Page 268 - Programming Microcontrollers in C

Timer Operations 253

on time is poor and the maximum on time can be up to one clock
cycle shy of the base period. If you wish to have good performance
at both ends, minimum on time at the same time as maximum on
time, you can examine the on time in your program and if it is less
than 50% of the base time period, control the PWM by an interrupt
on reset. Otherwise, control the PWM by an interrupt on set.
Implementation of the code for this approach is left to the exercises.
Output compare operations OC2 through OC4 and the
programmable timer subsystem I4O5 are all identical. These versatile
timers can be used to generate complex timing waveforms that are
useful in keeping time, running stepper motors, etc. The examples
shown above demonstrate how these outputs can be converted into a
PWM digital to analog output. The coupling between OC1 and the
other outputs provides an excellent mechanism for synchronizing
two time events for the outside circuitry. Of course, these subsystems
can create events in time, but they are completely unable to measure
time. We will later examine the analysis of timed events that can be
measured with the help of the input capture subsystems.

EXERCISES
1. Write the code that will determine from the on time for the PWM
the interrupt operation that will allow the minimum off and maxi­
mum on time simultaneously.
2. Modify the code in Listing 5.4 to eliminate the need for a condi­
tional test if (OC1D.OC1D7 = = 1).

Input Capture Subsystems

On the MC68HC11EX family, there are three input capture timers
and one timer that can be programmed as either an input capture or
an output compare. These sub-systems are used to measure time
interval. The same 16-bit timer used in the output compare systems
is used to support the input captures. When an input occurs on one of
the input capture pins, the value contained in the 16-bit timer is saved
in a register designated for that pin, a flag is set, and the processor
can request an interrupt. With input captures, we can measure period
and hence frequency (or speed). One of the leading applications where
the input capture is used is in automatic braking systems. We shall