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Section 3 Exception Handling
3.1
Overview
Exception handling is performed in the H8/3637 Series when a reset or interrupt occurs.
Table 3.1 shows the priorities of these two types of exception handling.
Table 3.1
Exception Handling Types and Priorities
Priority
Exception Source
Time of Start of Exception Handling
High
Reset
Exception handling starts as soon as the reset state is cleared
Low
Interrupt
When an interrupt is requested, exception handling starts after
execution of the present instruction or the exception handling
in progress is completed
3.2
Reset
3.2.1
Overview
A reset is the highest-priority exception. The internal state of the CPU and the registers of the on-
chip peripheral modules are initialized.
3.2.2
Reset Sequence
As soon as the
RES
pin goes low, all processing is stopped and the H8/3637 Series enters the reset
state.
To make sure the chip is reset properly, observe the following precautions.
•
At power on: Hold the
RES
pin low until the clock pulse generator output stabilizes.
•
When an external clock or ceramic oscillator is used, also, at power on the
RES
pin must be
held low for the crystal oscillator oscillation stabilization time shown in table 14.3 in section
14, Electrical Characteristics.
•
Resetting during operation: Hold the
RES
pin low for at least 18 system clock cycles.
Reset exception handling begins when the
RES
pin is held low for a given period, then returned to
the high level. Reset exception handling takes place as follows.
•
The CPU internal state and the registers of on-chip peripheral modules are initialized, with the
I bit of the condition code register (CCR) set to 1.