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PCA9545A: Total Effective Pull-up resistance when the switch is connected.

Karan Gadhia
Prodigy 10 points
Part Number: PCA9545A


Hi,

I would like to know what would be the effective pull-up resistance value when the switch "activates" and connects the main i2c bus to one of the channel.

The setup I have has the Vcc @ 3.3 V. All the pull-ups are 1.5K pulled to the same 3.3V source.

Something like this:

Would the effective pull-up resistance of the "active" connected bus be 750 Ohms (1.5K || 1.5K)?

Also on the connected bus does the voltage level on one port reflect as it is on the second port? I mean if I gradually increase/decrease the voltage from 0-3.3V should it reflect as is on other port?
If not what are the high-low voltage thresholds for individual ports (master and channelx) that would turn on the other port? (w.r.t my setup values)

Thank you.

  • +1
    Guru 317480 points

    This device is a passive switch and directly connects the SCx/SDx pins together (for low voltages; see figure 9-2 for the limit). So the pull-up resistors of all active channels act in parallel.

    If there is no master on the four channels on the right, their pull-up resistors are needed only when the channel is idle, so they can be made weaker (10K or so).

  • 0
    TI__Guru 74781 points

    Hi Karan,

    Clemen's is correct. 

    The main concern is when you have a multiple channels enabled and effectively have many resistors in parallel. Any device driving the line would need to be able to sink the sums of those currents and also have a VoL low enough to be recognized by devices downstream to be a logic low (VoL needs to be lower than ViL). 

    For that reason, pull up resistors should be sized closer to their maximum pull up resistor values when possible. This app note discusses how to calculate a pull up resistor:

    /cfs-file/__key/communityserver-discussions-components-files/138/I2C-Bus-Pullup-Resistor-Calculation.pdf

    You can treat each channel or segment separate from the other and just lean towards the maximum pull up values to lower VoLs and large IoLs.

    -Bobby