Page 145 - Programming Microcontrollers in C

130 Chapter 3 What Are Microcontrollers?

counter that counts down from it the value stored in it. When the
counter underflows, a flag is set, and an interrupt can be executed.
The 15-bit timer is a strictly Motorola name, and it is even
simpler than the 8-bit timer. This timer has a 15-bit minimally
programmable prescaler. An interrupt can be taken from two
locations in this ripple counter.
A second class of timer is the 16-bit timer. This timer is often
called a general purpose timer. These timers contain a 16-bit counter
that is clocked by the system clock. There are two associated
subsystems: the first is called an input capture system, and the second
is the output compare.
The input capture system simply captures the value of the system
timer counter when an input occurs. These inputs can set a flag or
request an interrupt so the input can be processed either synchronously
or asynchronously. The important fact is that the exact time of the
input relative to the 16-bit clock is saved when the input occurs.
Applications for input capture systems are interpulse period
measurements or frequency measurements.
The output compare system allows the programmer to specify a
time relative to the 16-bit counter when an output is to occur. This
time is calculated by adding the time offset value to the current value
of the 16-bit counter. This result is stored in the output compare
register. When the 16-bit counter counts to the value in the output
compare register, the output occurs, a bit is set, and an interrupt can
be processed if desired.
Input capture and output compare functions are sometimes called
high-speed inputs and outputs. The number of input captures and
output compare systems vary from as few as one each to as many as
16 programmable timers, each of which can be either input capture
or output compare.
There is another style of timer subsystem that is used on high-end
microcontrollers. This system is called the timer processor unit (TPU).
In most conventional computers, the contents of a memory location
are called an operand, and the processor has built-in operators that
operate on the operands. A TPU is also a computer, but rather than
using memory location contents as operands, time is the main operand
used by the TPU. Most TPUs contain many complex systems to
implement their operation. The TPU of the M68300 family and the
M68HC16 family contains sixteen registers, each of which can be