In SPI protocol, how many maximum slaves can I connect to master(nucleo STM32F103RB)?
Basically, SPI was intended as point-to-point protocol, i.e. one master, one slave.
You can add more slaves by using more or less elaborate Slave-Select mechanisms, that selects one slave out of <n>.
However, when using software slave select, you might run into trouble using interrupts or DMA for transfers.
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