
Overview of Interrupts
M30240 Group
Rev.1.00 Sep 24, 2003 Page 336 of 360
4.1.3.1 Interrupt Request Bit
The interrupt request bit is located in the interrupt control register of each interrupt. It is set to “1” by
hardware when an interrupt is requested. After the interrupt is accepted and jumps to the correspond-
ing interrupt vector, the request bit is set to “0” by hardware. The interrupt request bit can also be set
to “0” by software. (Do not set this bit to “1”).
4.1.3.2 Interrupt Enable Flag
The interrupt enable flag (I flag) is located in the flag register (FLG). A non-maskable interrupt can be
enabled or disabled using the interrupt enable flag (I flag). Setting this flag to “1” enables all maskable
interrupts; Setting it to “0” disables all maskable interrupts. This flag is set to “0” after reset.
Figure 4.3shows the timing change in the I flag to the interrupt.
When the content of the I flag is changed, the acceptance of an interrupt request occurs in the follow-
ing timing:
When changing the I flag using the REIT instruction, the acceptance of the interrupt takes effect as
the REIT instruction is executed.
When changing the I flag using one of the FCLR, FSET, POPC, and LDC instructions, the accep-
tance of the interrupt is effective as the next instruction is executed.
Figure 4.3:
Timing reflecting the change in the I flag to the interrupt
4.1.3.3 Interrupt Priority Level
A non-maskable interrupt can be enabled or disabled using the interrupt priority level selection bit, and
processor interrupt priority level (IPL).
The interrupt priority level selection bit is located in the interrupt control register of each interrupt. The
IPL is located in the flag register (FLG). When an interrupt request occurs, the interrupt priority level
is compared to the IPL. Use the interrupt priority level select bit to set the interrupt priority level. The
interrupt is enabled only when the priority level of the interrupt is higher than the IPL. Therefore, setting
the interrupt priority level to “0” disables the interrupt.
abled, according to the contents of the IPL. Note: The interrupt enable flag (I flag), interrupt request
bit, interrupt priority select bit, and IPL are independent of each other.
Previous
instruction
REIT
Interrupt sequence
Time
Interrupt request generated
Determination whether or not to
accept interrupt request
Previous
instruction
FSET I
Interrupt sequence
Next instruction
Interrupt request generated
Determination whether or not to
accept interrupt request
When changed by REIT instruction
When changed by FCLR, FSET, POPC, or LDC instruction
(If I flag is changed from 0 to 1 by REIT instruction)
(If I flag is changed from 0 to 1 by FSET instruction)
Time