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
120
Section 6: DRAM
Document
1250_1125-UM100CB-R
Combining the bank, row and column masks with the chip select selection allows for more complicated mixing
on a single channel.
shows a configuration using 128 MByte single chip select DIMM on chip select
0, and a 128 MByte dual chip select DIMM (also called a two bank DIMM using the term bank to refer to the
physical banks) on chip select 1 and 2. Mixed-cs mode is used to allow the first DIMM to be standalone and
the other to be interleaved. In this case the switch between the two physical banks is made based on address
bit 15, so the full column addressed page is contiguous in the address space.
C
HOOSING
I
NTERLEAVE
P
ARAMETERS
The best method of interleaving the memory is highly system dependant, so there is no single rule about how
the memory controller should be configured. However, there are some general guidelines that give a starting
point. The general goal is to maximize the number of banks (internal and physical) of the memory that are in
use, which in turn leads to more overlap of transactions and less unused time on the channels. There are
different costs involved in switching between banks: if the driver of the databus switches from one device to
another (a change in physical banks) there must be an additional turnaround cycle which is not needed for a
bank to bank switch in the same device (a change in internal banks). There are several measurable
parameters: memory bandwidth and average access latency are normally the most important, but maximum
access latency may also be important. Some things (e.g. a faster memory clock) will improve all of them, but
many of the options improve one at the expense of others. In particular reductions in average memory latency
are often at the expense of increasing the maximum latency.
In most cases the system will use more than one physical bank of memory. These can be put on different chip
selects on the same channel or on the different channels. From a performance standpoint using both memory
channels is almost always the correct choice. The peak memory bandwidth increases because the two
channels have physically separate data buses and operate independently of one another. In some cases using
both channels may not be possible on the BCM1250 (for example if the system has a single DIMM slot then a
two physical-bank DIMM is necessarily on a single channel), but both channels should be used if possible (to
continue the example: a second DIMM slot should go on the other channel in preference to using the remaining
two chip selects of the first channel). On the BCM1125/H, two DIMM slots with two physical-bank DIMMs
should be used if possible and full chip select interleaving should be enabled.
Table 67: Example for 128 MByte + 64 MB + 64 MB Mixed_CS Mode
CS0
Row Bits [26:15], Column Bits [14:9,6:5,4:3], Bank Bits [8:7]
Row
00000000_00000111_11111111_10000000_00000000
Column
00000000_00000000_00000000_01111110_01100000
Bank
00000000_00000000_00000000_00000001_10000000
CS1,2
Row Bits [26:16], CS Interleave Bit [15], Column Bits [14:9,6:5,4:3], Bank Bits [8:7]
CS Interleave
00000000_00000000_00000000_10000000_00000000
Row
00000000_00000111_11111111_00000000_00000000
Column
00000000_00000000_00000000_01111110_01100000
Bank
00000000_00000000_00000000_00000001_10000000