Page 145 - Understanding Automotive Electronics

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4 MICROCOMPUTER INSTRUMENTATION AND CONTROL

Interrupts

Interrupts cause the An efficient alternative to polling uses control circuitry called an
CPU to jump to a spe- interrupt. An interrupt is an electrical signal that is generated outside of the
cific location in the pro- CPU and is connected to an input on the CPU. The interrupt causes the
gram. By signaling the CPU to temporarily discontinue the program execution and to perform some
microprocessor for ser- operation on data coming from an external device. A slow ADC, for instance,
vice only when needed, could use an interrupt line to tell the processor when it is finished converting.
interrupts are more effi- When an interrupt occurs, the processor automatically jumps to a designated
cient than polling. program location and executes the interrupt service subroutine. For the
ADC, this would be a subroutine to read in the conversion result. When the
interrupt subroutine is done, the computer returns to the place where it left
off in the program as if nothing had happened. (Recall the previous
discussion on the jump-to-subroutine instruction.) Interrupts reduce the
amount of time the computer spends dealing with the various peripheral
devices.
Another important use for interrupts is in timekeeping. Suppose that a
system is being used that requires things to be done at particular times; for
instance, sampling an analog signal is a timed process. A special component
called a timer could be used. A timer is a device that works like a digital
watch. A square-wave clock signal is counted in counter registers like the one
discussed in Chapter 3. The timer can be programmed to turn on the
interrupt line when it reaches a certain count and then reset itself (start over).
It may be inside the CPU itself or it may be contained in peripheral devices in
the microcomputer system. Timers have many automotive applications (as
shown later).
Such a technique is sometimes used to trigger the output of a new
number to a DAC at regular intervals. The microcomputer simply programs
the timer for the desired amount of time by presetting the counter to some
starting value other than zero. Each time the timer counts out the programmed
number of pulses, it interrupts the computer. The interrupt service subroutine
then gets the new binary number that has been put into memory by the
microcomputer and transfers this number to the DAC data latches at the input
to the DAC.
Vectored Interrupts
All of the interrupt activity is completely invisible to the program that
gets interrupted. In other words, the interrupted program doesn’t know it was
interrupted because its execution continues without program modification
with minimum delay. Interrupts allow the computer to handle two or more
things almost simultaneously. In some systems, one interrupt line may be
used by more than one device. For instance, two or more ADCs may use the
same interrupt line to indicate when either is ready. In this case, the computer
doesn’t know which device caused the interrupt. The computer could poll all

132 UNDERSTANDING AUTOMOTIVE ELECTRONICS