Microcomputer Circuits and Processes

A typical bubble unit contains 300 loops, each with 4096 cells,
around which bubbles circulate. Two tracks communicate with the
loops: the input track which carries bubbles from a generator - a loop
of conductor carrying the input current pulse which locally reverses the
bias field and creates a bubble - and the output track. The latter leads
to a bubble detector, a magnetoresistive. device. When a magnetic
bubble passes by this device the change in magnetic field strength
causes the resistance of the device to change, which in turn changes a
current passing through the device. To complete the memory, a bit of
control is needed to synchronize bubble production and loading, or
unloading and detecting. Then, with the magnetic field rotating at
50kHz, data may be got from the memory in about 0.1ms. So bubble
memory is not particularly slow, it can store vast amounts of data, and,
unlike disk storage, has no moving mechanical parts to fail. It is nonvolatile
and so saves its contents when the computer is switched off or if
power fails.
APPLICATION NOTE - SEVEN-SEGMENT DISPLAY
How does a calculator-type display work? There may be up to ten~digits
used to display a large number. How can such a display be hooked up
to the computer? Begin with a single digit. A typical display is made of
seven light-emitting diodes arranged in a pattern of a figure '8'. This is
called a seven-segment display. It is normally driven by a special chip
that takes in a 4-bit number and produces numbers 0 to 9 and also a
decimal point on the display. The arrangement is shown in figure 4.10.
4-bit
input
driver
display
8
each of the segments
light-emitting diode
is a
Figure 4.10
The seven-segment LED display shown connected to a driver-decoder chip which is fed
by a 4-bit number 0000 (0) to 1001 (9 10 ).
We obviously have to connect this to an output buffer of our system
to supply a 4-bit number for the display, and must then decide which
CPU signal to use to enable the buffer. The best choice is the combination
ofM/IO, WR, and in-out decode, and is shown in figure 4.11.
buffer driver display
4 lines of
data bus
~8
Figure 4.11
Single digit display, arranged as
output port of our simple computer.
Part of computer board I
(see figure 2.31) I
I
from decoder I
inverted
WR 1--+-----4....----.....1
M/IO
E
latch
50