LSM6DS3 pin capacitance

GTS.1 · ‎2020-08-04

Hi,

I am using the LSM6DS3 with I2C at 400 kHZ. I would like to know the pin capacitance value so as to calculate the value of the required pull up resistors. I was unable to find this in the data sheet.

Thanks

Peter BENSCH · ‎2020-08-05

Well, the I2C specification gives some hints how the pull-up resistors depend on speed, capacitance, vcc etc. So for 400kHz and some 100pF for the pins plus your traces (except they are a lot longer than about 30cm) you should be fine with 4.7k. Depending on the power budget of the whole system you could also go even lower, e.g. down to 3.3k...1.5k, but of course this increases the total power consumption at smaller values.
It might be also a good idea to check the I2C signal quality with a scope.
Good luck!
/Peter

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Peter BENSCH · ‎2020-08-05

You are probably talking about the capacitance of the I2C pins, right?

Well, that usually doesn't matter with the LSM6DS3, as the capacitance of the traces/wires are a bigger contributor to the signal quality when running at 400kHz.

Have you also checked the appnote AN4650, which also gives you some recommendations?

Good luck!

/Peter

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GTS.1 · ‎2020-08-05

Hi Peter,

I took a look at the Appnote for which you have provided the link. The recommendation given is for the SDx and SCx pins when using it in Mode 2. But I'm trying to find a pull up resistance value to use with the SDA and SCL pins. I had tried the LSM6DS3 with a microcontroller and I was getting zeroes on the I2C lines till I added 4.7k pull up resistors. The 4.7k was something I had in hand and just tried it. I need a more concrete/calculated value as this would be used on a custom board.

Thanks

Peter BENSCH · ‎2020-08-05

Well, the I2C specification gives some hints how the pull-up resistors depend on speed, capacitance, vcc etc. So for 400kHz and some 100pF for the pins plus your traces (except they are a lot longer than about 30cm) you should be fine with 4.7k. Depending on the power budget of the whole system you could also go even lower, e.g. down to 3.3k...1.5k, but of course this increases the total power consumption at smaller values.
It might be also a good idea to check the I2C signal quality with a scope.
Good luck!
/Peter

In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.