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LSF0108: i2c signal degradation

teboul mickael
Prodigy 100 points
Part Number: LSF0108
Other Parts Discussed in Thread: BQ4050, TCA9801

Tool/software:

I connected between SMARC module (IMX8 anao) and several i2c periferals the levelshifter LSF0108 to confert 1.8V to 3.3V:

There is 470k pullup on the IMX8 1.8V side (internal pullup).

If I put 10k pullup on 3.3V side, the signal is very bad:

And if I change pullup from 10k to 2.2k the signal seams better but the BQ4050 doesn't work...

Why the signal is very bad with 10k pullup ?

Is there too many devices on the bus ?

What is the best way to use the LSF0108 ?

  • +1
    Guru 277140 points

    Rising edges are slowed down by the low-pass filter formed by the pull-up resistors and the trace/device capacitances. (A rough estimate of the capacitance would be 10 pF per device and 1 pF per cm of trace.)

    You have to adjust the pull-up resistors so that the waveforms turn out right. Faster rising edges result in a larger current through the pull-ups, and some devices might not be able to sink that current without a large increase of VOL. (I'd guess that the 1.8 V device has the weakest outputs, but the bq4050's outputs are specified only for 1.5 mA.) Please note that the LSF is a passive switch, so the pull-up resistors on both sides act in parallel.

    You would be able to improve the signal quality with an I²C buffer like the TCA9801.