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P82B715: How many are I2C devices supported?

Part Number: P82B715

Hi Experts,

I want to know how many I2C devices can be supported with P82B715. Below is the example in the datasheet and three devices are used.

The total bus capacitance is defined around 3000pF on the datasheet. Does this mean that it is Ok to just estimate the cable capacitance and connector capacitance ,etc and we do not need to limit the number of P28B715?

Regards,

Uchikoshi

  • Uchikoshi,

    Thanks for reaching out! There are two main considerations when determining how many P82B715 devices you can add in a star/multipoint configuration.

    One limitation includes what you have mentioned: the number of P82B715 devices connected in a star or multipoint architecture by their Lx/Ly ports is limited by the total capacitance of the buffered bus system, which cannot exceed 3000 pF.

    If you are intending to approach this limit by adding numerous P82B715s in this type of connection, you should either determine the approximate maximum capacitance of your setup via robust testing and/or simulation, or you should approximate the total capacitance of the system. One useful approximation is that each device connected to the buffered bus presents 10 pF of load capacitance and 10 pF of trace capacitance. This is in addition to capacitance added by cables, with Cat5 cables adding about 50 pF/m, theoretically allowing a cable length of up to 60 meters if all other capacitance is ignored. (This capacitance value is also important for determining the value(s) of the pull-up resistor(s) applied to the system. See the P82B715 datasheet section 9.2.2.2 for more information on this.) A natural consequence of your question is that, as the number of devices connected in star/multipoint architecture increases, the maximum cable length for the system decreases. Additionally, propagation delays across the line may limit performance to below 400 kHz.

    The second important consideration that is likely to limit the number of these devices is current sinking. Each device added to the architecture adds additional current that the master/slave devices must sink. The P82B715 does not redrive the I2C signal, so an increased current draw along the cable (Lx/Ly) side also results in current draw on the I2C (Sx/Sy) side (and this also results in cable noise propagating onto the board’s I2C lines). This figure shows a simplified model of how current splits between the cable side and board side:

    Note that 10% of the ILx current passes onto the I2C bus through Sx (and same for ILy and Sy) due to the configuration of the strong pull-down. Each time you add a P82B715, you must add a board-side I2C pull-up resistor to the system. These are in parallel to each other, and adding more causes increased current flow. As a design consideration, you are limited based on the maximum IOL current ratings of your master and slave devices. The maximum rating for the P82B715 itself is 60 mA, indicating a potential ISx of 6 mA.

    Regards,

    Danny Bacic