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IDT DDR Controller
DDR Registers
79RC32438 User Reference Manual
7 - 15
November 4, 2002
Notes
When data bus multiplexing is enabled, the address range allocated to a DDR chip select should be
expanded to twice the space allocated in a 32-bit mode system or four times the space allocated in a 16-bit
mode system by programming the corresponding base and mask registers. The 32-bit Mode section in
Figure 7.10 illustrates this address range expansion for a 32-bit mode system using 32M x 8 x 4 bank (i.e.,
1Gb) DDRs. Eight of these DDRs create a 64-bit data bus that interfaces to the RC32438’s 32-bit DDR data
bus through external bus switches as shown in Figure 7.11. The total space allocated for the DDR chip
select is 1GB. When an access is made to the lower 512 MB, the DDROEN[1:0] signals are asserted.
During writes to this region, the DDRDM[3:0] signals are used to select enabled byte lanes. When an
access is made to the upper 512 MB, the DDROEN[3:2] signals are asserted and DDRDM[7:4] are used to
select enabled byte lanes during writes.
Memory Range
Memory Range
Figure 7.10 DDR Data Bus Multiplexing Address Range Expansion
Figure 7.11 32-bit Bank DDR Data Bus Multiplexing
Operation in 16-bit mode parallels that in 32-bit mode. Using the same DDR devices as in the above
example, a 16-bit mode system with data bus multiplexing has 4 regions per chip select as shown in the 16-
bit mode section of Figure 7.10. Eight of these DDRs create a 64-bit data bus that interfaces to the
RC32438’s 16-bit DDR data bus through external bus switches as shown in Figure 7.12. When an access is
made to the lower 256 MB, DDROEN[0] is asserted and DDRDM[1:0] are used to select byte lanes during
writes. When an access is made to the next 256 MB, DDROEN[1] is asserted and DDRDM[3:2] are used to
select byte lanes during writes. The pattern continues for the upper two memory regions.
256 MB
DDRDM[1:0]
DDROEN[0]
256 MB
DDRDM[3:2]
DDROEN[1]
256 MB
DDRDM[5:4]
DDROEN[2]
256 MB
DDRDM[7:6]
DDROEN[3]
512 MB
DDRDM[3:0]
DDROEN[1:0]
512 MB
DDRDM[7:4]
DDROEN[3:2]
32-bit
Mode
16-bit Mode
External
DDR
Bank
(32-bits)
DM
DQS
D
B
OE
B
OE
RC32438
4
4
32
4
DDRDM[3:0]
DDRDQS[3:0]
DDRDATA[31:0]
DDRDM[7:4]
DDROEN[1:0]
DDROEN[3:2]
2
2
CS
External
DDR
Bank
(32-bits)
DM
DQS
D
CS
DDRCSNx