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
124
Section 6: DRAM
Document
1250_1125-UM100CB-R
The FCRAM includes a write mask or variable write (VW) function. The memory controller always does a full
burst write and will set the bits accordingly (A[14:11] = 4'b1010 during LAL command).
DIMMs
All standard DDR SDRAM DIMMs of 64+8bit data path are supported, except those that use x4 parts (which
replace DM pins with extra DQS pins). Both registered and unbuffered DIMMs can be used (for registered
DIMMs the timing parameters must be adjusted to take account of the additional cycle for the latch on the
address and command lines). The controller provides four chip selects per channel, allowing two DIMM slots
that use dual physical bank modules (i.e. need two chip selects) or four DIMMs with single physical bank
modules (each using a single chip select). Software should read the DIMM configuration information and
program the memory controller accordingly.
On a single channel it is not possible to mix registered and unregistered DIMMs.
L
ARGER
M
EMORY
S
YSTEMS
The memory controller can support larger memory systems by externally decoding 8 chip selects per channel.
This may limit the maximum speed of the memory and reduces the number of available configurations. The
increased loading requires use of registered DIMMS in the large memory system. This mode is enabled by
setting the large_memory bit in the DRAM type field of the
mc_drammode
register. Application Note 1250-
AN600-R “BCM1250 Big Memory System” describes the design and implementation of a board using the large
memory extension.
The extension works by configuring the memory controller to only use two chip selects, and driving two
additional address bits on the other two chip select outputs. Externally each of the chip selects is used to
enable a 2-to-4 decoder to generate the actual chip selects for the physical memory banks (the delay through
the decoder must be taken in to account when timing analysis is done, but since the chip select signals will be
registered on the DIMM this should not be a critical timing parameter). Internally the memory controller
accounts for the switch of chips when the decoded chip select changes, but it only keeps track of the standard
four open pages for the two internal chip selects (potentially degrading performance by not making use of open
pages).
The external connections are:
•
M_CS_l[0] and M_CS_l[1] are the two chip select lines. They should be connected to the active low
enable input of the two halves of a dual 2-to-4 decoder.
•
M_CS_[[3:2] provide two additional address bits to qualify the chip selects. They should be connected to
the address inputs of the 2-to-4 decoder
•
The (active low) outputs of the 2-to-4 decoder provide 8 chip selects for DRAMs.
The internal configuration should be:
•
The full address space should be covered using the
mc_cs0
and
mc_cs1
address selection registers.
The
mc_cs2_start
,
mc_cs2_end
,
mc_cs3_start
and
mc_cs3_end
registers must all be set to zero.
•
Two additional row address bits (corresponding to row_addr[14:13]) should be set in the
mc_cs0_row
and
mc_cs1_row
registers. These address bits will be output on the M_CS_l[3:2] pins to qualify the chip
select. Note that these bits must come from the memory address bits [34:28].
•
The large_memory bit in the
mc_drammode
register should be set.
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