6-7
INTERRUPT SYSTEM
6.6 INTERRUPT PRIORITIES
Each of the seven 8XC251Sx interrupt sources may be individually programmed to one of four
priority levels. This is accomplished with the IPH0.x/IPL0.x bit pairs in the interrupt priority high
(IPH0) and interrupt priority low (IPL0) registers (Figures 6-3 and 6-4 on page 6-8). Specify the
priority level as shown in Table 6-4 using IPH0.x as the MSB and IPL0.x as the LSB.
A low-priority interrupt is always interrupted by a higher priority interrupt but not by another in-
terrupt of equal or lower priority. The highest priority interrupt is not interrupted by any other in-
terrupt source. Higher priority interrupts are serviced before lower priority interrupts. The
response to simultaneous occurrence of equal priority interrupts (i.e., sampled within the same
four state interrupt cycle) is determined by a hardware priority-within-level resolver (see Table
6-5).
NOTE
The 8XC251Sx Interrupt Priority Within Level table (Table 6-5) differs from
MCS
®
51 microcontrollers. Other MCS
251 microcontrollers may have unique
interrupt priority within level tables.
Table 6-4. Level of Priority
IPH0.
x
(MSB) IPL0.
x
(LSB) Priority Level
0 0 0 Lowest Priority
011
102
1 1 3 Highest Priority
Table 6-5. Interrupt Priority Within Level
Priority Number Interrupt Name
1 (Highest Priority) INT0#
2 Timer 0
3INT1#
4 Timer 1
5 Serial Port
6 Timer 2
7 (Lowest Priority) PCA
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