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IDT DDR Controller
DDR Registers
79RC32438 User Reference Manual
7 - 14
November 4, 2002
Notes
DDR 0 Alternate Mask Register
Figure 7.8 DDR 0 Alternate Mask Register (DDR0AMASK)
DDR 0 Alternate Mapping Register
Figure 7.9 DDR 0 Alternate Mapping Register (DDR0AMAP)
DDR Data Bus Multiplexing
The DDR controller supports data bus multiplexing when the Data Bus Multiplexing (DBM) bit is set in
the DDRC register. Data bus multiplexing allows the RC32438’s 16-bit or 32-bit data bus
1
to interface to
DDR memory systems having a data bus width of 64-bits. This is necessary when interfacing the RC32438
to standard DDR memory modules such as DDR DIMMs and SODIMMs.
To support data bus multiplexing, external bus switches must be placed between the RC32438 and
external DDR memory banks. These bus switches are used to isolate unused data bits and strobes from
the RC32438 allowing 16-bit or 32-bit data quantities to be read from a 64-bit bus. The RC32438
DDROEN[3:0] pins are output enabled for these buffers.
MASK
Description:
Address Mask.
This field determines which bits of the upper 16-bits of the address participate in
address comparisons. When a bit is set in this field, then the corresponding address bit partici-
pates in address comparisons. When a bit is cleared in this field, then the corresponding address
bit is masked and does not participate in address comparisons.
When the MASK field is zero, the alternate DDR space is disabled and does not appear in the
memory map.
Initial Value:
0x0
(this field is not modified due to a warm reset)
Read Value:
Previous value written
Write Effect:
Modify value
MAP
Description:
Map Address.
This field contains the DDR mapping address for transactions mapped to DDR
chip select zero using the alternate address mapping range. Address bits that participated in
address comparison are substituted with values in this field.
Initial Value:
0x0
(this field is not modified due to a warm reset)
Read Value:
Previous value written
Write Effect:
Modify value
1.
Mode is determined by the state of the Data Bus Width (DBW) bit in the DDRC register.
DDR0AMASK
0
31
16
MASK
16
0
DDR0AMAP
0
31
16
16
MAP
0