
SDRAM Controller
MCF5272 ColdFire
®
Integrated Microprocessor User’s Manual, Rev. 3
9-6
Freescale Semiconductor
9.4
SDRAM Banks, Page Hits, and Page Misses
SDRAMs can have up to four banks addressed by SDBA1 and SDBA0. The two uppermost address lines
of the memory space are mapped to SDBA1 and SDBA0. Specific address lines mapped depend on the
size of the SDRAM array and are defined in the SDCR.
Each of the four bank address registers holds the page address (lower bits of row address) of an activated
page. Each bank can have one open page. A device with two banks can have two open pages. A device
with four banks can have four open pages.
The lower addresses of the row address are compared against the page address register content. If it does
not match, the SDRAM controller precharges the open page on the accessed bank and activates the new
required page. After this, the SDRAM controller executes the
READ
or
WRITE
command. Concurrently, the
page address register of the bank is updated. This is called a page miss.
After a bank is activated, it remains activated until the next page access causing a page miss.
A precharge of a deactivated bank is allowed and simply ignored by the SDRAM.
If a memory access is to an open page only the
READ
or
WRITE
command is issued to the SDRAM. This is
called a page hit.
In two-page SDRAMs, banks 2 and 3 are invalid and must not be addressed. To avoid address aliasing, the
user should restrict the chip select address range to the space available in the SDRAMs.
9.5
SDRAM Registers
The SDRAM configuration register (SDCR) and the SDRAM timing register (SDTR) are described in the
following sections. Note that SDRAM provides a mode register that is not part of the SDRAM controller
memory model. The SDRAM mode register is automatically configured during initialization.
9.5.1
SDRAM Configuration Register (SDCR)
SDCR is used to configure the SDRAM controller address multiplexers for the type of SDRAM devices
used on the system board.
describes SDCR fields.
15
14
13
12
11
10
8
7
6
5
4
3
2
1
0
Write
—
MCAS
—
BALOC
GSL
—
REG INV SLEEP ACT INIT
Reset
0
00
00
001
0
00
0
1
0
0
0
R/W
Read/Write
Read-only
R/W
Addr
MBAR + 0x0182
Figure 9-3. SDRAM Configuration Register (SDCR)