C8051F326 Datasheet, PDF (50/140 Pages) List of Unclassifed Manufacturers – Full Speed USB, 16 kB Flash MCU Family

C8051F326/7
6.3. Interrupt Handler
The CIP-51 includes an extended interrupt system supporting a total of 8 interrupt sources with two priority
levels. The allocation of interrupt sources between on-chip peripherals and external inputs pins varies
according to the specific version of the device. Each interrupt source, with the exception of USB0, has one
or more associated interrupt-pending flag(s) located in an SFR. USB0 interrupt sources are located in the
USB registers. See Section 12.8 for more details about the USB interrupt. 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 prede-
termined 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.)
Each interrupt source can be individually enabled or disabled through the use of an associated interrupt
enable bit in an SFR (IE-EIE2). 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.
Note: Any instruction which clears the EA bit should be immediately followed by an instruction which has two or more
opcode bytes. For example:
// in 'C':
EA = 0;
EA = 0;
// clear EA bit
// ... followed by another 2-byte opcode
; in assembly:
CLR EA ; clear EA bit
CLR EA ; ... followed by another 2-byte opcode
If an interrupt is posted during the execution phase of a "CLR EA" opcode (or any instruction that clears the
EA bit), and the instruction is followed by a single-cycle instruction, the interrupt may be taken. If the EA bit
is read inside the interrupt service routine, it will return a '0'. When the "CLR EA" opcode is followed by a
multi-cycle instruction, the interrupt will not be taken.
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.
6.3.1. MCU Interrupt Sources and Vectors
The MCU supports 8 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 6.4 on page 52. Refer to the data
sheet section associated with a particular on-chip peripheral for information regarding valid interrupt condi-
tions for the peripheral and the behavior of its interrupt-pending flag(s).
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