I have an application where I'd like the SPI slave to send an interrupt to the master. The hardware is fixed, and there is no way to add a separate pin for the interrupt.
So what I wonder is if there is a conventional way of sending an interrupt from SPI slave to SPI master using only the four SPI pins.
I have started work on defining such a transport protocol, but before I dive into the code I'd like to know if something like this exists as a standard.
If it doesn't I'll be happy to share my thoughts and code.