Socket AM3

Socket AM3 is a CPU socket for AMD processors. AM3 was launched on February 9, 2009 as the successor to Socket AM2+, alongside the initial grouping of Phenom II processors designed for it. The sole principal change from AM2+ to AM3 is support for DDR3 SDRAM.

Like the previous AMD socket, the "AM3 Processor Functional Data Sheet" (AMD document number 40778) has not been made publicly available. The "Family 10h AMD Phenom™ Processor Product Data Sheet" (document 446878) has, but contains only a brief list of features of the Phenom, and does not contain any substantive technical data regarding socket AM3.

Compatibility

Socket AM3 breaks compatibility with AM2/AM2+ processors due to a subtle change in key placement. The AM3 socket has 941 pin contacts in a different layout while AM2+ processors have 940 pins. Tom's Hardware removed the two obstructing key pins from an AM2+ Phenom processor in order to fit it into an AM3 socket. The processor did not work in the AM3 socket, but still worked in an AM2+ socket, suggesting that compatibility issues run deeper than merely the key pins. It is likely because the built-in memory controller in AM2/AM2+ processors only supports DDR2 (unlike AM3 processors, which support both DDR2 & DDR3 memory). In fact, a few motherboards were manufactured that supported both DDR2 and DDR3, however only one type could be used at a time. Despite using an AM3 socket and despite using a northbridge and southbridge found on AM3 motherboards, they support any AM2, AM2+, or AM3 processor.

Socket AM3+

AM3+ is a modification of the AM3 Socket designed for CPUs which use the AMD Bulldozer microarchitecture and retains compatibility with AM3 processors. The Vishera line of AMD CPUs also all use Socket AM3+.

The AM3+ Socket specification contains a few noteworthy design changes over its AM3 predecessor. The 942 pin count for the AM3+ is an increase of one compared to the AM3 Socket layout. The AM3+ Socket has larger pin socket diameter of 0.51 mm compared to 0.45 mm with the AM3 Socket. There is a faster serial link of 3400 kHz from the CPU to the power controller, compared to 400 kHz. The AM3+ Socket offers improved power regulation and power quality specifications, including an increased maximum current support of 145 A versus 110 A. There is also a redesigned CPU cooler retention harness allowing for slightly better airflow for CPU cooling, while retaining cooler backward compatibility.

Some manufacturers have brought AM3+ support to some of their AM3 motherboards via a simple BIOS upgrade. Mechanical compatibility has been confirmed and it is possible for AM3+ CPUs to fit in AM3 boards, provided they can supply enough peak current. Another issue is the use of the sideband temperature sensor interface for reading the temperature from the CPU. Therefore, some CPU PWM fan headers may only run at full speed. Also, certain power-saving features may not work, due to lack of support for rapid VCore switching.