Can Serial be safely used in an SPI interrupt routine I have anecdotal experience that using Serial.print inside a slave's SPI interrupt routine (ISR) causes issues with transmission. It also seems that interrupts may be called more than once before the main loop can react, so it is difficult to print info from all interrupts. Is this because Serial is inherently unsafe, or too slow? Can Serial be used safely inside the ISR routine?
Serial should be avoided in ISRs because interrupts are disabled inside ISRs, and Serial transmission uses interrupts to operate. Better to have the ISR set a flag that is read in the next iteration of loop() to print the desired info.