Not clear why the build in LED shares the pin with the CPI clock. Are there any concerns about using this pin as SCLK?
Are there any concerns about using this pin as SCLK?
No because there is a buffer between the pin and the LED, so there is no interaction.
Earlier Arduinos did not have this and very occasionally this lead to problems of not getting enough voltage on the clock pin.
The buffer was added to the R3 versions of the Uno and Mega (pin 13 of Mega is not SCK, but it's still somewhat relevant to this discussion), but on the Nano, Pro Mini, and LilyPad the LED is still connected directly to pin 13 via a resistor.
Thanks.
It's actually rather useful, as the LED indicates when a SPI transfer is occurring.