Specifications

C8051F330/1
Rev. 1.1 81
9.3. Interrupt Handler
The CIP-51 includes an extended interrupt system supporting a total of 13 interrupt sources with two priority levels.
The allocation of interrupt sources between on-chip peripherals and external inputs pins varies according to the spe
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cific version of the device. Each interrupt source has one or more associated interrupt-pending flag(s) located in an
SFR. When a peripheral or external source meets a valid interrupt condition, the associated interrupt-pending flag is
set to logic
1.
If interrupts are enabled for the source, an 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 predetermined address to
begin execution of an interrupt service routine (ISR). Each ISR must end with an RETI instruction, which returns pro
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gram 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 regardless of the interrupt's enable/disable state.)
Each interrupt source can be individually enabled or disabled through the use of an associated interrupt enable bit in
an SFR (IE-EIE1). However, interrupts must first be globally enabled by setting the EA bit (IE.7) to logic
1 before the
individual interrupt enables are recognized. Setting the EA bit to logic 0 disables all interrupt sources regardless of
the individual interrupt-enable settings.
Some interrupt-pending flags are automatically cleared by the 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.
9.3.1. MCU Interrupt Sources and Vectors
The MCUs support 13 interrupt sources. 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. MCU interrupt sources, associated vector addresses, priority order
and control bits are summarized in
Table 9.4 on page 83. Refer to the datasheet section associated with a particular
on-chip peripheral for information regarding valid interrupt conditions for the peripheral and the behavior of its inter-
rupt-pending flag(s).