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TCA9803: pull up resistor value for tca9803

Tanveer Ahmed
Prodigy 80 points

Part Number: TCA9803

Hello,

I am using Lantronix QSOM Open-Q 610 uSOM I2C which is at 1.8V. To convert this 1.8V I2C to 3.3V and 2.5V, I am using two TCA9803 ICs.

To a single I2C Rail, I have connected two TCA9803 ICs (one for level shifting 1.8V to 3.3V and the other for level shifting 1.8V to 2.5V).
For the 3.3V, I have a total of four slaves (at the B-Side without any pull ups); and for the 2.5V, only one slave is connected (at the B-Side without any pull ups).
I am using pull ups only on the input side as mentioned in "Figure 31. Parallel Use Case" of TCA9803 Datasheet.
What should be the pull resistor value considering 200pF Bus capacitance?
Currently, I am using 1.8k (and tried 1k also), but the I2C is not detecting.

Shall I use the 4.7k as standard pull up Resistors (as mentioned in the datasheet images as 5k OHM)?
Also, please find attached screenshot from my schematics.I2C_Detect_Screenshot.pngI2C_Level_Shifter_TCA9803.png

  • 0
    TI__Guru 55808 points

    Hi Tanveer,

    I have an application note that helps to calculate the correct PU resistor sizing for the I2C bus here.

    Your circuit looks fine. Even with 1k, 1.8k, PU resistors, that should be enough for a 200pF load. 

    I know I2C-detect is used to find all I2C devices on the bus, or at least this is my understanding. I.e. send out all available I2C addresses to detect what devices are currently on the bus. 

    What device are you attempting to detect? 

    Do you have an o-scope capture of pins 2/3/7/6 of the TCA9803? 

    Regards,

    Tyler