Emi Data Write Registers (Edwr) - Motorola DSP56009 User Manual

4.2.4

EMI Data Write Registers (EDWR)

The EMI Data Write Register (EDWR) is a single 24-bit write-only register that is
mapped to two memory addresses and is used when writing data to memory. Data
to be transferred to external memory is written into either EDWR0 or EDWR1. The
contents of the written register are transferred to the Data Register Buffer for
memory writes. All transfers to/from EDWR0 or EDWR1 are 24-bit transfers.
Writing to EDWR0 ($FFEA) triggers an EMI memory write operation that will use
EBAR0 and EWOR to generate the word address. Writing to EDWR1 ($FFEA)
triggers an EMI memory write operation that will use EBAR1 and EWOR to generate
the word address.
4.2.5 EMI Data Read Register (EDRR)
The EMI Data Read Register (EDRR) is a single 24-bit read-only register that is
mapped to two memory addresses and is used when reading data from memory.
Data to be received from external memory arrives in the EDRR, and can be read from
either the $FFEA (EDRR0) or the $FFEE (EDRR1) memory location. The contents of
the Data Register Buffer are transferred to the EDRR at the end of a memory read if
the EDRR is empty. All transfers to/from the EDRR are 24-bit transfers. Reading
EDRR0 ($FFEA) when the ERTS bit (in the ECSR) is set triggers an EMI memory read
operation that will use EBAR0 and EOR to generate the word address. Reading
EDRR1 when the ERTS bit (in the ECSR) is set triggers an EMI memory read
operation that will use EBAR1 and EOR to generate the word address. See Section
4.2.3 EMI Offset Register (EOR) on page 4-8 for a description of the operation when
ERTS is cleared.
4.2.6 EMI Data Register Buffer (EDRB)
Data pack and unpack procedures during memory accesses are performed in the
24-bit EMI Data Register Buffer (EDRB). Since the EMI data bus is either 4- or 8-bits
wide and the words for transfer are 8-, 12-, 16-, 20-, or 24-bits wide, the data word
must be sliced (unpacked) into data-bus-width segments for storage in memory, or
must be packed when reading memory. When writing 8-bit or 16-bit words to
external memory, only the most significant 8 or 16 bits of the EDRB contents are
transferred. When reading 8-bit or 16-bit words from external memory, the words are
left-aligned and zero-extended (to the right) before being transferred to the EDRR.
When writing 12-bit or 20-bit words to external memory via a 4-bit data bus, only the
most significant 12 or 20 bits of the EDRB contents are transferred. Similarly, when
MOTOROLA DSP56009 User's Manual
External Memory Interface
EMI Programming Model
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