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MICROCOMPUTER INSTRUMENTATION AND CONTROL 4

the devices each time an interrupt occurs to see which one needs service, but
as discussed, polling may waste time. A better way is to use vectored
interrupts.
Vectored interrupts tell In computer parlance, a vector is a memory location that contains another
the CPU which specific address that locates data or an instruction. It may be a specific memory location
device needs service, and that contains the address of the first instruction of a subroutine to service an
also may implement a interrupt; it may be a register that contains the same type address. In this
priority of service specific case, an interrupt vector is a register that peripherals use to tell the
scheme. Vectored inter- processor which device interrupted it. When a peripheral causes an interrupt, it
rupts allow a microcom- writes a code into the interrupt vector register so that the processor can tell
puter to handle a which device interrupted it by reading the code. The decoder for an interrupt
number of different vector usually includes circuitry that allows each device to be assigned a
tasks quickly. different interrupt priority. If two devices interrupt at the same time, the
processor will service the most important one first.
The vectored interrupt enables the microcomputer to efficiently handle
the peripheral devices connected to it and to service the interrupts rapidly.
Interrupts allow the processor to respond to things happening in peripheral
devices without having to constantly monitor the interfaces. They enable the
microcomputer to handle many different tasks and to keep track of all of them.
A microcomputer system designed to use interrupts is called a real-time
computing system because it rapidly responds to peripherals as soon as requests
occur. Such real-time systems are used in digital instrumentation and control
systems.

MICROCOMPUTER APPLICATIONS IN AUTOMOTIVE SYSTEMS

There is a great variety of applications of microprocessors in automobiles.
As will be explained in later chapters of this book, microprocessors find
applications in engine and driveline control, instrumentation, ride control,
antilock braking and other safety devices, entertainment, heating/air
conditioning control, automatic seat position control, and many other
systems. In each of these applications, the microprocessor serves as the
functional core of what can properly be called a special-purpose
microcomputer.
Although these applications are widely varied in operation, the essential
configuration (or architecture) has much in common for all applications. Figure
4.21 is a simplified block diagram depicting the various components of each of
the automotive systems having the applications listed previously. In this block
diagram, the microprocessor is denoted MPU. It is connected to the other
components by means of three buses: address bus (AB), data bus (DB), and
control bus (CB). Each bus consists of a set of wires over which binary
electrical signals are transmitted. By way of illustration, in early automotive
application, the DB consists of 8 wires, the AB is typically 8 to 16 wires, and
the control bus is a set of 3 or 4 wires.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 133