Ram Speed Confusion

[Raptor Games]

I'm planning a build for the upcoming ryzen 7 5700g (apu), and have watched many youtube videos on it, most of them use 3600-4200 ram and say that apus need faster ram, but on amd's website they say 3200 is the max, I'm confused, can someone help/explain. Thanks.

[VioDuskar]

anything above 3200 requires you to enable XMP profiles within the BIOS of your motherboard. otherwise they will run at 3200 or lower. 

[Eigenvektor]

3200 MHz is the maximum speed sanctioned by the JEDEC standard. Anything above that is an overclock, though many 3200 MHz sticks also require XMP, because the standard would be 3200 MHz at a CAS latency of 20, but you'd typically buy 3200 MHz CL 16. RAM sticks that are faster than that usually include an XMP profile, so all you have to do is enable that in the BIOS and the RAM will run at overclocking settings validated by the manufacturer.

[Chris Pratt]

3200MHz is the highest JEDEC standard supported. RAM faster than that is overclocked, which is what XMP/DOCP is for. In other words, if you just stick in some RAM, the highest speed it will go to is 3200MHz. To get the actual rated speed above that, you have to enable XMP/DOCP, which is a profile for overclocking the RAM to hit its rated speed.

 

However, Ryzen has an additional wrinkle in that the RAM speed is linked to the infinity fabric, the communication layer between the CCDs in the CPU. This is referred to as the FCLK in the BIOS, and there's an upward limit to how fast it can go. 1800MHz is basically guaranteed. 1900MHz is usually achievable, and in some cases, Zen 3 can go as high as 2000MHz. Since this should be 1:1 with the clockspeed of your RAM, that means you want RAM somewhere in the range of 3600-4000MHz, depending on how high you can get the FCLK to go. The recommended and safest bet is 3600MHz, as you get most of the performance without having to roll the dice that you'll end up wasting money on higher clocked RAM you can't actually utilize.

 

It may seem weird that I said that the FCLK and RAM need to be 1:1, but the RAM speed is double the FCLK. That's because those RAM clockspeeds are effective, not actual. DDR stands for double data rate, which means you get two transfers per clock cycle. 3600MHz RAM, for example is actually 3600 MT/s (mega-transfers per second) and only 1800MHz (2x1800=3600). Manufacturers just call it MHz because consumers are more familiar with that terminology, and it's effectively the same as if it was actually 3600MHz, with just one transfer per cycle.