Microcomputer Circuits and Processes

iased that way). This low, when fed back to the counter, tells it to stop
counting. The counter now holds 0101, the binary value of the input
voltage. The conversion is complete, and the 4-bit number can be
enabled on to the computer's data bus and read into memory.
Figure 4.27 shows this conversion; the step output of the D-to-A
converter as the counter increments until the D-to-A voltage equals Jlin'
V iṉ
-------
I
I
I
1
I
I
I
D-to-A output voltage
I Comp ••• ",
output
Time
I conversion
done
Figure 4.27
The technique oflinear ramp conversion. The D-to-A output increases in equal steps
until it is equal to the input voltage. Then, the comparator signals that the conversion is
done. The counter, which had been driving the D-to-A converter, is stopped, and holds
the binary value of the input voltage.
The comparator's output is also shown dropping low at this point.
In a working circuit, a little more logic is needed, to reset the counter
before each conversion is started and to signal the CPU when the
conversion is done, so the result can be enabled on to the bus.
It is quite a straightforward procedure, but rather slow. For a large
input voltage, the counter may need to count up to a large number. An
8-bit converter would need a maximum of 255 counts. A typical clock
driving the converter may run at 1MHz, needing 1 JlS per count, or up
to 255 JlS for a single conversion. Such pedestrian behaviour may be fine
if you want to measure the height of water in a river once per hour, but
totally inadequate to record the behaviour of a millisecond pulsar.
Microprocessor-controlled measuring systems are usually used in
the two extremes of either very infrequent (once per hour, day ... ) or
very frequent (thousands or millions per second). These are areas where
humans would not perform very well. The problem with quick or
transient measurements is illustrated in figure 4.28.
You know that a certain signal has a large initial peak and then
decays down. You start the ramp converter at the instant the signal
appears (you cannot anticipate), but by the time the counter has
counted a hundred or so pulses, the input voltage has long since
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