7.4 Printing in Interrupts

Printf is a slow function to execute for a couple of reasons. The simplest explanation is the serial nature of this function – for a string such as “Hello World”, each character needs to be sent to the UART hardware. Some devices have internal First-In First-Out (FIFO) structures that allow a few characters to be queued at a time, but these structures are limited in size. Once the buffer (FIFO or not) is full, the printf must wait for the buffer to have space for the next character before processing the next.

Because of the speed of printf, it is poor practice to print within an interrupt handler. When the microcontroller is in the interrupt handler, it can't execute its ordinary operations or handle other interrupts (of the same or lower priority). It is better practice to set a flag within an interrupt and perform the print elsewhere.