Interrupt System
The AT8xC51SND2C, like other control-oriented computer architectures, employ a pro-
gram interrupt method. This operation branches to a subroutine and performs some
service in response to the interrupt. When the subroutine completes, execution resumes
at the point where the interrupt occurred. Interrupts may occur as a result of internal
AT8xC51SND2C activity (e.g., timer overflow) or at the initiation of electrical signals
external to the microcontroller (e.g., keyboard). In all cases, interrupt operation is pro-
grammed by the system designer, who determines priority of interrupt service relative to
normal code execution and other interrupt service routines. All of the interrupt sources
are enabled or disabled by the system designer and may be manipulated dynamically.
A typical interrupt event chain occurs as follows:
•
•
•
An internal or external device initiates an interrupt-request signal. The
AT8xC51SND2C, latches this event into a flag buffer.
The priority of the flag is compared to the priority of other interrupts by the interrupt
handler. A high priority causes the handler to set an interrupt flag.
This signals the instruction execution unit to execute a context switch. This context
switch breaks the current flow of instruction sequences. The execution unit
completes the current instruction prior to a save of the program counter (PC) and
reloads the PC with the start address of a software service routine.
•
The software service routine executes assigned tasks and as a final activity
performs a RETI (return from interrupt) instruction. This instruction signals
completion of the interrupt, resets the interrupt-in-progress priority and reloads the
program counter. Program operation then continues from the original point of
interruption.
Table 50. Interrupt System Signals
Signal
Name
Alternate
Function
Type Description
External Interrupt 0
See section "External Interrupts", page 39.
INT0
I
I
I
P3.2
P3.3
-
External Interrupt 1
See section “External Interrupts”, page 39.
INT1
KIN0
Keyboard Interrupt Input
See section “Keyboard Interface”, page 198.
Six interrupt registers are used to control the interrupt system. 2 8-bit registers are used
to enable separately the interrupt sources: IEN0 and IEN1 registers (see Table 53 and
Table 54).
Four 8-bit registers are used to establish the priority level of the different sources: IPH0,
IPL0, IPH1 and IPL1 registers (see Table 55 to Table 58).
Interrupt System
Priorities
Each of the interrupt sources on the AT8xC51SND2C can be individually programmed
to one of four priority levels. This is accomplished by one bit in the Interrupt Priority High
registers (IPH0 and IPH1) and one bit in the Interrupt Priority Low registers (IPL0 and
IPL1). This provides each interrupt source four possible priority levels according to
Table 51.
36
AT8xC51SND2C
4341D–MP3–04/05
ATMEL [ ATMEL ]
