MSC8144E Reference Manual, Rev. 3
12-2
Freescale
Semiconductor
DDR SDRAM Memory Controller
12.1
Architecture
The DDR SDRAM controller controls processor and I/O interactions with system memory. It
supports JEDEC-compliant DDR1 and DDR2 memories.
Note:
A bank is a physical bank specified by a chip select; a logical bank is one of the four or
eight sub-banks in each SDRAM chip. A sub-bank is specified by the two or three bits
on the memory bank address (
MBA
) pins during a memory access. The memory
interface supports two physical banks of 32/40-bit words or 16/24-bit wide memory.
As shown in Figure 12-1, requests are received from the internal mastering device, and the
address is decoded to generate the physical bank, logical bank, row, and column addresses. The
transaction is then loaded into the input staging queue with the decoded information. The lower
two entries of the input queue are compared with values in the row open table to determine
whether the address maps to an open page. If the address from either entry does not map to an
open page, an activate command is issued for that entry, with the lowest entry having priority.
Commands are always issued from the lowest input queue entry.
Programmable parameters give a variety of memory organizations and timings. Using optional
error checking and correcting (ECC) protection, the DDR memory controller detects and corrects
all single-bit errors, detects all double-bit errors within the 32-bit or 16-bit data bus, and detects
all errors within a nibble. The controller allows as many as 16 pages to be open simultaneously.
The amount of time (in clock cycles) the pages remain open is programmed via the
DDR_SDRAM_INTERVAL[BSTOPRE] bit (see Table 12-27 on page 12-49).
Read and write accesses to memory are burst oriented; accesses start at a selected location and
continue for four higher locations. Accesses to closed pages start with the registration of an
ACTIVE
command followed by a
READ
or
WRITE
. Accessing open pages does not require an
ACTIVE
command. The address bits registered with the activate command specify the logical bank and
row to be accessed. The address coincident with the
READ
or
WRITE
command specify the logical
bank and starting column for the burst access.
The data interface is source synchronous, so the source of the data provides a clocking signal to
synchronize data reception. These bidirectional data strobes (
MDQS[0–4]
) are inputs to the
controller during reads and outputs during writes. The DDR SDRAM specification requires the
data strobe signals to be centered within the data tenure during writes and to be offset by the
controller to the center of the data tenure during reads. These delays are implemented by the
DDR SDRAM memory controller for both reads and writes. The address and command interface
is also source synchronous, although 1/8 cycle adjustments are provided for adjusting the clock
alignment. When ECC is enabled, 1 clock cycle is added to the read path to check ECC and
correct single-bit errors. ECC generation does not add a cycle to the write path.
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