Spd

What is SPD (Serial Presence Detection)?

SPD, or Serial Presence Detection, basically refers to the Read Only Memory (ROM) chip integrated on the memory modules to report different properties of the module such as its speed, size and more.

Technically, it refers to an added chip in the memory that holds 128 Hex bytes of info about it.

It also refers to a specific communication method followed by the DIMMs with the BIOS by using two special pins to transmit the data serially.

Understanding SPD (Serial Presence Detection)

What is SPD

Serial Presence Detection is actually the info held in the Electrically Erasable Programmable Read Only Memory (EEPROM) chip during the boot up process of the computer system.

Ideally, the SPD communicates different information regarding the features and capacities of the memory module to the BIOS, such as:

The information is typically placed on the EEPROM chip by the manufacturer of the memory module.

If there is no such SPD information available in the BIOS, the computer will assume it all and continue with the bootup process.

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However, all these are necessary for the configuration of the memory controller of the module to ensure optimum performance and reliability.

It is also necessary for the BIOS to identify the memory module correctly during POST or Power On Self Test so that the motherboard may know what specific timings to be used based on the characteristics of the memory chip.

Typically, the Parallel Presence Detection (PPD) method was followed with the previous 72-pin SIMMs.

The standard was changed to Serial Presence Detection in the 168-pin DIMM models and later at the same time when the SDRAM was introduced.

This specific standard is more useful than the earlier PPD method because it can encode a lot more information in comparison.

Data is communicated by the Dual Inline Memory Modules or DIMMs in this method by using two pins instead of one, which helps in transferring data in serial as opposed to the former parallel method, where each pin denoted a single bit.

Key Features

Memory SPD Failure

SPD failures or errors are typically the results of the conflicts between the Basic Input Output System (BIOS) setting and the Random Access Memory (RAM) timings.

There can be different types of SPD failures, for example:

Some computer systems in which the memory modules are soldered directly to the motherboard may not have an SMBus interface.

This will not allow them to access the raw SPD data usually hardcoded in the BIOS firmware directly and result in such a failure.

Also, in systems running on an Intel Xeon chipset with Temperature Sensor on DIMM (TSOD) support, the hardware TSOD polling may interfere with the process of collecting SPD data.

This will result in incorrect or missing SPD information and, eventually, in an SPD failure.

Apart from that, in a few specific platforms, such as in ASUS, SPD Write is disabled by default. This will once again prevent accessing the raw data in SPD and result in a failure.

All such failures and errors may result in data corruption and even unpredictable system crashes.

Where is SPD in the BIOS?

Though in a few specific BIOS settings you may get the SPD located under the Extreme Tweaker tab, such as in ASUS, ideally, it resides on the Synchronous Dynamic Random Access Memory (SDRAM) module and communicates with the BIOS.

Conclusion

The SPD or Serial Presence Detection helps in communicating the features, capacity and characteristics of the memory module used in a computer system to the BIOS.

This helps the BIOS to identify the memory much more easily during POST. SPD with a high speed makes the computer system run much faster.