TCA9548A: Where to place pullup resistors

Matthew Olson

Part Number: TCA9548A

Tool/software:

Hello,

I am using the TCA9548 to communicate with "slave" boards that are to be connected to the "master" board via up to 3 foot long cables. The master PCB has the TCA9548 connecting to slave boards using the I2C for communication. The question is where is the best place to locate the pullup resistors. On the master board or on the slave board? Thanks for the help.

Matt

1 month ago

0 Kameron Hill 1 month ago

TI__Mastermind 24440 points

Hi Matt

With the TAC9548A in the design I think you need to place pull up resistors near the slave and master device. ( Basically put pull up resistors on both sides of the switch)

Put a pair on the master side for the main SDA and SCL and other pairs on the desired downstream channels connected to the slave devices

Here is a app note that gives a overall idea of a i2c switch on a bus.

https://www.ti.com/lit/an/slva704/slva704.pdf?ts=1753381288943&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTCA9548A%253FkeyMatch%253DTCA9548A%2526tisearch%253Duniversal_search%2526usecase%253DGPN

Please let me know if you have any other questions though

Regards,

Kameron

0 Matthew Olson 1 month ago in reply to Kameron Hill

Prodigy 10 points

Hi Kameron,

Thanks for the speedy reply. I don't think I worded the question properly. What I have is the secondary side of the TCA9548 (the one with 8 I2C lines) running to devices via a cable (up to 3 feet away). I would like to know if the pullup resistors should go on the board with the TCA9548 or the remote device board . See drawing , in either position A (master board), or position B (slave board.

Thanks,

Matt

+1 Kameron Hill 1 month ago in reply to Matthew Olson

TI__Mastermind 24440 points

Hi Matt

Thanks for the additional detail on your question.

I would recommended to place the pull up resistors in position B( on slave board)

This is because if you place the pull up resistors closer to the slave device the voltage at the slave pin rise faster because the pull up is directly driving the local capacitance at that end.

Conversely if the pull up was placed at the TCA9548A sides the Logic High signals has to "charge" the entire cable length which slows rise time at the slave.

Please let me know if you have any other questions and I can try to help!

Regards,

Kameron

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TCA9548A: Where to place pullup resistors