Microcomputer Circuits and Processes

That is the principle of the converter. The commercial DAC 08 chip
from Precision Monolithics incorporates transistors as switches. Currents
can be switched faster than voltages (the DAC 08 takes only 85 ns
to respond to a change in binary number), which is another reason why
the R-2R ladder technique is popular.
Interfacing a digital-to-analogue converter with a microprocessor is
very easy; it can simply be hung on the data bus via a latch which is
enabled by the usual output signal. A binary number to be converted is
put on to the databus, then the latch is enabled and the data allowed in
and passed to the converter. When the enable signal is removed, the
binary data is of course held in the latch. This has the purpose of
holding the D-to-A output volt~ge steady while other information is on
the data bus. This is shown in figure 4.23.
digital-to-analogue
~---oVout
latch--~
enable 1--_---,.,,__ .....•
Figure 4.23
Connecting a D-to-A converter to a microprocessor is simple. A single latch is required;
the latch enable signal is an output signal generated by the microprocessor.
A rather nice use ofthe digital-to-analogue converter you might like
to try involves graphics: draw~ng pictures on an x-y plotter, or on an
oscilloscope screen using the x-plates and y-plates. Two converters are
needed, one for the x-direction, and the other for the y-direction. As
shown in figure 4.24, each latch is controlled by its own output signal.
It works like this: data destined for the x-position are put on the bus,
and the x-output signal latches this into the x-latch, and the data are
converted by the x digital-to-analogue converter. The x-output signal
returns low, and the x-data is latched safe. The y-position data is then
put on the bus, and latched into the y converter via the y-output signal,
and converted. The plotter pen is now at point (x, y). Then the new x
position is obtained, then the new y position. The plotter pen thus steps
from point to point, first along the x-axis, then up the y-axis. Lines
drawn this way are in reality made up of small steps.
The theory of D-to-A conversion sounds nice and simple. When you
get around to building a converter, you will learn a lot about
electronics, solving many problems. One of the problems involves
'glitches'. These are unwanted spikes which appear at the output of a
D-to-A converter when the input number changes. They are due to
circuit imbalances when the transistors actually switch the currents.
This explanation is incomplete, but the headaches you get trying to deglitch
are quite real.
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