In the M453 series related audio applications, SPI1 or SPI2 are used as I2S function.

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The following figure shows the SPI1 and SPI2 block diagram of the M453 series. The SPI function pins can be Master or Slave.
MOSI (Master Output Slave Input): This pin is the output when the SPI is Master; it is the input when the SPI is the Slave.
MISO (Master Input Slave Output): This pin is the input when the SPI is Master; this pin is the output when the SPI is the Slave.

When SPI1 and SPI2 are used as I²S function, the definition of MOSI and MISO are not applicable. Under the I2S function setting, regardless of whether the I2S is set to I2S Master or Slave, the DI (Data Input) pin is the input of the MISO (I2S_DI) pin, and the DO (Data Output) pin is output from the MOSI (I2S_DO) pin.