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

FHopp.1
Associate III

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?

11 REPLIES 11
Uwe Bonnes
Principal III

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

FHopp.1
Associate III

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
Associate III

0693W000007Dl03QAC.jpg

FHopp.1
Associate III

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
Principal III

Are you driving SCK and MOSI as Open-Drain?

FHopp.1
Associate III

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

FHopp.1
Associate III

I use push-pull configuration.

Uwe Bonnes
Principal III

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
Associate III

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