Si1000/1/2/3/4/5
12. Interrupt Handler
The Si1000/1/2/3/4/5 microcontroller family includes an extended interrupt system supporting multiple
interrupt sources and two priority levels. The allocation of interrupt sources between on-chip peripherals
and external input pins varies according to the specific version of the device. Refer to Table 12.1, “Interrupt
Summary,” on page 131 for a detailed listing of all interrupt sources supported by the device. Refer to the
data sheet section associated with a particular on-chip peripheral for information regarding valid interrupt
conditions for the peripheral and the behavior of its interrupt-pending flag(s).
Each interrupt source has one or more associated interrupt-pending flag(s) located in an SFR or an indi-
rect register. When a peripheral or external source meets a valid interrupt condition, the associated inter-
rupt-pending flag is set to logic 1. If both global interrupts and the specific interrupt source is enabled, a
CPU interrupt request is generated when the interrupt-pending flag is set.
As soon as execution of the current instruction is complete, the CPU generates an LCALL to a predeter-
mined address to begin execution of an interrupt service routine (ISR). Each ISR must end with an RETI
instruction, which returns program execution to the next instruction that would have been executed if the
interrupt request had not occurred. If interrupts are not enabled, the interrupt-pending flag is ignored by the
hardware and program execution continues as normal. (The interrupt-pending flag is set to logic 1 regard-
less of the interrupt's enable/disable state.)
Some interrupt-pending flags are automatically cleared by hardware when the CPU vectors to the ISR.
However, most are not cleared by the hardware and must be cleared by software before returning from the
ISR. If an interrupt-pending flag remains set after the CPU completes the return-from-interrupt (RETI)
instruction, a new interrupt request will be generated immediately and the CPU will re-enter the ISR after
the completion of the next instruction.
12.1. Enabling Interrupt Sources
Each interrupt source can be individually enabled or disabled through the use of an associated interrupt
enable bit in the Interrupt Enable and Extended Interrupt Enable SFRs. However, interrupts must first be
globally enabled by setting the EA bit (IE.7) to logic 1 before the individual interrupt enables are recog-
nized. Setting the EA bit to logic 0 disables all interrupt sources regardless of the individual interrupt-
enable settings. Note that interrupts which occur when the EA bit is set to logic 0 will be held in a pending
state, and will not be serviced until the EA bit is set back to logic 1.
12.2. MCU Interrupt Sources and Vectors
The CPU services interrupts by generating an LCALL to a predetermined address (the interrupt vector
address) to begin execution of an interrupt service routine (ISR). The interrupt vector addresses associ-
ated with each interrupt source are listed in Table 12.1 on page 131. Software should ensure that the inter-
rupt vector for each enabled interrupt source contains a valid interrupt service routine.
Software can simulate an interrupt by setting any interrupt-pending flag to logic 1. If interrupts are enabled
for the flag, an interrupt request will be generated and the CPU will vector to the ISR address associated
with the interrupt-pending flag.
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