TCA9548A: Enable 4 channel at same time for slave device, how to design pull up at slave channel?

Hello James,

Each channel should have a pull up resistor. When the channels are enabled at the same time you need to be mindful of the capacitance, since I2C has a bus capacitance limit of 400pF. For finding the right pull up resistors value, please check out our app note.

Thanks,

Nir 

Hi Nir, 

Thanks for your response. When we enable 4 channel at the same time, SC/SD [0:3] each channel have external pull up 2.2K ohm. But we measured the waveform, SC/SD output Vol rise to 800mV. We try to keep a pull up 2.2 ohm at SC/SD0, SC/SD[1:3] removed pull up, Vol can be drop to 400mV.

My question, when all channel enable at the same time and each channel have pull up, why does Vol rise up? I do not understand, would you help to explain this behavior?   

Thanks,

James 

Hello James,

Can you please share a schematic? 

Thanks,

Nir 

Hi Nir,  I try to insert schematic to E2E in TI, but it can't insert successful. It seem no allow in our rule, is it possible through e-mail send schematic to you? 

Thanks,

James

Hello James,

Yes, you can email it to me. 

n-gilgur@ti.com

Thanks,

Nir 

Hello James,

Thank you for sharing the schematic, I see no issues with it. 

Enabling all 4 channels at once results in a spike of parasitic capacitance which may cause the issue you are seeing.

Does Vol rises and settles at 800mV, or it shows up as a spike?

Also, can you please share the waveform you are referencing showing the voltage rising?

Thanks,

Nir