IA6805E2
29 August 2007
Microprocessor Unit
As of Production Version 00
Figure 10. Interrupt Mode Diagram
Timer Interrupt:
If the timer mask bit (TCR6) and the interrupt mask bit (I) of the condition code register are
cleared, each time the timer decrements to zero ($01 to $00 transition) an interrupt request is
generated. When the interrupt is recognized, the current state of the machine is pushed onto
the stack and the condition code register I-bit gets set masking further interrupts until the
present one is serviced. The program counter is then loaded with the contents of the timer
interrupt vector, which contains the location of the timer interrupt service routine. The
contents of $1FF8 and $1FF9 specify the address for this service routine. If the MPU is in
the wait mode and a timer interrupt occurs, then the contents of $1FF6 and $1FF7 specify
the service routine. When the timer interrupt service routine is complete, the software
executes an RTI instruction to restore the machine state and starts executing the interrupt
program.
Software Interrupt:
Software interrupt is an executable instruction regardless of the state of the interrupt mask
bit (I) in the condition code register. SWI is similar to hardware interrupts. It executes after
the other interrupts if the interrupt mask bit is zero. The contents of $1FFC and $1FFD
specify the address for this service routine.
Low Power Modes:
The low power modes consist of the stop instruction and the wait instruction. The
following paragraphs explain these modes of operation.
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