Serial Presence Detect - Stored Information

Stored Information

In order for a memory module to support SPD, the JEDEC standards require certain parameters to be placed in the lower 128 bytes of an EEPROM located on the memory module. These bytes contain timing parameters, manufacturer, serial number and other useful information about the module. This data allows a device utilizing the memory to automatically determine key parameters of the module. For example, the SPD data on an SDRAM module might provide information about the CAS latency, allowing this to be correctly set without user intervention.

The SPD EEPROM is accessed using SMBus, a variant of the I²C protocol. This reduces the number of communication pins on the module to just two: a clock signal and a data signal. The EEPROM shares ground pins with the RAM, has its own power pin, and has three additional pins (SA0–2) to identify the slot, which are used to assign the EEPROM a unique address in the range 0x50–0x57. Not only can the communication lines be shared among 8 memory modules, the same SMBus is commonly used on motherboards for system health monitoring tasks such as reading power supply voltages, CPU temperatures, and fan speeds.

(SPD EEPROMs also respond to I²C addresses 0x30–0x37 if they have not been write protected, and an extension uses addresses 0x18–0x1F to access an optional on-chip temperature sensor.)