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P1 block diagram MICRO_SPI1_SS pin name needs a note added for clarification
this image: https://docs.particle.io/assets/images/p1-pinout1.png
Notice how the SPI_SS starts on PA4, when it should be for PC2.
The way that SPI is currently pinned out is correct, we do not use the PA4 SS pin in an hardware SPI driven sense. SS is only user controlled as a GPIO, thus the default one is A2 (PC2) https://docs.particle.io/reference/firmware/photon/#spi
Yes, we list out the P1 module pin names... but don't necessarily need to use MICRO_SPI1_SS in a hardware sense, but the DAC1 function of that pin was thought to be more important to route to A6/DAC pin at the time. Rather than obfuscating the P1 module pin name, we chose to leave it as-is. It's a bit confusing understandably, so perhaps we need to add a note to the P1 datasheet to help clarify this.
The location of SPI1_SS on A2 is done that way to be pin compatible with the Spark Core.
For P1 products, I suppose you could wire up PA4 as your SPI SS pin to be used in code as SPI.begin(DAC) or SPI.begin(A6) or SPI.begin(DAC1), but it would just use that pin as a GPIO, and not SPI hardware SS control. However you would not be able to use the DAC1 anymore for your product if you needed it.
