
Resets and Interrupts
Data Sheet
M68HC11E Family — Rev. 5
100
Resets and Interrupts
MOTOROLA
5.5.1  Interrupt Recognition and Register Stacking 
An interrupt can be recognized at any time after it is enabled by its local mask, if 
any, and by the global mask bit in the CCR. Once an interrupt source is recognized, 
the CPU responds at the completion of the instruction being executed. Interrupt 
latency varies according to the number of cycles required to complete the current 
instruction. When the CPU begins to service an interrupt, the contents of the CPU 
registers are pushed onto the stack in the order shown in 
Table 5-5
. After the CCR 
value is stacked, the I bit and the X bit, if XIRQ is pending, are set to inhibit further 
interrupts. The interrupt vector for the highest priority pending source is fetched 
and execution continues at the address specified by the vector. At the end of the 
interrupt service routine, the return-from-interrupt instruction is executed and the 
saved registers are pulled from the stack in reverse order so that normal program 
execution can resume. Refer to 
Section 4. Central Processor Unit (CPU)
. 
5.5.2  Non-Maskable Interrupt Request (XIRQ) 
Non-maskable interrupts are useful because they can always interrupt CPU 
operations. The most common use for such an interrupt is for serious system 
problems, such as program runaway or power failure. The XIRQ input is an 
updated version of the NMI (non-maskable interrupt) input of earlier MCUs. 
Upon reset, both the X bit and I bit of the CCR are set to inhibit all maskable 
interrupts and XIRQ. After minimum system initialization, software can clear the X 
bit by a TAP instruction, enabling XIRQ interrupts. Thereafter, software cannot set 
the X bit. Thus, an XIRQ interrupt is a non-maskable interrupt. Because the 
operation of the I-bit-related interrupt structure has no effect on the X bit, the 
internal XIRQ pin remains unmasked. In the interrupt priority logic, the XIRQ 
interrupt has a higher priority than any source that is maskable by the I bit. All 
I-bit-related interrupts operate normally with their own priority relationship. 
When an I-bit-related interrupt occurs, the I bit is automatically set by hardware 
after stacking the CCR byte. The X bit is not affected. When an X-bit-related 
interrupt occurs, both the X and I bits are automatically set by hardware after 
Table 5-5. Stacking Order on Entry to Interrupts  
Memory Location
SP
SP–1
SP–2
SP–3
SP–4
SP–5
SP–6
SP–7
SP–8
CPU Registers
PCL 
PCH 
IYL 
IYH 
IXL 
IXH 
ACCA 
ACCB 
CCR 
F
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.