Tuesday, May 31st 2022
AMD Zen 4 & Socket AM5 Explained: PCIe Lanes, Chipsets, Connectivity
There has been a fair bit of confusion about AMD's upcoming AM5 platform when it comes to connectivity and we're going to break things down and explain what the difference is between the B650 and X670 boards. We're also going to cover the processor connectivity, since that's an integral part of any motherboard these days. All the information in this article is based on what we've been told by various sources during Computex.
Connectivity from the Processor
Starting with the Zen 4 processor's lanes, all of its PCIe lanes are PCIe 5.0 and there are a total of 28 lanes. The first 16 PCI Express lanes will be used for a single x16 PCIe slot, or they can be split into two x8 slots. AMD's requirements only enforce PCIe 5.0 on the X670E boards, which means PCIe 4.0 will be applicable to lower cost motherboards.
The new Zen 4 Ryzen processors will have eight general purpose lanes, of which at least four will be required to be dedicated to an M.2 storage slot (always Gen 5). The other four lanes are up to the motherboard manufacturers. Some boards will use these to implement Thunderbolt 4 (Intel Maple Ridge JHL8540) or USB4 (ASMedia ASM4242). If none of these options are used, these lanes can go towards an additional M.2 slot.
With integrated graphics becoming standard on Zen 4, the first generation of AM5 processors will offer four dedicated display outputs, with HDMI 2.1 and DisplayPort 2.0 being supported, but neither being required as far as we understand. There are also four USB 3.2 Gen 2 (10 Gbps) ports and at least one USB 2.0 port coming from the processor. Three of the USB 3.2 ports also support DP Alt Mode, something we've seen several announced boards supporting on at least one USB-C port. This seems to be up to the motherboard manufacturers to implement once again.
The remaining four PCIe lanes are used for connecting to the chipset. Just to clarify, on the processor side these do support PCI-Express 5.0, the chipset only supports PCIe 4.0, so the link negotiation mechanism will downgrade the link to Gen 4.
Chipset Connectivity
The way AMD presented their AM5 chipset options at Computex, it seemed that these each is an independent designs, based on its own silicon. In reality AMD has partnered with ASMedia to create a single chipset, called "Promontory 21," which is used in various configurations. For the X670 and X670E they are daisy-chaining a pair of B650 chipsets together, for additional connectivity options.
Promontory 21 offers a total of 16 PCI-Express lanes. Four of these are used to connect to the CPU, over a Gen 4 interface, as mentioned before. In the X670/X670E daisy-chained configuration the secondary chipset connects to the primary chipset, it has no direct link to the processor. This means that on the primary chipset another four lanes are used up, leaving eight usable PCIe lanes, whereas the secondary chipset has 12 usable PCIe lanes. Four of the lanes are PCIe 3.0, although these are muxed interfaces with SATA 6 Gbps. This allows the motherboard manufacturers to choose how they want to implement those interfaces and as we've seen, ASRock has gone for eight SATA ports, whereas most other board makers appear to be going for six on their X670 and X670E motherboards.
In other words, B650 motherboards will have a total of eight usable PCIe 4.0 lanes and four PCIe 3.0 or SATA 6 Gbps interfaces. X670 and X670E motherboards will have 12 PCIe 4.0 lanes and up to eight PCIe 3.0 or SATA 6 Gbps interfaces. In addition to this, each chipset will have six USB 3.2 Gen 2 (10 Gbps) interfaces, where the first two can be combined into a single USB 3.2 Gen 2x2 (20 Gbps) interface. This means X670 and X670E boards can have a total of 16 USB 3.2 Gen 2 (10 Gbps) ports, or two USB 3.2 Gen 2x2 (20 Gbps) ports and 12 USB 3.2 Gen 2 (10 Gbps) ports, including the USB 3.2 Gen 2 (10 Gbps) ports from the processor. Finally there's support for up to six USB 2.0 ports from the chipset. As a side note, any motherboard with more than two USB 3.2 Gen 2x2 (20 Gbps) ports, will be using a third party host controller or a hub.
Compared to Intel Alder Lake
Compared to Intel's Z690 chipset, which has support for a total of 28 PCIe lanes, AMD has clearly decided to scale things back a little bit. In all fairness, Intel doesn't support more than 12 PCIe 4.0 lanes from the Z690 chipset and four of those lanes are shared with SATA 6 Gbps ports. Intel wins by having support for an additional 12 PCIe 3.0 lanes though, but two of those are shared with an Ethernet MAC, something AMD doesn't do, as the company relies on PCIe based Ethernet controllers. It's worth noting that Intel has a wider bus to some of its chipsets, as their CPUs support eight DMI 4.0 lanes. Comparing AMD's B650 chipset with Intel's B660, AMD comes out slightly ahead if high-speed interfaces matter, as the B660 chipset only supports six PCIe 4.0 lanes and eight PCIe 3.0 lanes, although none of its four SATA 6 Gbps ports are shared with PCIe.
The Z690 chipset supports a total of 10 USB 3.2 Gen 2 (10 Gbps) interfaces, but as with AMD, two interfaces are combined to create a single 20 Gbps interface, which means up to four USB 3.2 Gen 2x2 (20 Gbps) ports are supported. The B660 chipset supports two USB 3.2 Gen 2x2 (20 Gbps) plus two USB 3.2 Gen 1 (10 Gbps) ports, or six 10 Gbps ports. That said, Intel doesn't have any USB ports inside the CPU, which makes this something of a draw, depending on how the motherboard makers implement the connectivity options on their motherboards.
What we obviously don't know at this point is how much of a performance penalty there will be for AMD having two chipsets, especially when it comes to high-speed storage devices. We've seen some concerns about this implementation in virtualized environments and how these devices will appear to the OS in such a case, but we don't share those concerns. We expect the primary chipset to appear as PCIe bridge to the host system, a mechanism that is part of the specification and has been supported and used for many years. These are things we're going to have to wait and see how they play out, but AMD clearly deemed the tradeoffs reasonable enough versus the cost of developing multiple different chipsets.
Starting with the Zen 4 processor's lanes, all of its PCIe lanes are PCIe 5.0 and there are a total of 28 lanes. The first 16 PCI Express lanes will be used for a single x16 PCIe slot, or they can be split into two x8 slots. AMD's requirements only enforce PCIe 5.0 on the X670E boards, which means PCIe 4.0 will be applicable to lower cost motherboards.
With integrated graphics becoming standard on Zen 4, the first generation of AM5 processors will offer four dedicated display outputs, with HDMI 2.1 and DisplayPort 2.0 being supported, but neither being required as far as we understand. There are also four USB 3.2 Gen 2 (10 Gbps) ports and at least one USB 2.0 port coming from the processor. Three of the USB 3.2 ports also support DP Alt Mode, something we've seen several announced boards supporting on at least one USB-C port. This seems to be up to the motherboard manufacturers to implement once again.
The remaining four PCIe lanes are used for connecting to the chipset. Just to clarify, on the processor side these do support PCI-Express 5.0, the chipset only supports PCIe 4.0, so the link negotiation mechanism will downgrade the link to Gen 4.
Chipset Connectivity
The way AMD presented their AM5 chipset options at Computex, it seemed that these each is an independent designs, based on its own silicon. In reality AMD has partnered with ASMedia to create a single chipset, called "Promontory 21," which is used in various configurations. For the X670 and X670E they are daisy-chaining a pair of B650 chipsets together, for additional connectivity options.
In other words, B650 motherboards will have a total of eight usable PCIe 4.0 lanes and four PCIe 3.0 or SATA 6 Gbps interfaces. X670 and X670E motherboards will have 12 PCIe 4.0 lanes and up to eight PCIe 3.0 or SATA 6 Gbps interfaces. In addition to this, each chipset will have six USB 3.2 Gen 2 (10 Gbps) interfaces, where the first two can be combined into a single USB 3.2 Gen 2x2 (20 Gbps) interface. This means X670 and X670E boards can have a total of 16 USB 3.2 Gen 2 (10 Gbps) ports, or two USB 3.2 Gen 2x2 (20 Gbps) ports and 12 USB 3.2 Gen 2 (10 Gbps) ports, including the USB 3.2 Gen 2 (10 Gbps) ports from the processor. Finally there's support for up to six USB 2.0 ports from the chipset. As a side note, any motherboard with more than two USB 3.2 Gen 2x2 (20 Gbps) ports, will be using a third party host controller or a hub.
Compared to Intel Alder Lake
The Z690 chipset supports a total of 10 USB 3.2 Gen 2 (10 Gbps) interfaces, but as with AMD, two interfaces are combined to create a single 20 Gbps interface, which means up to four USB 3.2 Gen 2x2 (20 Gbps) ports are supported. The B660 chipset supports two USB 3.2 Gen 2x2 (20 Gbps) plus two USB 3.2 Gen 1 (10 Gbps) ports, or six 10 Gbps ports. That said, Intel doesn't have any USB ports inside the CPU, which makes this something of a draw, depending on how the motherboard makers implement the connectivity options on their motherboards.
What we obviously don't know at this point is how much of a performance penalty there will be for AMD having two chipsets, especially when it comes to high-speed storage devices. We've seen some concerns about this implementation in virtualized environments and how these devices will appear to the OS in such a case, but we don't share those concerns. We expect the primary chipset to appear as PCIe bridge to the host system, a mechanism that is part of the specification and has been supported and used for many years. These are things we're going to have to wait and see how they play out, but AMD clearly deemed the tradeoffs reasonable enough versus the cost of developing multiple different chipsets.
86 Comments on AMD Zen 4 & Socket AM5 Explained: PCIe Lanes, Chipsets, Connectivity
And new software, frankly, is not making significant strides towards needing uberpowerful CPUs outside of the content production industry.2013 wants its talking point back. Maybe by 2050 we'll finally see ARM desktops commonplace!They didnt thinkt he consoomers would worry that much about either rDNA or AM4, if you think they've learned their lesson I have a bridge to sell you.
X670E - 2 packages - ATX
X670 - 2 packages - ATX
B650E - 1 package - ATX/mITX (conjecture)
B650 - 1 package - ATX/mITX
A6XX - chipset less - mITX (conjecture)
I don’t believe we will see many microATX so I left it out.
Ain't competition great!
This.
Oh yes, gimme dat USB-C output!
Who cares about space-wasting honking huge connectors for our IGP's when our USB-C outputs can cheaply and easily used with displayport connections?I do agree it's weird the laptops from both sides have this while the desktops dont
Cost - they can bring all this in with X770, whilst launching AM5 at a cheaper price point.
Unlike intels side of things, AMD is planning for these boards and CPU's to be inter-compatible for several years: So these first gen boards need to cut the most expensive, least likely to be used peripheral options to lower the platform cost and save something for the definitely coming second generation chipsets.
At a guess the 700 series chipsets will be gen 5, with native USB 4 - clear and simple marketing and distinction for end users, stick with 600 series for cheaper builds and 700 series for the high end connectivity users
I too want native USB 4 (one of the few things that i would upgrade for, although i could just do a PCI-E card)
On the PCI-E lanes... while i see this as unlikely, we did see lane variance on AM4.
I wonder if they left the potential for more lanes in future chipsets, or if they plan on doing 5.0 -> twice as many 4.0
As someone that doesn't go for the high end, i can't agree more with Brian. AMD changed a lot, GPU's and CPU's for the worst in this last year.
Waited 2 years for AM4 but still had lots of problems, was impacted by system freezes (solved by agesa update eventually, but it took a year, set the system aside and built and used an intel x299 instead because system freezes are unacceptable), was impacted by USB drop outs (just now fully solved by Agesa update this past month), still having other minor issues but they are mostly ignorable. Really want more PCIe slots. Slots. M.2 uses up a lot of lanes from chipset, but its not flexible, whereas a slot you can put in a m.2 in via a card, or any other device.
With 2x m.2 coming from the CPU I really don't need any more coming from the chipset and even if I do I'd prefer the flexibily of simple generic slots. I think a lot of these supposed "features" like extra m.2 with "heatsinks" (aka a chunck of painted metal) are just ways to upcharge the boards
The original quote he quoted was how CPU's are aging more now then ever which is actually the complete opposite (try and run windows vista era programs/games on a 1997 cpu and try to run windows windows 7 era games/programs on a 1999 era computer and then claim "CPU's are aging worst now")
I'm honestly interested in getting your technical evaluation of it, since you clearly know something AMD doesn't even know.The system freezes only affected a minority of users from my understanding, it's one issue I didn't have. The USB dropout was fixed much longer back than that, unless it was specific to your board/maker.
The PCIe/M.2 layout it up to the board makers, take it up with them, it's not AMD that's imposing limitations here.
One thing that keeps me wondering is how they will de-feature A620 platform. With only one x4 chipset downlink disabled there would still be a lot of I/O in play... So much that most consumers wouldn't really need to jump into B650 territory. There probably will be more cuts to convince people. USB3 2x2 unavailable? One CPU x4 link disabled, Intel H510/H610 style?With AM4 providing almost zero chipsetless boards, AM5 has even less chance to make it common.
AM4 at least did have a built-in storage controller with two lanes configurable as SATA or PCIe.
No AM5 leak even mentions cpu-attached SATA, and that's a big no-no for consumer-oriented board. External controllers are always an option, but combined costs would make simply going for A620 more reasonable.
Remind me in 1 to 2 years I guess
I expect instead they will be inefficiently used on m.2 slots. Companies like ASRock who are the king of i/o options should at least have optional routing to PCIE slots shared with the m.2 slots I hope like on my b450 pro 4. Good to see they will still offer 8 ports as well albeit only on what will probably be really pricey boards.
Also why does the chipset still only have 4 lanes?
I assume pcie gen 3 will be dead on this gen, so cheaper gen 3 boards will be history. Have they figured out a way to make gen 4/5 boards with same number of PCB layers as gen 3 yet?On the basic point he has nailed it, in short becoming an industry for people who love the newest tech, but the budget side of things has gone to crap.
There should be a budget option on AM5 option for gen 3 PCIE, DDR4, using existing ATX standards and not having VRM's over spec'd. 90% of people would be satisfied with that.
I screen grabbed this.
Flexibility is king and should be the fore front of board design. Or should we all comply and be stuffing our machines with m.2 drives?
I can answer your question of the uses.
1 - Easy install NVME drives, onboard slots are fiddly and hard to cool.
2 - Discrete sound.
3 - Capture card.
4 - I/O card whether its SATA, SAS, NVME or other.
5 - Wifi/Ethernet card.
6 - USB expandability.
7 - Additional GPUs e.g. for virtualization.
Also there is no evidence M.2 slots are cheaper than PCIE slots for manufacturing. Cheaper boards tend to have a higher PCIE slot to M.2 ratio which if anything suggests the opposite. ASUS themselves are always over priced, I brought a PCIE M.2 adaptor for £8 although is now £12.
www.amazon.co.uk/gp/product/B084GDY2PW/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1
4 slots? :)
www.amazon.co.uk/4-Port-Controller-Expansion-Adapter-Converter-4-Port-PH44/dp/B09CTZ8QJM/ref=pd_lpo_1?pd_rd_i=B09CTZ8QJM&th=1
I have sat wondering what has led to the manufacturers making these decisions, I think part of the problem is they are too heavily influenced by the review industry, reviewers seem to be backing the trend of a new system design that promotes heavy use of M.2, and just a single GPU installed with no other PCIE devices. Normal consumers dont have access to media departments and designers.
To the cost point, Moore's Law Is Dead is reporting that 2 X670 chipsets are cheaper than a single X570, if true that already goes some way towards absorving the price increase from moving to pcie gen5 and extra layer pcbs.
The argument of "what gamer needs pcie gen5" is also completely flawed, you could also say "what gamer needs a new gen system when the last gen is selling at a discount" or the classic "what gamer needs more than 4 cores". Software that makes use of higher speeds/performance will be available as higher speeds/performance becomes available, not the other way around. Granted we haven't really scratched the surface of what gen4 nvme is able to do because it was also a short lived spec, but that doesn't mean we can't continue to move forward on the hardware side while the software catches up.
I thought that video cards doesn't even use full 16x 4.0 lanes.
This thought came in my mind thinking that maybe they could use the spare lanes for other purpose. Or maybe couldnt' they check if the device connected has 8 or 16 lanes and dedicate the spare lanes for other connections?
I clearly am talking from a point of view of a ignorant in this matter