Motorola DSP56305 Manuel d'utilisateur télécharger pdf (Page 46)

Program Control

Using Fast Interrupts

MOTOROLA Optimizing DSP56300/DSP56600 Applications 3-17

instructs the assembler to try and compact the argument into a

1-word opcode. Without it, the assembler may use the 2-word

version.

Note: If a 2-word opcode is used, the value of _CONT1 is 3 (and not

2). This is because the extra word pushes the RTS instruction

address one position forward.

3.7 USING FAST INTERRUPTS

Once the PIC (Program Interrupt Controller) decides to pass an

interrupt request and interrupt the core, two instruction words are

fetched from the interrupt vector space and enter the pipeline for

execution. As explained in Section 7 of the

DSP56300

and

DSP56600 Family Manuals

, interrupt execution is of two types:

• Fast interrupts—None of the two interrupt words is a

“change of flow” instruction (JSR, JSCLR, etc.). The program

controller automatically continues to fetch instructions from

the address at which it was interrupted and execution

resumes with no software overhead. There is no pipeline

flush or stall— the pipeline behaves as if the two interrupt

words were originally part of the program sequence.

• Long interrupts—If one of the instructions is of a

change-of-flow type, execution cost is much higher.

Normally, the minimum activity includes a jump to a

subroutine, which is relatively a long instruction since it

pushes data to the stack. Normally at the subroutine end

there is a corresponding “RTI” instruction that pops data

from the stack and reconstructs the PC and the SR.

The pipeline is optimized for very high performance (minimum

stall) for fast interrupts, so the user is advised to try using them

whenever possible. Some specialized instructions were added to the

instruction set in order to help the user perform many operations

using fast interrupts. Also for this reason, many instructions have

1-opcode versions, generally at the expense of argument.

A frequently encountered interrupt activity is driving data to and

from a peripheral device triggered by an interrupt at the

peripheral's request. For example, a serial peripheral interrupts the

core when data is received, and expects it to be moved (generally to

a memory location), or the interrupt occurs when the serial device is

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Motorola DSP56305 Manuel d'utilisateur Page 46

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