Clock Generation

Athlon 64 CPUs use a base clock (also referred as HyperTransport, HT or HTT clock – some people also call this clock as external clock or FSB clock, even though technically these names are wrong) of 200 MHz to generate its internal clock, multiplying it by a fixed rate (called clock multiplier), which varies according to the CPU. To find out this multiplier, just divide your CPU internal clock by 200.

For example, Athlon 64 X2 5000+ works internally at 2.6 GHz (2,600 MHz). This clock rate is achieved by multiplying its 200 MHz base clock by 13. If you don’t know your Athlon 64 internal clock rate, take a look at the tables available on our All Athlon 64 Models tutorial.

Like SDRAM, DDR memories need a clock signal to work. On Athlon 64 systems this clock is generated by dividing the CPU internal clock rate by a fixed value (called memory divider), which varies according to the CPU and memory used.

This divider will be integer value (rounded up) necessary to deliver the memory clock. To find it, divide the CPU internal clock rate in MHz by the memory real clock rate. Keep in mind that on DDR and DDR2 memories the rated memory clock is actually the double the memory clock: DDR400 memories run at 200 MHz, DDR2-667 memories run at 333 MHz, DDR2-800 memories run at 400 MHz and so on.

Let’s give some examples. On an Athlon 64 3800+ (which runs at 2.4 GHz) using DDR400 memories, this divider will be of 12 (2400/200). As you can see, on Athlon 64 using DDR400 memories this math is very easy as the memory divider will be the same value as the clock multiplier.

But if DDR333 memories were used instead, a different divider needs to be used, otherwise the CPU would deliver a clock signal with a frequency higher than the maximum theoretical clock rate supported by the memory. As DDR333 memories work at 166 MHz, we come to 14.45 as the result of 2400/166. So the CPU will use a divider of 15 (the next integer value). This is really interesting. If you make the math back (2400/15) you will find that DDR333 memories with Athlon 64 3800+ run actually at 160 MHz (320 MHz DDR), not 166 MHz (333 MHz DDR).

With DDR2-based Athlon 64 the same idea applies. For example, Athlon 64 X2 4800+ based on socket AM2 runs at 2.4 GHz. So if you use DDR2-800 memories with it, we find 6 as the memory divider (2400/400). But if DDR2-667 memories were used instead, the divider would be 8 (2400/333 = 7.20, the next integer value is 8). With this processor DDR2-667 memories run at 300 MHz (600 MHz DDR), not 333 MHz (667 MHz DDR).

Just a final example, Athlon 64 X2 5000+ (2.6 GHz) uses a memory divider of 7 for DDR2-800 memories (2600/400 = 6.5, the next integer value is 7). So DDR2-800 memories installed on Athlon 64 X2 5000+ systems run at 371 MHz (742 MHz DDR), not 400 MHz (800 MHz DDR). With DDR2-667 memories the divider will be 8 (2600/333 = 7.8, the next integer value is 8). So DDR2-667 memories installed on this system run at 325 MHz (650 MHz), not at 333 MHz (666 MHz DDR).

By now you know all basic clock specs from your CPU: its base clock rate (200 MHz), its internal clock rate, its clock multiplier and its memory divider.

But all other clocks used on your motherboard are also derived from the CPU base clock (HTT clock).

HyperTransport bus clock will be the HTT clock multiplied by 5, making its 1,000 MHz clock rate (a.k.a. “2,000 MHz” or “4 GB/s”). Older Athlon 64 CPUs have their HyperTransport bus running at 800 MHz (a.k.a. “1,600 MHz” or “3.2 GB/s”), so with these CPUs the HyperTransport clock rate is achieved by multiplying the HTT clock by four.

The clock rates used by all slots found on the motherboard also come from the CPU base clock (HTT clock). PCI Express clock is 100 MHz, AGP clock is 66 MHz and PCI clock is 33 MHz, which are obtained diving the base clock by 2, by 3 and by 6, respectively (usually on AGP-based systems the PCI clock is achieved by dividing the AGP clock by two, and on PCI Express-based systems the PCI clock is achieved by dividing the PCI Express clock by three). Serial ATA ports need a 100 MHz clock signal, usually coming from the PCI Express clock, which comes from the base clock, as we’ve seen.

Overclocking-targeted motherboards can allow you to configure the clock division for these devices or, better still, use a separated clock generator for them. We will explain you why in the next page.


Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.