Usage of SPI inside an ISR

Question

I learned, that SPI-code uses interrupts. So is it true then, that I can't use SPI related code inside an ISR?

Background: I want to capture one or more revolution speeds with an MCP23S17. I rewired the interrupt lines to INT1 and INT2 accordingly to get the interrupts at the Arduino.

I thought it was good to fetch the port registers from the MCP23S17 inside the ISR, but this apparently doesn't work.

Is the only thing I can do in my ISR set a flag for another test in the main loop? And let a routine called from lood() then poll the registers from the port expander? This opposes the idea to fetch the relevant data which issued the interrupt ASAP.

• Ignacio Vazquez-Abrams: This is why you don't use port expanders for timing-sensitive applications.
• Ariser: And instead of port expanders i shall use what?
• Majenko: Real IO pins on the microcontroller.
• Nick Gammon: Please post your code. Majenko is perfectly correct. SPI does not use interrupts, at least during SPI.transfer(). I learned, that SPI-code uses interrupts. - where did you learn that?
• Nick Gammon: apparently doesn't work. - I hate that expression. What happened? What did you expect to happen? In what way did they differ?

Answer 1

SPI doesn't use interrupts. It references the interrupt flag to know if a transfer has been completed, but it doesn't actually have interrupts enabled:

SPDR = data;
asm volatile("nop");
while (!(SPSR & _BV(SPIF))) ; // wait
return SPDR;

It should be perfectly possible to use SPI within the interrupt - indeed there is portions of the SPI API that deal specifically with this - see https://www.arduino.cc/en/Reference/SPIusingInterrupt for instance.

• Ariser: Ok, I missed that. Then there must be another thing I apparently have botched...