Microcomputer Circuits and Processes

to the digit select. A very short program has to be written to scan across
all of the digits in the display, making sure each gets its correct number.
But that is cheaper than the previous suggestion.
APPLICATION NOTE - KEYBOARD SCANNING AND INTERFACE
A keyboard may be a typewriter-type board just like the one on a
personal computer, it could be a music synthesizer's 4- or 5-octave
keyboard, or it could be a small 16-key pad. To take this last example, it
will have 16 switches and need the computer to determine the keys
pressed. Each key is given a number, here 0 to 15, and when a key is
pressed, say 3, the computer has to save the number 3 in one of its
registers or in memory.
The arrangement is shown in figure 4.15. The computer sends a 4-bit
number (0 to 15) via an output buffer to a decoder, just like the ones
discussed in Chapter 2.
4-bit numbers
from output
port
key s't WI Ches
- 0 - -
- 1 - -~
•.
2 - ~
DECODER
3 '-'
·
·• 1-- •••
15 ~ ~
common line
to CPU
-;.
signals any key
is pressed
Figure 4.15
A decoder chip is used to scan a keyboard made of 16 key switches.
Remember, a decoder receives a 4-bit number, and then makes one
of its 16 output lines go high. If the processor has output 0011 (3 10 ),
then the decoder makes its output 3 high, which is connected to key
switch 3. Notice that all the key switch outputs are connected together.
If key 3 is pressed then the high gets through to this common line. If 4 is
pressed the high does not get through. A high on the common line
means number 3. The common line is connected directly to the
microprocessor, so that the microprocessor knows that a key has been
pressed, and stops. Now the microprocessor has been programmed to
output not just 3, but all numbers 0 to 15 in order, over and over. So if
key 3 is pressed continually then the common line will remain low until
the processor has got round to outputting 0011. Then it will go high,
and the microprocessor will be stopped. Its last output, 0011, will be in
some register which can be got at and so used in another part of the
program to implement whatever pressing key 3 is meant to do.
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