SPD EEPROM Specification
In addition to the description of the transmission interface, I2C information, the PC SDRAM Serial Presence Detect (SPD) Specification specification defines the information recorded in the EEPROM by the manufacturer. The need for such a specification was caused by the fact that the main problems when installing SDRAM in computer systems are associated with incorrect interaction between the BIOS on the motherboard and the EEPROM on the memory module.
The fact is that many Taiwanese motherboard manufacturers using the Intel 440BX chipset install their own BIOS, which has the function of disabling the reading of information from the EEPROM. In this case, the system parameters are set manually when entering SETUP, or the BIOS itself sets the system parameters for working with standard memory.
EPOS specialists quite often encounter situations when motherboards (for example, Intel) “refuse” to work with memory modules that do not have a correctly programmed EEPROM memory:
EEPROM may not be correctly programmed by the manufacturer.
A chipset running an incorrect BIOS may incorrectly set system parameters.
To eliminate operational errors, Intel has established uniform rules for all manufacturers. Each manufacturer must enter information about himself in bytes 61 … 127 EEPROM. In accordance with the technical requirements of Intel SPD EEPROM must be correctly programmed by the manufacturer and never change. Therefore, during normal operation with the SDRAM module, the possibility of accidentally erasing or changing data is excluded. This is a kind of insurance against the use of slow DIMM modules in Intel systems. However, despite all the precautions, manufacturers write data to the EEPROM with obviously false information. EEPROM is like paper on which anything can be written.
The number of major manufacturers of memory chips (brand-name-manufacturers) is limited. In order to distinguish them from other manufacturers “with a name”, they are also called Major-brands, or simply “major”. The vast majority of “majors” also produce memory modules based on their own chips, which in the SDRAM industry are considered quality standards and are called Major / Major (“major in major”). Such firms include such well-known global manufacturers as Samsung, Hyundai, LG, Siemens, which most accurately fulfill all the requirements of the PC SDRAM Serial Presence Detect (SPD) specification (revision 1.2A, December, 1997).
Perhaps, with truly German scrupulousness and pedantry, Siemens approaches the programming of SPD EEPROM. The following is the programming table for the SPD memory modules manufactured by Siemens. One of the most important characteristics for the user is the memory performance, which is determined by the latent state of CAS CL and described by two bytes – 18th and 27th. An equally important parameter is the type of memory defined by byte 2. If “02hex” is in this byte, then the memory is EDO, if “04 hex” is SDRAM. The actual DIMM speed is contained in byte 126. For a clock frequency of 100 MHz, its value should be “64 hex”. In case the value is “66hex” – this DIMM is 66 MHz. The SPD-EEPROM version number is located in byte 62. The current version is 1.2 (December 1997).
For buyers of SDRAM-modules, as well as for companies-builders of computers, information that is in separate digits is important. Having this information, you can determine the manufacturer of the SDRAM module, the date of its manufacture, serial number and recommended frequencies of its use.