Page 191 - Programming Microcontrollers in C

176 Chapter 4 Small 8-Bit Systems

0325 26 04 BNE $032B
0327 A6 01 LDA #$01 hrs=1;
0329 B7 C0 STA $C0 }
}
032B 20 E0 BRA $030D }
032D 81 RTS }

void __TIMER(void)
07F8 03 2E {
032E 1D 08 BCLR 6,$08 TCST.RTIF=0;
0330 3C C3 INC $C3 if (++count==122)
0332 B6 C3 LDA $C3
0334 A1 7A CMP #$7A
0336 26 04 BNE $033C
{
Timers
0338 3C C2 INC $C2 sec++;
033A 3F C3 CLR $C3 count=0;
}
033C 80 RTI }

07FE 03 00
At the beginning of the interrupt service routine there is an entry
07f8, which has a value of 032e, in the address column. This entry
places the address of the timer interrupt service routine 0x032e
into the timer vector 0x07f8. The code generated in the interrupt
service routine is straightforward and little different from what one
would expect an assembly language programmer to do. Note, how­
ever, that the return at the end of the interrupt service routine is an
RTI instruction. This is the instruction that causes the microcontroller
to restore the processor status to the state that existed when the inter­
rupt occurred. The normal return from subroutine RTS does not restore
the processor state. An RTI must be used to return from interrupt
service routines, and any function identified with a vector pragma
will be assumed to be an interrupt service routine by the compiler.
It was noted earlier that this timer routine is inaccurate. It is inac­
curate only because 122 periods of 8.192 milliseconds each total
0.999424 seconds. This seemingly small error will cause big problems