Page 300 - Programming Microcontrollers in C

Summary 285

There are three input/output routines that have been written for
this program. These routines, putchar(), dprint(), and
do_crlf(), can be used with other systems with a serial input/
output system. The Cosmic compiler does provide the usual I/O
routines like printf(), gets(), puts(), etc. It does not provide
a basic putchar() and getchar() which is used by all of these
library routines. The reason that these routines are not provided by
the compiler is the wide variety of what the programmer will want to
implement with the built-in SCI ports on the MC68HC11 family.
The putchar() shown above will work in most instances. The
dprint() routine is a recursive routine that converts an integer
into an ASCII string and sends it to the SCI port.
There is one final modification to the program. In the last lines of
the PWM timer routine, count1 and tick1 are processed to set
tick1 to be TRUE each second. This flag is then used to control the
writing of the motor speeds to the terminal screen.

Summary

There has been no attempt to work all of the peripherals on the
MC68HC11. The various peripherals are similar to those on the other
parts that we have discussed in other chapters or will discuss later.
We have seen several timer applications both in the MC68HC11 and
in the MC68HC05. We will see other timer applications in the
following chapters.
We have seen detailed use of the output compare timer subsystem
to make a pulse width modulation digital-to-analog converter system.
Depending on the program, the system allowed excellent performance
in either short on times or maximum on times, but not both without the
addition of a significant amount of code. We will see a system in the
next chapter that provides excellent performance for both minimum
and maximum on times. This performance is not a limitation of the
MC68HC11, merely a limitation of the programs presented so far.
The input capture subsystem has been used to measure motor
speed in a simple DC motor controller. This system used a primitive
reed switch to measure the rotation of the motor shaft, and the
performance of the switch was poor. A debouncing system was
developed that prevented input captures to occur for a specified time
after the first input was detected. This approach uses an output