55 Ohms between GND and 3.3V in my design

[seridj_mse]

Hi,

I have designed and assembled a board that uses 8 Differential I2C PCA9615 IC with an mux TCA9548APWR but with I2C termination resistors 390-100-390 i have 55 ohms between GND and 3.3 (16 resistor 440 ohms in parallèle ).

so the question is if its normal and i can continue my design or its a big problem and i should solve it and how i can do that ????


Let me know if you see anything in the schematic or layout that might an issue.

Patiently waiting for your input.

Thanks!

SparkFun QwiicBus - EndPoint - COM-16988 - SparkFun Electronics

The SparkFun QwiicBus EndPoint is the fastest and easiest way to extend the range of your I2C communication bus.

www.sparkfun.com

Reply

 

[KlausST]

Hi,

(16 resistor 440 ohms in parallèle ).

I see 18 termination circuits, and I see 880 Ohms each.
* Don´t know how you come to 16 and 440 Ohms (which makes 27.5 Ohms in total).

--> Your values don´t match.

****
Basically if you connect n circuits with x Ohms in parallel, then the resulting resistance is x/n. So yes, this is quite normal.

****
Is it a big problem? I don´t see any. It´s just physics.
What do you want to solve?

You designed the circuit, so you should have known from the beginning. And you will have your reasons for this design. We don´t know them.
****

Let me know if you see anything in the schematic or layout that might an issue.

The schematic is by far not complete.
From a brief look on the snippet, I don´t see any big mistake.

Klaus

 

[seridj_mse]

Hi,


I see 18 termination circuits, and I see 880 Ohms each.
* Don´t know how you come to 16 and 440 Ohms (which makes 27.5 Ohms in total).

--> Your values don´t match.

****
Basically if you connect n circuits with x Ohms in parallel, then the resulting resistance is x/n. So yes, this is quite normal.

****
Is it a big problem? I don´t see any. It´s just physics.
What do you want to solve?

You designed the circuit, so you should have known from the beginning. And you will have your reasons for this design. We don´t know them.
****

The schematic is by far not complete.
From a brief look on the snippet, I don´t see any big mistake.

Klaus

hi, thanks for ur replay , for termination circuits its about only second pictures ,bcs the first one is just the all design for module Sparfun but its not a problem , yes i know its just physique and its logic but i dont know mybe its not design to use in parallel or something like this , i want just to be sure its ok , i think its ok thanks

 

[FvM]

If you have more than two PCA9615 in a multi-drop topology, termination resistors are only placed at both end positions, quite similar to a RS-485 bus.

If you have 8 separate differential I2C connections, each needs SCL and SDA termination networks at both ends. Overall quiescent current will be respectively high.

Are you sure that your application needs this technique?

--- Updated Apr 15, 2021 ---

PCA9615 datasheet suggests 120 and 600 ohm resistors which already reduces quiescent current by 1/3.