Page 195 - Programming Microcontrollers in C

180 Chapter 4 Small 8-Bit Systems

microcontroller crystal frequency. An input capture operation de­
tects the occurrence of an input and transfers the contents of the
16-bit counter into the input capture register. This transfer will al­
ways set a flag, and it can cause a CPU interrupt if desired. With an
input capture system, precise measurement of time interval is pos­
sible. Details such as phase between two waveforms can be determined
or slight differences in frequencies between several signals can be
detected. The input capture provides far more accurate time mea­
surement than can be obtained with either synchronous polling or
asynchronous interrupt time measurements. There is a tiny inherent
delay between the occurrence of the input and the setting of the input
capture register. Such a measurement made by polling an input would
require that the computer have a free running counter available to
interrogate when the input is detected. Then, the computer would
have to be assigned totally to the job of watching the input for the
impending transition. When the transition is detected, the value of
the counter would have to be read to determine the time of the tran­
sition. Of course, this sequence of operations would require several
computer clock cycles per test, and also several cycles would be re­
quired to read the counter. Therefore, the accuracy of the time
measurement would be compromised by these necessary time delays.
An asynchronous interrupt method to determine the time inter­
val is better than a polled method, but even this method has built-in
errors that make it an impractical means to measure time intervals
accurately. The input capture register input system resolves most of
the problems associated with accurate measurement of time inter­
vals with a microcomputer.
Another type of timing problem exists. Suppose that the time
that an event is to occur has been calculated. If the time of occur­
rence is to be accurate, we have a situation like that discussed above.
The processor will have to spend all of its time watching the clock to
determine when the correct time has arrived. Any time spent on other
tasks during this measurement interval will be a latency during which
the processor cannot determine if the specified time has arrived. In
this case, the accuracy of the event time will be degraded by the time
spent on other tasks.
The 16-bit timer avoids this type of problem nicely. An output
compare system is used. The time of occurrence is calculated rela­