
Central Processor Unit (CPU)
Data Sheet
M68HC11E Family — Rev. 5
78
Central Processor Unit (CPU)
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MOTOROLA
operation of the CPU continues uninterrupted until the I bit is cleared. After any 
reset, the I bit is set by default and can only be cleared by a software instruction. 
When an interrupt is recognized, the I bit is set after the registers are stacked, but 
before the interrupt vector is fetched. After the interrupt has been serviced, a 
return-from-interrupt instruction is normally executed, restoring the registers to the 
values that were present before the interrupt occurred. Normally, the I bit is 0 after 
a return from interrupt is executed. Although the I bit can be cleared within an 
interrupt service routine, "nesting" interrupts in this way should only be done when 
there is a clear understanding of latency and of the arbitration mechanism. Refer 
to 
Section 5. Resets and Interrupts
. 
4.2.6.6  Half Carry (H) 
The H bit is set when a carry occurs between bits 3 and 4 of the arithmetic logic 
unit during an ADD, ABA, or ADC instruction. Otherwise, the H bit is cleared. Half 
carry is used during BCD operations. 
4.2.6.7  X Interrupt Mask (X) 
The XIRQ mask (X) bit disables interrupts from the XIRQ pin. After any reset, X is 
set by default and must be cleared by a software instruction. When an XIRQ 
interrupt is recognized, the X and I bits are set after the registers are stacked, but 
before the interrupt vector is fetched. After the interrupt has been serviced, an RTI 
instruction is normally executed, causing the registers to be restored to the values 
that were present before the interrupt occurred. The X interrupt mask bit is set only 
by hardware (RESET or XIRQ acknowledge). X is cleared only by program 
instruction (TAP, where the associated bit of A is 0; or RTI, where bit 6 of the value 
loaded into the CCR from the stack has been cleared). There is no hardware action 
for clearing X. 
4.2.6.8  STOP Disable (S) 
Setting the STOP disable (S) bit prevents the STOP instruction from putting the 
M68HC11 into a low-power stop condition. If the STOP instruction is encountered 
by the CPU while the S bit is set, it is treated as a no-operation (NOP) instruction, 
and processing continues to the next instruction. S is set by reset; STOP is 
disabled by default. 
4.3  Data Types 
The M68HC11 CPU supports four data types: 
1.
Bit data 
2.
8-bit and 16-bit signed and unsigned integers 
3.
16-bit unsigned fractions 
4.
16-bit addresses 
F
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n
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