MC68HC11G5 Datasheet, PDF (61/195 Pages) Motorola, Inc – High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcontroller

Freescale Semiconductor, Inc.
5.2.5 Interrupt Mask Bits in Condition Code Register
On reset, both the X bit and the I bit in the CCR are set to inhibit all maskable interrupts and XIRQ.
After minimum system initialization, software may clear the X bit by a TAP instruction, thus enabling
XIRQ interrupts. Thereafter, software cannot set the X bit. Thus, an XIRQ is effectively a non-
maskable interrupt. Since the operation of the I bit related interrupt structure has no effect on the
X bit, the internal XIRQ pin remains effectively non-masked. In the interrupt priority logic, the XIRQ
interrupt is a higher priority than any source that is maskable by the I bit. All I bit related interrupts
operate normally with their own priority relationships.
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 the I bit
are automatically set by hardware after stacking the CCR byte. A return from interrupt (RTI)
instruction restores the X and I bits to their pre-interrupt request state.
5.2.6 Priority and Masking Structure
Interrupts obey a fixed hardware priority circuit to resolve simultaneous requests. However, one I bit
related interrupt source may be elevated to the highest I bit priority position in the resolution circuit.
Six interrupt sources are not masked by the I bit in the condition codes register and have the fixed
priority relationship:
1. Reset
2. Clock failure monitor
3. COP failure
4. Illegal opcode
5. SWI
6. XIRQ
SWI is actually an instruction and has highest priority other than reset in the sense that once the SWI
opcode is fetched, no other interrupt can be honoured until after the first instruction in the SWI
service routine has been executed. The scenario is similar for the illegal opcode.
Each of these sources is an input to the priority resolution circuit. The highest I bit masked priority
input to the resolution circuit is assigned under software control (via the HPRIO register) to be
connected to any one of the remaining I bit related interrupt sources. In order to avoid timing races
the HPRIO register may only be written while the I bit related interrupts are inhibited (I bit in CCR
set). An interrupt which is assigned to this high priority position is still subject to masking by any
associated control bits or by the I bit in the CCR. The address from which the interrupt vector is
fetched is not affected by assigning a source to this higher priority position.
RESETS, INTERRUPTS AND LOW POWER MODES
5-9
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