SPD (Serial Presence Detect)

Serial Presence Detect (SPD) refers to a standardized way to automatically access information about a computer memory module. It is the serial version of Parallel Presence Detect (PPD).

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. (There are three additional pins to provide each EEPROM with a unique device address.) The same SMBus is commonly used on motherboards for system health monitoring tasks such as reading power supply voltages, CPU temperatures, and fan speeds.


Enhanced Performance Profiles (EPP)
EPP is an enhancement of SPD and provides additional performance-specific information needed for overclocking applications. The EPP information is stored in the same EEPROM, but in bytes 99-127 which are unused by standard SPD.