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UG129 (v1.1.2) June 24, 2008
Example Interrupt Flow
R
3. The PicoBlaze microcontroller recognizes the interrupt and preempts the ADD s0,s1
instruction. The current PC, which points to the ADD s0 s1 instruction, is pushed
onto the CALL/RETURN stack. Likewise, the ZERO and CARRY flags are preserved.
Furthermore, the INTERRUPT_ENABLE flag is cleared disabling any further
interrupts. Finally, the PC is loaded with all ones (3FF) and the PicoBlaze
microcontroller performs an interrupt service routine call to the last location in the
instruction store. If using a 1Kx18 block RAM for instruction store, the last location is
3FF. If using a smaller instruction store, then the interrupt vector is still located in the
last instruction location. The PicoBlaze microcontroller also asserts the
INTERRUPT_ACK output, indicating that the interrupt is being acknowledged.
4. The interrupt vector is always located in the last location in the instruction store. In this
example, the program jumps to the interrupt service routine (ISR) via the JUMP isr
instruction.
5. When completed, exit the interrupt service routine (ISR) using the special RETURNI
instruction. Do not use the RETURN instruction, which is used with normal subroutine
calls. The RETURNI ENABLE instruction returns from the interrupt service routine and
re-enables the INTERRUPT input, which was automatically disabled when the
interrupt was recognized. Using RETURNI DISABLE also returns from the interrupt
service routine but leaves the INTERRUPT input disabled.
6. The RETURNI instruction restores the preserved ZERO and CARRY flags saved
during Step (3). Likewise, the RETURNI instruction pops the top of the
CALL/RETURN stack into the PC, which causes the PicoBlaze microcontroller to
resume program executing the instruction that was preempted by the interrupt, ADD
s0,s1 in this example.
Figure 4-3: Interrupt Timing Diagram
ADD s0,s1INPUT s1,01 JUMP isr TEST s7,02
3FF isr
Address of
ADD s0,s1
...
Interrupt
recognized
Call to interrupt
vector, assert
INTERRUPT_ACK
Jump to interrupt
service routine
ADD s0,s1 instruction
pre-empted. PC saved to
stack. Flags preserved.
Interrupt disabled.
Begin executing
interrupt service
routine
5 clock cycles
CLK
INSTRUCTION
ADDRESS[9:0]
INTERRUPT
INTERRUPT_ACK
PREEMPTED
CALL/RETURN
Stack
Preserved
ZERO Flag
ZERO
Flag
Preserved
CARRY Flag
CARRY
Flag
INTERRUPT_ENABLE
1
0
UG129_c4_03_051404
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