ST92P141K4D0 View Datasheet(PDF) - STMicroelectronics
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ST92P141K4D0 Datasheet PDF : 179 Pages
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ST92141 - INTERRUPTS
ARBITRATION MODES (Cont’d)
3.5.2 Nested Mode
The difference between Nested mode and Con-
current mode, lies in the modification of the Cur-
rent Priority Level (CPL) during interrupt process-
ing.
The arbitration phase is basically identical to Con-
current mode, however, once the request is ac-
knowledged, the CPL is saved in the Nested Inter-
rupt Control Register (NICR) by setting the NICR
bit corresponding to the CPL value (i.e. if the CPL
is 3, the bit 3 will be set).
The CPL is then loaded with the priority of the re-
quest just acknowledged; the next arbitration cycle
is thus performed with reference to the priority of
the interrupt service routine currently being exe-
cuted.
Start of Interrupt Routine
The interrupt cycle performs the following steps:
– All maskable interrupt requests are disabled by
clearing CICR.IEN.
– CPL is saved in the special NICR stack to hold
the priority level of the suspended routine.
– Priority level of the acknowledged routine is
stored in CPL, so that the next request priority
will be compared with the one of the routine cur-
rently being serviced.
– The PC low byte is pushed onto system stack.
– The PC high byte is pushed onto system stack.
– If ENCSR is set, CSR is pushed onto system
stack.
– The Flag register is pushed onto system stack.
– The PC is loaded with the 16-bit vector stored in
the Vector Table, pointed to by the IVR.
– If ENCSR is set, CSR is loaded with ISR con-
tents; otherwise ISR is used in place of CSR until
iret instruction.
Figure 19. Simple Example of a Sequence of Interrupt Requests with:
- Nested mode
- IEN unchanged by the interrupt routines
Priority Level of
Interrupt Request
0
1
INT 0
CPL=0
INT0
CPL6 > CPL3:
INT6 pending
INTERRUPT 0 HAS PRIORITY LEVEL 0
INTERRUPT 2 HAS PRIORITY LEVEL 2
INTERRUPT 3 HAS PRIORITY LEVEL 3
INTERRUPT 4 HAS PRIORITY LEVEL 4
INTERRUPT 5 HAS PRIORITY LEVEL 5
INTERRUPT 6 HAS PRIORITY LEVEL 6
2
INT 2
INT 2
CPL=2
INT6
CPL=2
3
INT2
4
INT3
INT4
5
INT 5
ei CPL=5
INT 3
CPL=3
INT2
CPL2 < CPL4:
Serviced next
INT 4
CPL=4
6
INT5
INT 6
CPL=6
7
MAIN
CPL is set to 7
MAIN
CPL=7
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