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通过将NSS配置为GPIO，在通过SPI操作从设备时，就可以通过软件来选中和释放从设备了。虽然比起硬件自动置位要麻烦，但问题毕竟解决了。 进一步阅读长达900页的Manual，我发现，文中对于SPI hard模式的描述并非大多数人所认为的硬件SPI，原文如下： Slave select (NSS) pin management There are two NSS modes: ● Software NSS mode: this mode is enabled by setting the SSM bit in the SPI_CR1 register (see Figure 209). In this mode, the external NSS pin is free for other application uses and the internal NSS signal level is driven by writing to the SSI bit in the SPI_CR1 register. ● Hardware NSS mode: there are two cases: – NSS output is enabled: when the STM32F20xxx is operating as a Master and the NSS output is enabled through the SSOE bit in the SPI_CR2 register, the NSS pin is driven low and all the NSS pins of devices connected to the Master NSS pin see a low level and become slaves when they are configured in NSS hardware mode. When an SPI wants to broadcast a message, it has to pull NSS low to inform all others that there is now a master for the bus. If it fails to pull NSS low, this means that there is another master communicating, and a Hard Fault error occurs. – NSS output is disabled: the multimaster capability is allowed. 当SPI配置为hard模式后，通过检测NSS可以实现的是自身主机和从机模式的切换，而不是大多数人所认为的自动NSS。。。也就是说：在一个多SPI系统中，STM32 SPI通过NSS检测，一旦发现系统中无NSS低信号，自己就输出低，从而成为主机；当系统中有NSS低信号时（及已经有其它SPI宣布为主机），自己就配置为从机。 所谓的hard模式的NSS，实际就是为了实现多机间通信的。 小结： 望文生义很可怕，Manual要仔细研读。 STM32的SPI NSS不论是配置为soft还是hard都是无法自动置位的，但这却是大多数应用所需要的。正如ST 论坛上RichardE所说：“Everything would be done by the peripheral. Fire and forget.”
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