As far as I can tell (SPI or I2C) and USART functions are
independent on the F874. There are 5 PortC pins for these
comms.

RC3 is the clock line for SPI or I2C
RC4 is Data In for SPI or bi-directional data for I2C
RC5 is Data Out for SPI
RC6 is USART Tx
RC7 is USART Rx

I can't (yet) see any reason why the USART can't be running
at the same time as either SPI or I2C (you have to choose
one as SPI and I2C share pins)

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