Motorola DSP56305 Manuel d'utilisateur télécharger pdf (Page 51)

4-2 Optimizing DSP56300/DSP56600 Applications MOTOROLA

Using the DMA

Conserving Core MIPS by Working In Parallel

The core may contend with the DMA in one of two cases only:

1. Accessing the same internal memory block (contiguous 256

RAM words or 3 K ROM words), in which case the DMA

stalls—otherwise simultaneous core and DMA access to the

internal memory is possible without any delays

2. Accessing an external address through Port A, in which case

either the core or the DMA stalls, depending upon the

programmed priority

These restrictions are not severe and allow high utility of the

parallel potential.

The following example demonstrates a double-buffering scheme.

The DMA loads one buffer from the external memory while the core

processes the data block loaded previously into a second buffer.

When the core finishes, the DMA fills the second area while the core

processes the data block that the DMA has just loaded. The DMA

reads the data from an external memory using DMA channel 0. The

block size is BLOCK_SIZE data words. The core uses R0 as the

pointer to the data area under work, and R1 to point to memory

locations where the top addresses of the two memory areas are

stored. Modulo 1 Addressing mode is used with R1 to quickly load

R0 with the block address and switch between the two memory

areas. In this application, it is up to the user to stop processing the

data in mid-block if the data transferred is not an exact multiple of

the block size.

;============ general definitions and initialization.

;initial address of the first data area

BASE_AREA1 equ 512

BASE_AREA2 equ 1024 ;initial addr. of 2nd data area

BLOCK_SIZE equ 512 ;size of each data area

AREA_POINTER equ 16 ;two consecutive addresses where the

;values BASE_AREA1,2 will be stored for

;use every area switch.

;the base address of the external

;memory from which the DMA will load

EXTERNAL_BASE equ 32768

;total size of the memory to be loaded

TOTAL_DATA_SIZEequ 51200 ;

;check if TOTAL_DATA_SIZE is divided exactly by BLOCK_SIZE

IF ((TOTAL_DATA_SIZE%BLOCK_SIZE)==0)

NUMBER_OF_TRANSequTOTAL_DATA_SIZE/BLOCK_SIZE

ELSE

NUMBER_OF_TRANSequ(TOTAL_DATA_SIZE/BLOCK_SIZE)+1

ENDIF

move #AREA_POINTERS,r1

1 2 ... 46 47 48 49 50 51 52 53 54 55 56 ... 111 112

Commentaires sur ces manuels

Pas de commentaire

Motorola DSP56305 Manuel d'utilisateur Page 51

Commentaires sur ces manuels

Produits connexes et manuels pour Processeurs Motorola DSP56305