2021-02-03 03:27 AM
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?
2021-02-03 03:29 AM
Is your SPI clk slow enough to fight against tau = R(Pullup) * C(parasitics)?
2021-02-03 03:34 AM
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.
2021-02-03 03:37 AM
2021-02-03 03:40 AM
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.
2021-02-03 03:55 AM
Are you driving SCK and MOSI as Open-Drain?
2021-02-03 04:18 AM
No, they are driven as push/pull. Does this cause problems?
2021-02-03 04:21 AM
I use push-pull configuration.
2021-02-03 04:40 AM
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.
2021-02-03 04:58 AM
OK, thanks. Open drain than should actually be possible?