Microcomputer Circuits and Processes

These equally spaced voltages are fed to a line of comparators. The
second input to each comparator is connected to a common point, and
~O' the signal to be converted, is fed in here. Now, if ~n is zero, all the
comparators give output logic low, as may be seen from the comparator
characteristics shown in figure 4.26. If ~n increases to, say, 2.7 V, then
comparator 0 will go high, the others remaining low. And if ~n lies
between 4 and 6 V then both comparators 0 and 1will go high. So as the
input voltage rises, each comparator will, in turn, go logical high.
Figure 4.33 shows the possible comparator outputs for any ~n.
Input voltage range
(V)
Comparator
321 0
outputs
Output from
priority encoder
o to 1.9
2 to 3.9
4to 5.9
6to 7.9
above 8
o 0 0 0
000 1
001 1
o 1 1 1
1 1 1 1
000
001
010
011
lOO
I'overflow'
bit
Figure 4.33
Flash conversion of any positive input voltage. The comparator outputs don't form a
particularly nice binary series, but the priority encoder puts this right, as shown by its
outputs. Note that the last line in the table is the 'overflow' condition, where the only
information provided by the circuit is that Vln is more than 8 V.
The only difficulty is that each comparator does not respond to a
range of voltages, but is always on if ~n is bigger than its own turn-on
voltage. That means the four comparator outputs do not form a very
nice pattern. This is cleared up by the final part of the flash converter,
the 'priority encoder'. When two inputs are high, for example, it chooses
the one that came from the comparator with the bigger turn-on voltage.
It then produces a regular binary output as shown in figure 4.33. This is
then enabled on to the data bus and stored in memory.
Flash conversion involves making comparisons in parallel, and for
each ~n just one of these comparisons is used in making the output
binary number. There is no counting or approximating.
This example using four comparators could classify any input
voltage into one of four intervals. Its resolution is pretty poor. The 8-bit
resolution which is a standard minimum resolution for most work
would require 2 8 = 256 intervals and hence 256 comparators and some
257 resistors all carefully matched. Even with the use of laser-trimmed
integrated resistors, resistor matching and making good comparators is
not easy. Hence the high cost of flash conversion.
You may have thought that in the above example with four
comparators, there were five intervals of voltage, the last being 8 to
10 V. That is not true, since any signal greater than 7.9 V will turn on the
last comparator; 55 V would do equally well. The last comparator
(number 3) only tells you if the input signal is outside the interval
6-7.9 V, and in this way it is an overflow indication.
Finally, what about the speed of conversion? This is simply the sum
of the time it takes the comparator to respond (25 ns for the NE529) and
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