W79E804A/803A/802A
If any of these conditions are not met, then the LCALL will not be generated. The polling cycle is
repeated every machine cycle, with the interrupts sampled in the same machine cycle. If an interrupt
flag is active in one cycle but not responded to, and is not active when the above conditions are met,
the denied interrupt will not be serviced. This means that active interrupts are not remembered; every
polling cycle is new.
The processor responds to a valid interrupt by executing an LCALL instruction to the appropriate
service routine. This may or may not clear the flag which caused the interrupt. In case of Timer
interrupts, the TF0 or TF1 flags are cleared by hardware whenever the processor vectors to the
appropriate timer service routine. In case of external interrupts, INT0 and INT1, the flags are cleared
only if they are edge triggered. In case of Serial interrupts, the flags are not cleared by hardware. The
Watchdog timer interrupt flag WDIF has to be cleared by software. The hardware LCALL behaves
exactly like the software LCALL instruction. This instruction saves the Program Counter contents onto
the Stack, but does not save the Program Status Word PSW. The PC is reloaded with the vector
address of that interrupt which caused the LCALL. These address of vector for the different sources
are as follows:
VECTOR LOCATIONS FOR INTERRUPT SOURCES
SOURCE
External Interrupt 0
External Interrupt 1
Serial Port
VECTOR ADDRESS
0003h
SOURCE
VECTOR ADDRESS
000Bh
Timer 0 Overflow
0013h
Timer 1 Overflow
001Bh
0023h
Brownout Interrupt
002Bh
I2C Interrupt
0033h
KBI Interrupt
003Bh
Comparator 2 Interrupt
Watchdog Timer
Comparator 1 Interrupt
PWM Brake Interrupt
0043h
-
-
-
-
004Bh
0053h
005Bh
0063h
006Bh
0073h
007Bh
Table 12-1: Vector locations for interrupt sources
Execution continues from the vectored address till an RETI instruction is executed. On execution of
the RETI instruction the processor pops the Stack and loads the PC with the contents at the top of the
stack. The user must take care that the status of the stack is restored to what it was after the hardware
LCALL, if the execution is return to the interrupted program. The processor does not notice anything if
the stack contents are modified and will proceed with execution from the address put back into PC.
Note that a RET instruction would perform exactly the same process as a RETI instruction, but it
would not inform the Interrupt Controller that the interrupt service routine is completed, and would
leave the controller still thinking that the service routine is underway.
Publication Release Date: July 16, 2007
- 61 -
Revision A2
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