Why mix USB2 and USB3 on a motherboard

Question

A conceptual question, I suppose. If USB3 port is backwards compatible with UBS2 & 1, why do manufacturers continue to include UBS2 ports on motherboards with USB3 support? To me, there seems to be no reason to have 6 USB2 ports and 2 USB3 ports at the same time. Can't they simply include 8 USB3 ports? Or just have 4 USB3 ports if bandwidth is thought to be an issue? I just can't think of any answer.

Answer 1

It's a limitation of the motherboard chipset. For example, an Intel H81-based motherboard can support up to 2 USB 3.0 ports and 8 USB 2.0 ports.

Answer 2

There are various reasons for this

---

For newer motherboards, there simply aren't enough die area for the complexity of USB 3.0+ standards. There are also not enough PCI-Express lanes for the USB 3.0 controller to operate at full speed for that many ports. For example the Skylake Z170 chipset has only 26 High Speed IO lanes, of which 6-10 can be dedicated for USB.

A single PCIe 3.0 lane (maximum 985 MB/s) or two PCIe 2.0 lanes (maximum 2x500 MB/s) are barely enough for 2 USB 3.1 gen 1 ports (625 MB/s per port). So you need more lanes if you want more speed/more ports, and then you'll have less lanes for the graphics cards and SATA drives. Or you can simply include a few USB 2.0 ports along the 3.0 ones, because a single PCIe 3.0 lane is enough for almost 10 USB 2.0 ports. 3 USB 3.0 ports + 10 USB 2.0 ports are vastly more useful than 4 USB 3.0 ports because very few devices actually need USB 3.0 speed. Keyboards and mice only need USB 1.0 speed and most USB sticks don't even use 1/4 the USB 2.0 speed

---

For older motherboards and laptops, it's because they use different controllers. The first Intel microarchitecture that has a chipset with built-in USB 3.0 support is Ivy Bridge.

• Intel to Natively Support USB 3.0 on Ivy Bridge Chipsets
• Intel delivers USB 3.0 in its chips, finally
• Correction: Ivy Bridge and Thunderbolt - Featured, not Integrated

So in earlier mainboards the manufacturers must use a separate USB 3.0 controller if they want to provide USB 3.0 functionality.

If you notice you'll see old laptops often equipped with only 1 USB 3.0 port and 1-3 USB 2.0 ports. It's because most of the external controllers only support 2 ports, and one may be used internally for some purposes. Hence only one is exposed to users and the remaining ports are connected to the native USB 2.0 controller inside the chipset. Newer laptops will have more or all USB 3.0 ports.

This happens even with new motherboards. They'll need a separate controller if they want to provide more USB ports than supported, or adopt a newer USB standard. For example on the mainboard with Skylake Z170 chipset above there are 2 solutions mentioned in the article

• Use the ASMedia ASM1142 controller with 2 PCIe lanes to provide USB 3.1 Gen 2 ports
• Use Intel’s Alpine Ridge Thunderbolt controller to provide USB 3.1 Gen 2, Thunderbolt 3 and DisplayPort with 4 PCIe lanes + DisplayPort lanes

---

This is also the same reason many mainboards have different SATA port types with different colors. When SATA3 was introduced, no Intel chipset supported it so mainboard manufacturers used a separate chip, often from Renesas, for controlling SATA3. The resulting mainboard will have 2 SATA3 and 2 or 4 SATA2 ports.

Some mainboards also use a 3^rd party RAID SATA controller as the chipset they use don't support RAID. Those mainboards also have 4-8 SATA ports of the same type but if you want to use RAID, you must plug the cables into the correct ports that are connected to the RAID controller. Mainboards designed for people who need lots of SATA ports, to plug tens of hard disks or use RAID5 with 10 disks for example, also need separate controllers.