Microcomputer Circuits and Processes

Where exactly is this common line connected to the processor? An
obvious place would be one of the input lines. The program would
instruct the processor to look at this line from time to time. If it is low,
then no key is pressed. If it is high, then some key has been pressed. The
key number is the last number the processor has outputted.
Most processors have a special sort of input called an
INTERRUPT, which could have been used to hook up to the common
line. When the interrupt goes high, the program jumps to another
subprogram which 'services' the interrupt. Here the servicing would
involve getting the last number outputted from the register.
APPLICATION NOTE - DIGITAL-TO-ANALOGUE CONVERSION
You may wish to set up the computer so that it can control smoothly
the speed of a motor, the brightness of a light, or the position of a
drawing pen. In all of these applications, a smoothly varying voltage,
say between 0 and 10, is needed from a computer that has been designed
to work on logic levels 0 and 1 of 0 V and 5 V respectively. Some sort of
conversion is needed. Consider a digital-to-analogue converter which
takes in a 4-bit digital number and gives an output voltage between 0
and 15. You will need to know how an operational amplifier works,
adding up voltages at its input. Look at figure 4.16. Here an op-amp is
shown adding three input voltages.
Figure 4.16
A reminder of how op-amps can add several voltages. Note how the resistances R a to R c
determine by how much each voltage is multiplied before the addition.
ov
Note how the values of the resistors in each input determine by what
amount that input voltage is multiplied. If resistors are used to form a
binary sequence R, 2R, 4R, 8R and connected to, say, a 1V supply via
switches, then closing the switches in binary sequence will give an
output voltage that will rise by equal increments. Figure 4.17 shows the
circuit and a table of the output voltages for each binary ipput from
0000 to 1111.
Of course, the switching-is done by logic circuits which could be put
together with the resistors and op-amp on a single chip and which could
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