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Interrupt Priorities
10-7
Interrupts
10.5 Interrupt Priorities
The MSC1210 offers three levels of interrupt priority: highest, high, and low.
By using interrupt priorities, higher priority may be assigned to certain interrupt
conditions. The highest priority is reserved for the auxiliary interrupt that vec-
tors through address 0033
H
—the auxiliary interrupt is always of highest prior-
ity and no other interrupt may be assigned that priority.
All other interrupts may be assigned either high or low priority. For example,
assume the Timer 1 interrupt has been enabled to be automatically called ev-
ery instance Timer 1 overflows. Additionally, the serial interrupt has been en-
abled to be called every time a character is received via the serial port. Howev-
er, in this case, receiving a character is much more important than the timer
interrupt. Therefore, if the Timer 1 interrupt is already executing, the serial in-
terrupt must interrupt the Timer 1 interrupt. When the serial interrupt is com-
plete, control passes back to the Timer 1 interrupt and finally back to the main
program. This may be accomplished by assigning a high priority to the serial
interrupt and a low priority to the Timer 1 interrupt.
Interrupt priorities are controlled by the IP (B8
H
) or EIP (F8
H
) SFRs. These
SFRs have the following formats, as shown in Table 10−5 and Table 10−6.
Table 10−5.IP (B8
H
) SFR
Bit Name Bit Address Explanation of Function
7 − BF
H
Undefined
6 − BE
H
Undefined
5 − BD
H
Undefined
4 PS BC
H
Serial Interrupt Priority
3 PT1 BB
H
Timer 1 Interrupt Priority
2 PX1 BA
H
External 1 Interrupt Priority
1 PT0 B9
H
Timer 0 Interrupt Priority
0 PX0 B8
H
External 0 Interrupt Priority
Table 10−6.EIP (F8
H
) SFR
Bit Name Bit Address Explanation of Function
7 − FF
H
Undefined (set to 1)
6 − FE
H
Undefined (set to 1)
5 − FD
H
Undefined (set to 1)
4 PWDI FC
H
Watchdog Interrupt Priority
3 PX5 FB
H
External Interrupt 5 Priority
2 PX4 FA
H
External Interrupt 4 Priority
1 PX3 F9
H
External Interrupt 3 Priority
0 PX2 F8
H
External Interrupt 2 Priority