CHAPTER 14 INTERRUPT FUNCTIONS
14.4.2 Maskable interrupt request acknowledgment operation
A maskable interrupt request can be acknowledged when the interrupt request flag is set to 1 and the
corresponding interrupt mask flag is cleared to 0. A vectored interrupt is acknowledged in the interrupt enabled
status (when the IE flag is set to 1).
The time required to start the interrupt servicing after a maskable interrupt request has been generated is shown
in Table 14-3.
Refer to Figures 14-13 and 14-14 for the timing of interrupt request acknowledgement.
Table 14-3. Time from Generation of Maskable Interrupt Request to Servicing
Minimum Time
Maximum TimeNote
19 clocks
9 clocks
Note The wait time is maximum when an interrupt request is generated immediately before
BT or BF instruction.
1
Remark 1 clock:
(fCPU: CPU clock)
fCPU
When two or more maskable interrupt requests are generated at the same time, they are acknowledged starting
from the one assigned the highest priority by the priority specification flag.
A pending interrupt is acknowledged when the status where it can be acknowledged is set.
Figure 14-12 shows the algorithm of interrupt request acknowledgement.
When a maskable interrupt request is acknowledged, the PSW and PC are saved to the stack in that order, the IE
flag is reset to 0, and the data in the vector table determined for each interrupt request is loaded to the PC, and
execution branches.
To return from interrupt servicing, use the RETI instruction.
Figure 14-12. Interrupt Request Acknowledgment Program Algorithm
Start
No
xxIF = 1 ?
Yes (Interrupt request generated)
No
xxMK = 0 ?
Yes
Interrupt request pending
Interrupt request pending
No
IE = 1 ?
Yes
Vectored interrupt
servicing
xxIF: Interrupt request flag
xxMK: Interrupt mask flag
IE:
Flag to control maskable interrupt request acknowledgement (1 = enable, 0 = disable)
274
User’s Manual U15075EJ1V0UM00
NEC [ NEC ]
