-high enable (EN) input with an internal pull-up. This allows users to select when
    the repeater is active and isolate malfunctioning slaves on power-up reset. States should never be changed
    during an I2C operation. Disabling during a bus operation will hang the bus and enabling part way through a bus
    cycle may confuse the I2C parts being enabled. The EN input should only change state when the global bus and
    the repeater port are in an idle state to prevent system failures.

     

     

    Please note that the I2C bus has a 400 pF limit. While I do not believe you are violating this (the input capacitance of each PCA device is 7pF typ), please make sure that there are no other devices on the trace that might contribute to an excessive amount of capacitance.