Page 205 - Programming Microcontrollers in C

190 Chapter 4 Small 8-Bit Systems

creates a structure that contains two bytes. This structure is com­
bined with a type long in the union below. Remember that a union
provides space to hold its largest member and the different members
of the union will occupy the same memory space. Therefore, the
memory space to store the long l is the exact same memory to
store the structure b. b.hi will occupy the most significant byte of
l, and b.lo will occupy the least significant byte of l. It is now easy
to deal with the bytes of the 16-bit quantity when moving the data
around, and equally easy to invoke 16-bit arithmetic operations on
the long combination of the two bytes.

union both
{
long l;
struct bothbytes b;
};
The statement

union both time_count;
declares that time_count is a union of the type both. In the in­
terrupt service routine, the members of time_count are handled
with little difficulty, as shown below.
The type registera defined as ac above is unique to the
M68HC05 compiler. This type specifies that the variable ac will be
stored in the accumulator. There is also a registerx type for the
index register.
In the main program, the output compare interrupt is enabled
and the system interrupt is enabled with the CLI() instruction in
the initialization section. The program then enters an endless loop in
which the clock is serviced every time sec becomes 60. The param­
eter sec will be changed by the interrupt service routine every second
so that the system should be a satisfactory clock. This loop is the
initialization section.
The last instruction in the FOREVER loop is a WAIT() instruc­
tion. This instruction places the processor into the wait mode. In this
mode, processor operations are halted and the microcontroller op­
eration is configured to reduce energy consumption. The operation
of the internal timers proceed as usual. The part is removed from the
wait mode by either a reset or the occurrence of an interrupt. The