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SDRAM SPD Data Standards
The Serial Presence Detect (SPD) device is a small 8-pin 256-byte EEPROM chip, mounted on a 168-pin DIMM module or a 144-pin SO DIMM module. It contains important information about the module's speed, size, addressing mode and various other parameters, so that the motherboard memory controller (chipset) can better access the memory device. The SPD contents are controlled by the JEDEC standard. The Intel PC-100 standard uses the same data content, but includes Intel's distinct settings in the bytes designated for Vendor Specific entries. The PC133 standard was developed by a group of companies led by Via Technologies, IBM Microelectronics, Micron Semiconductor, NEC and Samsung. It is essentially based on the INTEL PC-100 specification with the enhancement of 133MHz operation at a Cas Latency of 3 (CL=3) and access time from clock (Tac) of 5.4nS.
This application note illustrates the JEDEC standard as well as the variance that is used by Intel and the PC133 group for 168-pin SDRAM DIMMs. It further explains RAMCHECK's screen indications regarding SPD data.
SPD CONTENTS - JEDEC Standard
The following are the SPD contents assigned for 168-pin SDRAM DIMMs as listed from the JEDEC standard No. 21-C. For ease of use, we have added typical CAS Latency values to the table itself.
Byte 126 - Intel specification for frequency
The JEDEC standard calls for Byte 126 and Byte 127 to be vendor specific, Intel uses these two bytes in the following manner.
This byte defines the clock frequency of the Intel SDRAM DIMM specification.
For 100MHz support, this byte should be programmed to 64h. For 66MHz support, this byte should be programmed to 66h, which is required for backward compatibility with existing BIOS code.
Byte 127 - Intel Specification details for 100MHz Support
This byte defines the SDRAM component and Clock interconnection details for the DIMMs as defined:
Bit 2 and Bit 1=CL3 and CL2 support as shown:
Typical PC-100 Timing Values
The following values appears in most PC-100 modules:
For CAS Latency 3, byte 9=A0h to indicate 10nS cycle time, and byte 10=60h to indicate 6nS access time from clock. Please note that the letter 'h' after 'A0' or '60' is used to indicate hexadecimal values.
If the high speed CAS Latency 2 is supported (as required, for example, for the AMD Athlon processors), byte 23=A0h to indicate a 10nS cycle time (at CL=2), and byte 24=60h to indicate a 6nS (or=65h to indicate 6.5nS) access time from clock. However, we have found that byte 24 may be left at 0 by some manufacturers.
Tthe SPD data of a PC133 modules is similar to the PC-100 SPD data, with the exception of byte 9 (SDRAM device cycle time CL=3) set to 75h (7.5nS) and byte 10 (SDRAM device access time from clock at CL=3) set to 54h (5.4nS).
In many PC-133 modules in the market we have found that the CAS Latency 2 speed (byte 23 and 24) are left at 0, as PC-133 is essentially supported for CAS Latency 3.
Byte 126 may be marked with 64h like a PC-100 module, or by 85h, as shown in the table above.
RAMCHECK identifies the setting for the Intel standard by reading Byte 126. RAMCHECK's display will reflect this setting in the test log, and also in the form of a summary screen after the Basic Test as shown below.
Other messages will include the following:
Please note that whenever RAMCHECK uses the notation "SPD=", it refers to information read from the SPD chip. As newer firmware revisions are developed, you will note that RAMCHECK will measure additional characteristics of the memory device. The acquired values will then be compared to the SPD data that is read and subsequently be displayed in the test log for comparative purposes.
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