I2C pullup resistors with the audio board and other devices.

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alialiali

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Hi everyone, sorry as usual for my cluelessness.

I am adding the Audio board to a design which currently uses I2C with 4.7k pull up resistors on each channel. I noticed the Audio board also has pull ups of 2.2k on these channels.

Using science(?!), I see that the combined resistors in parallel give me

1/R = 1/R1 + 1/R2

1/R = 1/2200 + 1/4700

and R at around 1499.

Will this be enough, or should I change my existing resistors to match the total 4.7k? Then maths fails me, because I can only see how a negative value would work.

Thanks as always for your help
 
4.7k is a usual value for 5V systems. The Teensy is a 3.3V system which should normally be happy with 2k2. Thus, you could even remove the 4k7 from the other circuit. There is no general rule about the pull-up resistor value in I2C systems, only rules of thumb since the signal quality depends not only on these resistor values but also of the parasitic inductance and capacitance of the wiring and the connected ICs. Under normal conditions (short wiring and/or boards stacked) and I2C speed at 400kHz, you should be fine with either 2k2 alone or 2k2 in parallel with 4k7.

The classic way of engineering would be to build everything together and to check the SDA and SCL lines with an oscilloscope. When you see that the pulses are malformed, you'd deduce from the kind of malformation if you need to increase or to decrease the resistor values and try again.
 
Thanks for the complete information. Very helpful.

The audio board is pluggable as an extra in my design so I should be able to operate without it. I'll try them in parallel and see how it works.
 
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