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CPU On-Die Termination

Common Options : Auto, 75 Ohms, 150 Ohms, 300 Ohms

Quick Review

The CPU On-Die Termination BIOS feature controls the impedance value of the termination resistors for the processor's on-die memory controller. This is different from DRAM Termination, which controls the impedance value of the termination resistors in the DDR2 / DDR3 chips. However, both work in tandem to reduce signal reflections on the memory bus.

You have a choice of three impedance values -75 Ohms, 150 Ohms and 300 Ohms. Sometimes, there is also an Auto option, which uses the default impedance value, or enables the auto-calibration circuit (if one's available).

A lower impedance value improves the resistor's ability to absorb signal reflections and thus improve signal quality. However, this comes at the expense of a smaller voltage swing for the signal and higher power consumption.

The proper amount of impedence depends on the memory type and the number of DIMMs used. Therefore, it is best to contact the memory manufacturer to find out the optimal amount of impedance for the particular set of memory modules. If you are unable to obtain that information, you can also follow these guidelines :

  • Single memory module / channel : 300 ohms
  • Two memory modules / channels : 150 ohms
  • Four memory modules / channels : 75 ohms

However, these guidelines are not foolproof. If you are unsure, you can try using a low impedance of 75 ohm and adjust upwards if you face any stability issues.


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