I met a problem when I try to use the SPI (3.3V) to drive the meganetic isolation ADum2400 which is powered by 5V.The SPI pin are remapped to SPI3 (PC10, PC11, PC12) and the CS pin are remapped to PB3 on the STM32F4-Discovery board. The same case occurs when I use USART pin to drive the RS485 interface chipset which is powered by 5V. Because I have to be compatible with the 5V power system for the old product. So could anyone suggest how to use the 3.3V I/O to drive the chipset which is powered by 5V. The configuration of Pin structures are PP and pull down. The test result from oscilloscope are attached. I have tried to use OD with no pull, there is no signal at all. What confused me most is that, the same pin struction configured for SCK, SDO, CS. But different test result. Only SCK works OK. In the attachment:
When I disconnected the SPI with the meganetic isolation, it works well. However, the SPI pin are also connect to the audio codec on discovery board. I doubt but can't understand.