AMD Geode LX CS5536 Handmatig - Pagina 7

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Application Note

46959A - March 2009

3.4

Other Information and Restrictions

The LX processor/DDR2 solution does not do DQS train-

ing. The LX processor's memory controller does not have

the adjustability to make this worthwhile, and the speeds

are slow enough that this is not a problem.

The memory must be organized with 4 component banks.

This means that only 512Mbits, and lower, devices are sup-

ported. In addition, the memory controller only supports 1

or 2 ranks. Any other configurations should be avoided.

The DRAM must support a CAS# Latency of 2 clocks. A

DIMM's SPD may not indicate that this is supported, due to

the higher typical operating frequencies of the memory.

The BIOS may determine that CL=2 is possible by consid-

ering the fundamental access time of the memory (i.e.,

cycle time multiplied by the normal CAS# Latency). The

reason for this restriction is that the LX processor's mem-

ory controller must operate with a Write Latency of 1 clock.

DDR2 defines CAS# Latency equal to Write Latency + 1.

AMD Geode™ LX Processor DDR2 BIOS Porting Guide

Supporting CL=2 at higher frequencies implies that the

memory must be of higher performance. To run the mem-

ory at 166MHz, this means that the memory should have

an access time of 12ns. 133MHz requires 15ns.

The typical LX processor/DDR2 implementation will run the

memory at lower frequencies than average. While 166MHz

is within the DDR2 specification, some memory vendors

may recommend operating with the DLL disabled. The cus-

tomer should investigate this with their memory supplier.

Some DRAM modules may operate at higher frequencies

by raising their supply voltage. The customer should con-

sult the memory manufacturer before taking this approach.

The BIOS may implement an algorithm that accounts for

the higher frequency. AMD has seen only limited success

with this technique.