Introduction - Motorola DSP56009 User Manual

External Memory Interface

Introduction

4.1 INTRODUCTION
The External Memory Interface (EMI) enables the DSP to access external dynamic
and/or static memory with no (or minimal) additional logic. The EMI permits simple
implementation of data-delay buffers in external memory and is often used for audio
sample storage, as required by digital reverberation algorithms. The EMI is designed
to connect directly to one or two page-mode DRAM devices of the following sizes: 64
K × 4, 256 K × 4, 1 M × 4, and 4 M × 4 bits. When using SRAMs, the EMI can directly
access up to 256 K × 8 bits. The data bus width can be 4- or 8-bits wide. Data words of
8-, 12-, 16-, 20- or 24-bits can be stored and retrieved via the EMI with automatic
packing and unpacking. In addition, the EMI can be configured to operate in the
Absolute Addressing mode. This allows connection to external memory devices for
program bootstrap and data storage, as well as general parallel access to external
memory-mapped peripheral devices.
4.1.1 Theory of Operation
The DSP views the EMI as a memory-mapped peripheral. The EMI functions as a
memory-mapped peripheral in which data transfers are performed by moving data
to/from data registers, and control is exercised by polling status flags in the
control/status register or by servicing interrupts. An external memory write is
executed by writing the data into the EMI Data Write Register (EDWR). This will
trigger the EMI operation in which the EDWR contents are transferred to the external
memory device. The EDWR is free for the next write operation when signalled by a
status bit or by an interrupt request. An external memory read is triggered by either
writing to the EMI Offset Register (EOR) or reading the EMI Data Read Register
(EDRR). This will trigger an EMI read operation in which the data is read from the
external memory device and is stored in the EDRR. The end of operation is signaled
by a status bit or by an interrupt request.
MOTOROLA DSP56009 User's Manual 4-3