Nested Interrupts - Fujitsu 8FX Hardware Manual

MB95630H Series
5.1.3

Nested Interrupts

Different interrupt levels can be assigned to multiple interrupt requests from
peripheral functions in the interrupt level setting registers (ILR0 to ILR5) to
process nested interrupts.
■ Nested Interrupts
During the execution of an interrupt service routine, if another interrupt request whose interrupt
level has priority over the interrupt level of the interrupt being processed is made, the CPU
suspends the current interrupt processing and accepts the interrupt request given priority. The
interrupt level of an interrupt request can be set to 0 to 3. If it is set to 3, the CPU does not
accept that interrupt request.
[Example: Nested interrupts]
In the following example of nested interrupts, assuming that the external interrupt is to be
given priority over the timer interrupt, the interrupt level of the timer interrupt is set to 2 and
that of the external interrupt to 1. If the external interrupt is generated while the timer interrupt
is being processed, they are processed as shown in Figure 5.1-2.
Initialize peripheral resources (1)
Timer interrupt occurs (2)
Resume main program
• While the timer interrupt is being processed, the interrupt level bits in the condition code
register (CCR:IL[1:0]) hold the same value as that of the interrupt level setting registers
(ILR0 to ILR5) corresponding to the timer interrupt (level 2 in the above example). If an
interrupt request whose interrupt level has priority over the interrupt level of the timer
interrupt (level 1 in the above example) is made, that interrupt is processed first.
• To temporarily disable nested interrupts processing while the timer interrupt is being
processed, disable interrupts by setting the interrupt enable flag in the condition code
register (CCR:I) to "0", or set the interrupt level bits (CCR:IL[1:0]) to "0b00".
• After the interrupt processing is completed, if the interrupt return instruction (RETI) is
executed, the value of the program counter (PC) and that of the program status (PS) are
restored, and the CPU resumes executing the program interrupted. In addition, the values of
the condition code register (CCR) return to the ones existing before the interrupt due to the
restoration of the value of the program status (PS).
MN702-00009-2v0-E
Figure 5.1-2 Example of Nested Interrupts
Main Program Timer Interrupt Processing
Interrupt level 2
(CCR:IL[1:0]=0b10)
Suspend
Resume
(8)
FUJITSU SEMICONDUCTOR LIMITED
External Interrupt Processing
Interrupt level 1
(CCR:IL[1:0]=0b01)
(3)
External interrupt
occurs
(6)
Process timer interrupt
(7)
Return from timer interrupt
CHAPTER 5 INTERRUPTS
5.1 Interrupts
(4)
Process external interrupt
(5)
Return from external interrupt
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