Page 162 - Embedded Microprocessor Systems Real World Design

Interrupts in Embedded Systems 5

I have deliberately left discussion of interrupts out of the preceding chapters, treat-
ing interrupt code as a “black box” that produces certain outputs. Part of the reason
for the brevity of Chapter 4 is that the real-time aspect of an embedded system often
depends on interrupts; such software is covered here. Interrupts in a real-time
system require a tight relationship between software and hardware.

Interrupt Basics

An interrupt is an input to a microprocessor that temporarily redirects the program
flow. An interrupt can now the processor when an analog-todigital converter
(ADC) has new data, when a timer rolls over, when a direct memory access (DMA)
transfer is complete, when another processor wants to communicate, or when
almost any asynchronous event happens. The interrupt hardware is initialized and
programmed by the system software. When an interrupt is acknowledged, that
process is performed by hardware internal to the processor and the interrupt
controller integrated circuit (IC) (if any). Figure 5.1 shows the sequence of events
performed by the hardware and software in response to an interrupt. When an
interrupt occurs, the on-chip hardware performs the following functions:

It saves the program counter (the address the processor was executing when the
interrupt occurred) on the stack. Some processors save other information as well,
such as register contents.
It executes an interrupt acknowledge cycle to get a vector from the interrupting
peripheral, depending on the processor and the specific type of interrupt.
It branches to a predetermined address specific to that particular interrupt.
The destination address is the interrupt service routine (ISR, or sometimes ISP for
interrupt service process). The ISR performs whatever functions are required and

143