ST92T163 ST Microelectronics, ST92T163 Datasheet - Page 52

• Manufacturer Part Number: ST92T163
• Description: 8/16-BIT FULL SPEED USB MCU FOR COMPOSITE DEVICES WITH 16 ENDPOINTS
• Manufacturer: ST Microelectronics

ST92163 - 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:
Figure 24. Simple Example of a Sequence of Interrupt Requests with:
- Nested mode
- IEN unchanged by the interrupt routines
52/224
0
1
2
3
4
5
6
7
Priority Level of
Interrupt Request
CPL is set to 7
INT5
MAIN
ei
CPL=5
INT 5
INT2
INT3
INT4
CPL=2
INT 2
INT0
CPL=0
INT 0
CPL2 < CPL4:
Serviced next
CPL=3
INT 3
INT6
CPL6 > CPL3:
INT6 pending
CPL=4
– All maskable interrupt requests are disabled by
– CPL is saved in the special NICR stack to hold
– Priority level of the acknowledged routine is
– 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
– The Flag register is pushed onto system stack.
– The PC is loaded with the 16-bit vector stored in
– If ENCSR is set, CSR is loaded with ISR con-
INT 4
INT2
clearing CICR.IEN.
the priority level of the suspended routine.
stored in CPL, so that the next request priority
will be compared with the one of the routine cur-
rently being serviced.
stack.
the Vector Table, pointed to by the IVR.
tents; otherwise ISR is used in place of CSR until
iret instruction.
CPL=2
INT 2
INTE RRUPT 0 HAS PRIORITY LEVEL 0
INTE RRUPT 2 HAS PRIORITY LEVEL 2
INTE RRUPT 3 HAS PRIORITY LEVEL 3
INTE RRUPT 4 HAS PRIORITY LEVEL 4
INTE RRUPT 5 HAS PRIORITY LEVEL 5
INTE RRUPT 6 HAS PRIORITY LEVEL 6
CPL=6
INT 6
CPL=7
MAIN