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Section 4 Exception Handling
4.1
Overview
4.1.1
Exception Handling Types and Priority
As table 4.1 indicates, exception handling may be caused by a reset, direct transition, trap
instruction, or interrupt. Exception handling is prioritized as shown in table 4.1. If two or more
exceptions occur simultaneously, they are accepted and processed in order of priority. Trap
instruction exceptions are accepted at all times in the program execution state.
Exception handling sources, the stack structure, and the operation of the CPU vary depending on
the interrupt control mode set by the INTM0 and INTM1 bits in SYSCR.
Table 4.1
Exception Types and Priority
Priority
Exception Type
Start of Exception Handling
High
Reset
Starts immediately after a low-to-high transition at the
5(6
pin, or when the watchdog timer overflows.
Trace*
1
Starts when execution of the current instruction or
exception handling ends, if the trace (T) bit is set to 1.
Interrupt
Starts when execution of the current instruction or
exception handling ends, if an interrupt request has been
issued.*
2
Direct transition
Started by a direct transition resulting from execution of a
SLEEP instruction.
Low
Trap instruction (TRAPA)*
3 Started by execution of a trap instruction (TRAPA).
Notes: 1. Traces are enabled only in interrupt control modes 2 and 3. (They cannot be used in
the H8S/2138 Series or H8S/2134 Series.) Trace exception handling is not executed
after execution of an RTE instruction.
2. Interrupt detection is not performed on completion of ANDC, ORC, XORC, or LDC
instruction execution, or on completion of reset exception handling.
3. Trap instruction exception handling requests are accepted at all times in the program
execution state.