Microcomputer Circuits and Processes

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) refresh
nd
utput-input
ircuits
transistors would do this job. Note that there is a common input and
output connection to the flip-flop; check for yourself that this works by
putting a logic level 1 at this connection, and work out the logic states
at the inputs and outputs of both inverters. Repeat for input logical O.
Figure 4.3(a) shows how four of these cells are connected with switches.
In reality, these switches are also transistors on an integrated chip and
are controlled by the row and column select logic. Data is written and
read via the line at the bottom of the diagram.
You will realize that in a flip-flop one halfis on while the other is oft
So two transistors are always at work. This type of memory needs a lot
~ of current, but is fast. A 2147 chip needs about 55 ns (nanoseconds) to
storage read or write a cell. More common RAM needs about 200 ns.
capacitor
V The second type of RAM memory cell is the dynamic RAM shown in
I_______
figure 4.4. Here the bit stored, 0 or 1,is stored as no charge, or charge on
a capacitor. A single transistor, shown on the diagram as a switch, is
needed to control the charging of this capacitor. At once you can se~
that these dynamic cells are much simpler and smaller than static cells.
Since they store information by storing charge, they do not need large
operating currents. They score over static RAM, but there is a small
problem: the charge on the capacitor leaks away, so special control
circuitry has to be installed, to continually top-up the capacitors, or
'refresh' them. This must happen every 20 ms or so. Refresh circuitry is
Figure 4.4 complex to build, and dynamic RAM controllers, chips dedicated to
A dynamic RAM cell is
topping up the dynamic RAM capacitors, are not cheap. The expense of
basically a capacitor whose
charging is controlled by this extra circuitry is only recovered when large memory systems of
a single transistor switch. hundreds of K are being built.
Bubble memory - mega storage
Figure 4.5
Ferrite slice with weak perpendicular
magnetic field showing domains.
Bubble memory units with capacities of up to 4 megabits are the subject
of research today. They are not semiconductor devices, but employ
magnetic materials in their technology. This is not as hard to understand
as semiconductor technology, so here is an outline of how a
bubble memory works. It illustrates how physics has been applied in a
high-engineering situation.
You are probably familiar with the idea of magnetic domains found
in all sorts of magnetic materials. When unmagnetized, the domains in
the material are randomly arranged in three dimensions, but as the
material is slowly magnetized, the domain walls move so that most of
the domains align to a common direction. When the material used is a
thin film (0.01mm thick) of ferrite (a magnetic oxide of iron, with other
metals), then the domains become two-dimensional, as shown in figure
4.5.
When the slice is immersed in a perpendicular magnetic field, the
domains oriented oppositely to the field shrink in size. As the 'bias field'
is increased, the few remaining domains become small cylinders, called
'bubbles', as shown in figure 4.6.
These bubbles are a few micrometres in size. If, in addition to the
bias field, there is a small field which is weak in one place and strong in
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