I have a question regarding SPI communications. I feel like I have a good understanding fundamentally of how SPI works. However, I'm often confused when implementing the slave select line of SPI.
Is the slave select line on a microcontroller, in general, enhanced via hardware? That is, is there anything different between a slave select output and an output controlling an LED? Do microcontrollers allow certain I/O to be toggled faster when used for SPI?
Best Answer
No difference, as far as drive strength or transition speed are concerned.
Some uC's I've worked with don't even have a dedicated SS pin. You can implement it in code using whichever pin is convenient.
However, some microcontrollers will toggle the SS line for you (without you having to toggle the pin in code). This can reduce the dwell time between SPI transactions, decreasing the total time elapsed during multiple-transaction transmissions.
Also, if you are designing an SPI slave device, it is very convenient to use a uC that has a dedicated SS pin, which is used by the uC's internal SPI module.
The STM32F1 ARM-based microcontrollers, for example, have a dedicated SS pin for each of their SPI busses, with the option to disable the SS functionality and free up the pin for general use.