AMD Geode LX CS5536 Manual - Page 2

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3.0

Solution

The method for initializing DDR2 memory on the processor

is to insert a CPLD and quick switches in the address and

BA signals. Figure 3-1 shows a block diagram of this

design. During initialization, the Enable signal opens

(default) the switches. BIOS tells the CPLD what pattern to

assert on the BA[n] and A[n] signals. Upon completion,

BIOS tells the CPLD to close the switches, giving control

over BA[n] and A[n] to the processor. Additional physical

and electrical details of the design are beyond the scope of

this document.

3.1

Hardware

This section explains the details of the initialization. First it's

important to delineate two unique versions of this hardware

technology.

3.1.1

On-DIMM Design

This hardware form-factor has a DDR pin assignment (only

SO-DIMM as of this writing), but contains DDR2 SDRAM

modules, and the CPLD. This type of design will be attrac-

tive for customers wanting to upgrade existing systems.

The only board change required is a lower memory volt-

age.

AMD Geode™

LX Processor

CS5536

Figure 3-1. AMD Geode™ LX Processor DDR2 Block Diagram

A[13]

A[12:0], BA[1:0]

Quick

Switches

Enable

I2C

CPLD

Because the CPLD is contained on the DIMM assembly,

the only bus available for communication is I2C. The

CPLD's I2C address is A0/A1 (i.e., the same as DIMM0).

The CPLD also contains the SPD information.

Also note that the CPLD uses CKE as its RESET# signal.

As a result, the list of BIOS changes may require moving

the assertion of CKE (e.g., if the SPD is accessed prior to

CKE).

3.1.2

On-board Design

This type of system will have the CPLD soldered onto the

motherboard, and will be able to use certain off-the-shelf

(OTS) DDR2 DIMMs. In this case, the CPLD does not con-

tain SPD information.

Because the communication is not limited to I2C, using I/O

to send data to the CPLD simplifies the CPLD design and

speeds up initialization.

The I/O addresses selected for the AMD Geode™ LX Pro-

cessor Refresh Reference Design Kit (RDK) board are

AC10h and AC11h. This requires a modification to the Vir-

tual PCI portion of the BIOS to identify the I/O range to an

operating system. As of this writing, the CPLD claims a

range of 8 bytes (i.e., AC10h-AC17h).

A[15:14],

BA[2]

A[12:0],

BA[1:0]

AMD Geode™ LX Processor DDR2 BIOS Porting Guide

Application Note

46959A - March 2009

DDR2

SDRAM