Page 333 - Programming Microcontrollers in C

318 Chapter 6 Large Microcontrollers

Lines 150 through 160 above are the interrupt service routine
OC3_Isr( ) and lines 161 through 166 comprise the PIT_Isr( ).
The main item that is observed here is the quality of the optimizer for the
compiler. In general, interrupt service routines, ISR, must save the com­
plete status of the machine prior to executing any code. The CCR along
with the PC are both saved by the interrupt sequence. The remainder of
the registers must also be saved if the ISR can use any of the additional
register resources of the computer. Usually this case will be found. Note,
for example, in OC3_Isr( ) the first instruction is

pshm k,z,y,x,d,e
This instruction causes the contents of all significant registers to be
saved on the stack, so that the status of the machine at the time the
interrupt occurred can be restored before control is returned to the
portion of the program that was interrupted. This restoration is com­
pleted by the two instructions
pulm k,z,y,x,d,e
rti
which refills all of the registers with the values they contained when
the ISR was entered, and the rti instruction restores the condition
code register to the value it had and the program counter is then
restored. Thus, the status of the machine is restored and the program
control is returned to the interrupted instruction. Note that the rou­
tine PIT_Isr( ) compiles to a much simpler routine. This routine
has simply two increment instructions and requires no register re­
sources. The optimizer recognizes this simple operation and does
not save the status beyond that saved when the interrupt occurred.
This routine has probably been pushed further than it should be,
for demonstration purposes. This routine demonstrates multiple in­
terrupts working simultaneously, keyboard input and output, and
generates a simple pulse-width modulation signal whose on period
is determined by a number entered from the keyboard. All of these
operations are quite similar to those shown in Chapter 5; however,
some major modifications in approach are required to meet the sev­
eral requirements of the peripheral components on the MC68HC16.
Let us now look at some other applications often used with
microcontrollers.