STM32 with SPI 1.8V connected to 3.5V MAX31865 cuases not working communication

[FHopp.1]

I have an 1.8V STM32 and an 3.5V MAX31865 which are connected via SPI, I used 10k Ohms pullups on data line to 3.5V. Now the problem is that communication dosen't work. When I increas STM32 voltage to 3.5 V everything is fine. Is there an limitation in logic high on STM32 side?

[Uwe Bonnes]

Is your SPI clk slow enough to fight against tau = R(Pullup) * C(parasitics)?

[FHopp.1]

Thanks for your fast answer. Hm, right now i uses 1000k as clock speed. I have no test points on schematic to check this with an oscillosope easily.

I can reduce clock speed. I will try.

[FHopp.1]

[FHopp.1]

I tried 1k 10k and 100k as clock rate, but all these combinations didn't work. If I lower the difference between STM32 and MAX31865 to 1.5V it works fine.

[Uwe Bonnes]

Are you driving SCK and MOSI as Open-Drain?

[FHopp.1]

No, they are driven as push/pull. Does this cause problems?

[FHopp.1]

I use push-pull configuration.

[Uwe Bonnes]

Of course, the pull up transistor to 1.8. Volt will win against the pull up to 3.5. Volt and so SCK/MOSI will not reach a level that the device will recognize as high.

[FHopp.1]

Yes, that was the reason. If i set down voltage from 3.5 to 3.0 its working

[FHopp.1]

OK, thanks. Open drain than should actually be possible?

[Uwe Bonnes]

On register level it is possible. Wheter Cube exposes this interface, I do not know.