Datasheet

Section 4 Exception Handling
Rev. 4.00 Jun 06, 2006 page 94 of 1004
REJ09B0301-0400
4.2 Reset
4.2.1 Overview
A reset has the highest exception priority.
When the RES pin goes low, all processing halts and the MCU enters the reset state. A reset
initializes the internal state of the CPU and the registers of on-chip supporting modules.
Immediately after a reset, interrupt control mode 0 is set.
Reset exception handling begins when the RES pin changes from low to high.
H8S/2138 Group and H8S/2134 Group MCUs can also be reset by overflow of the watchdog
timer. For details, see section 14, Watchdog Timer (WDT).
4.2.2 Reset Sequence
The MCU enters the reset state when the RES pin goes low.
To ensure that the chip is reset, hold the RES pin low for at least 20 ms when powering on. To
reset the chip during operation, hold the RES pin low for at least 20 states. For pin states in a reset,
see appendix D.1, Port States in Each Processing State.
When the RES pin goes high after being held low for the necessary time, the chip starts reset
exception handling as follows:
[1] The internal state of the CPU and the registers of the on-chip supporting modules are
initialized, and the I bit is set to 1 in CCR.
[2] The reset exception vector address is read and transferred to the PC, and program execution
starts from the address indicated by the PC.
Figures 4.2 and 4.3 show examples of the reset sequence.