BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
110
Section 6: DRAM
Document
1250_1125-UM100CB-R
C
HIP
S
ELECT
A memory controller channel has four active low Chip Selects:CS[3:0]. The address range that they become
active for is configurable. On the BCM1250 if the two channels are interleaved then the chip selects for each
channel are programmed to an identical address range, which is twice the size of the memory attached to one
of the channels. Within a channel there are three methods for generating the chip selects: interleaved-CS,
msb-CS and mixed-CS.
•
msb-CS
divides the memory space into contiguous or non-contiguous sub-spaces based on address
ranges. The start and end address for each select line is individually programmable. For example CS[3]
covers the top portion of memory space and CS[0] covers the bottom portion.The memory size covered
by each CS can be different.
•
interleaved-CS
decodes the chip select from two non-MSB memory address bits, for example the lower
column bits or the lower row bits. This increases the possibilities for having active banks. In this
configuration the channel must be assigned a contiguous memory address range and each chip select
covers the same memory size.
•
mixed_CS
allows interleaving between a pair of chip selects (either CS[0] and CS[1], or CS[1] and CS[2],
or CS[2] and CS[3]) and allows the others to be set using their start and end registers. This mode allows
devices to be interleaved when only two chip selects are in use.
Figure 15: Chip Select Options
shows some chip select options. On the left msb_cs is used to separate the four chip
selects into different regions. The diagram in the center shows a full interleaved_cs, which requires all the chip
select regions are the same size. On the right mixed_cs has the SDRAMs on chip select 0 and 1 interleaved
and keeps the SDRAMs on chip select 2 and 3 separate. For the memory covered by chip selects 0 and 1 there
can be twice as many active banks (although there maybe a chip-to-chip bubble on some accesses).
3
2
1
0
3
2
1
0
3
2
1
0
3
2
1
0
1
0
1
0
1
0
3
2
1
0
3
2
1
0
msb_cs
(can have all cs different size)
mixed_cs
0 and 1 interleaved
(cs 0 and 1 same size)
interleaved_cs
(all cs same size)
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