Page 340 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12
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11
Input/Output
An input routine is a program segment that inputs words from the outside world into
the computer, and an output routine is a program segment that does the reverse. It
outputs words from the computer to the outside world. Clearly, a computer that does not
have input and output routines, and the hardware to carry out these routines, would be
useless regardless of its power to invert matrices or manipulate great quantities of data.
Until now, we have implied that you should avoid knowing the details of these routines.
Even though we have left the discussion of input and output until near the end of this
book, it is really simple and should pose no problem to the reader.
In this chapter, we first describe how the basic input and output operations are
implemented in hardware and executed in software, using simple ports available in both
the 6812 'A4 and 'B32. We then discuss the use of buffers in input and output. To
describe synchronization, we describe a greatly simplified timer interrupt mechanism,
which is also available in both the 6812 'A4 and 'B32. We then discuss gadfly and
interrupt mechanisms. Finally, we introduce D-to-A and A-to-D converters.
Upon concluding this chapter, you should understand how basic input and output
operations are performed and be able to read and write input and output routines that use
simple synchronization mechanisms.
11.1 Input and Output Devices
In this section, we introduce a simplified hardware model used to understand input-output
routines. We also discuss simple input and output ports to provide enough background
for the later sections of this chapter.
Figure 11.1. Simplified Diagram of a Microcomputer (Identical to Figure 1.1)
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