DS_1209F_004
73S1209F Data Sheet
1.7.3.1 Interrupt Overview
When an interrupt occurs, the MPU will vector to the predetermined address as shown in Table 32. Once
the interrupt service has begun, it can only be interrupted by a higher priority interrupt. The interrupt
service is terminated by a return from the RETI instruction. When a RETI is performed, the processor will
return to the instruction that would have been next when the interrupt occurred.
When the interrupt condition occurs, the processor will also indicate this by setting a flag bit. This bit is
set regardless of whether the interrupt is enabled or disabled. Each interrupt flag is sampled once per
machine cycle, then samples are polled by the hardware. If the sample indicates a pending interrupt
when the interrupt is enabled, then the interrupt request flag is set. On the next instruction cycle, the
interrupt will be acknowledged by hardware forcing an LCALL to the appropriate vector address.
Interrupt response will require a varying amount of time depending on the state of the MPU when the
interrupt occurs. If the MPU is performing an interrupt service with equal or greater priority, the new
interrupt will not be invoked. In other cases, the response time depends on the current instruction. The
fastest possible response to an interrupt is 7 machine cycles. This includes one machine cycle for
detecting the interrupt and six cycles to perform the LCALL.
1.7.3.2 Special Function Registers for Interrupts
Interrupt Enable 0 Register (IEN0): 0xA8 Å 0x00
Table 19: The IEN0 Register
MSB
EAL
LSB
EX0
WDT
–
ES0
ET1
EX1
ET0
Bit
Symbol
EAL
WDT
–
Function
IEN0.7
IEN0.6
IEN0.5
IEN0.4
IEN0.3
IEN0.2
IEN0.1
IEN0.0
EAL = 0 – disable all interrupts.
Not used for interrupt control.
ES0
ET1
EX1
ET0
EX0
ES0 = 0 – disable serial channel 0 interrupt.
ET1 = 0 – disable timer 1 overflow interrupt.
EX1 = 0 – disable external interrupt 1.
ET0 = 0 – disable timer 0 overflow interrupt.
EX0 = 0 – disable external interrupt 0.
Rev. 1.2
33
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