Page 129 - Understanding Automotive Electronics

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4 MICROCOMPUTER INSTRUMENTATION AND CONTROL

The special memory locations pointed to by the stack pointer are called
stacks; if one considers memory locations as being slots stacked one atop the
other, it makes sense to think of the stack pointer stacking data like plates on a
shelf.
When the subroutine is After the program counter has been incremented and saved, the jump
completed, a return address is loaded into the program counter (step 9). The jump to the
instruction retrieves the subroutine is made, and the CPU starts running the subroutine (step 10).
saved program counter The only thing that distinguishes the subroutine from another part of the
value from the stack program is the way in which it ends. When a subroutine has run to
pointer and loads it into completion, it must allow the CPU to return to the point in the main
the program counter. program from which the jump occurred. In this way, the main program can
Execution of the main continue without missing a step. The return-from-subroutine (RTS)
program then resumes instruction is used to accomplish this. It is decoded by the instruction
from the point at which register, and increments the stack pointer as shown in Figure 4.11, step 1. It
the jump occurred. uses the stack pointer to address the stack memory to retrieve the old
program counter value from the stack (steps 2 and 4). The old program
counter value is loaded into the program counter register (steps 3 and 5),
and execution resumes in the main program (step 6). The return-from-
subroutine instruction works like the jump-to-subroutine instruction,
except in reverse.

EXAMPLE USE OF A MICROCOMPUTER
Let’s look at an example of how a microcomputer might be used to
replace some digital logic, and along the way learn about some more
microcomputer instructions.
Microcomputers can be The digital logic to be replaced in this example is a simple AND gate
used in place of discrete circuit. Now, no one would use a microcomputer to replace only an AND gate,
logic circuits such as because an AND gate costs a fraction of what a microcomputer costs. However,
AND gates. if the system already has a microcomputer in it, the cost of the AND gate could
be eliminated by performing the logical AND function in the computer rather
than with the gate. This is a perfectly legitimate application for a
microcomputer and is something that microcomputers do very well. Moreover,
this example well illustrates the use of a microcomputer.
Suppose there are two signals that must be ANDed together to produce
a third signal. One of the input signals comes from a pressure switch located
under the driver’s seat of an automobile; its purpose is to indicate whether
someone is occupying the seat. This signal will be called A, and it is at logical
high when someone is sitting in the seat. Signal B is developed within a
circuit contained in the seat belt and is at logical high when the driver’s seat
belt is fastened. The output of the AND gate is signal C. It will be at logical
high when someone is sitting in the driver’s seat and has the seat belt
fastened.

116 UNDERSTANDING AUTOMOTIVE ELECTRONICS