MC68HC711K4CFN4 Datasheet, PDF (38/80 Pages) Motorola, Inc – 8-Bit Microcontroller

5 Resets and Interrupts
All M68HC11 MCUs have three reset vectors and 18 interrupt vectors. The reset vectors are as follows:
• RESET, or Power-On Reset
• Clock Monitor Fail
• COP Failure
The 18 interrupt vectors service 22 interrupt sources (three nonmaskable, 19 maskable). The three non-
maskable interrupt sources are as follows:
• XIRQ Pin (X-Bit Interrupt)
• Illegal Opcode Trap
• Software Interrupt
On-chip peripheral systems generate maskable interrupts, which are recognized only if the global inter-
rupt mask bit (I) in the condition code register (CCR) is clear. Maskable interrupts are prioritized accord-
ing to a default arrangement; however, any one source can be elevated to the highest maskable priority
position by a software-accessible control register (HPRIO). The HPRIO register can be written at any
time, provided bit I in the CCR is set.
Nineteen interrupt sources in the M68HC11 K series devices are subject to masking by the global inter-
rupt mask bit (bit I in the CCR). In addition to the global bit I, all of these sources, except the external
interrupt (IRQ) pin, are controlled by local enable bits in control registers. Most interrupt sources in
M68HC11 devices have separate interrupt vectors; therefore, there is usually no need for software to
poll control registers to determine the cause of an interrupt.
For some interrupt sources, such as the SCI interrupts, the flags are automatically cleared during the
normal course of responding to the interrupt requests. For example, the RDRF flag in the SCI system
is cleared by the automatic clearing mechanism invoked by a read of the SCI status register while RDRF
is set, followed by a read of the SCI data register. The normal response to an RDRF interrupt request
would be to read the SCI status register to check for receive errors, then to read the received data from
the SCI data register. These two steps satisfy the automatic clearing mechanism without requiring any
special instructions.
Refer to the following table for a list of interrupt and reset vector assignments.
MOTOROLA
38
M68HC11 K Series
MC68HC11KTS/D