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AMD Nixes PCIe 4.0 Support on Older Socket AM4 Motherboards, Here's Why

(Image credit: AMD)

Third-gen Ryzen's support for PCIe 4.0 opens up a new world of faster connectivity, but according to a recent statement from the company, you won't be able to enjoy the faster transfer speeds on older AMD motherboards, taking a bit of the shine of AMD's vaunted backward compatibility with Ryzen processors on the AM4 socket. 

At CES 2019, word emerged from motherboard vendors that they could support PCIe 4.0 on older motherboards, like X470, X370, B350, and A320, and AMD confirmed that it would not restrict the faster standard on older motherboards. We've even seen the feature enabled on recent third-gen Ryzen-enabling BIOS updates, but now AMD has reversed the earlier decision:

Users today may find a PCIe 4.0 option available in their pre-X570 motherboards. However, users should expect this option to be disabled when final retail BIOSes are released to implement full performance and stability for new 3rd Gen Ryzen processors. As pre-X570 motherboards were not designed with PCIe 4.0 in mind, their designs may be incapable of running PCIe 4.0 signaling with the requisite stability and performance. To ensure a reliable and consistent experience in the field, PCIe 4.0 will not be an option ultimately available to pre-X570 motherboards. Users may continue with a beta BIOS if they desire, but performance and stability cannot be guaranteed.

The problem boils down to signal integrity. Motherboard vendors at Computex 2019 told us that motherboard traces, the electrical pathways that carry data signals, require wider spacing with PCIe 4.0 compared to PCIe 3.0. The wider spacing, coupled with the increased power requirements for the PCIe bus, necessitates placing the transmit and receive traces on different layers of the motherboard, whereas PCIe 3.0-capable motherboards can carry the signals on a single PCB layer. That means most new X570 motherboards, and their newer B- and A-series counterparts, will come with a minimum of six layers, while previous-gen motherboards would dip down to four layers. That has obvious cost implications that will manifest as higher pricing on new AM4 motherboards. (Edit: There is word that Gigabyte has a few four-layer boards, we're following up for clarification.)

PCIe 4.0 also requires other additives that support the higher transfer rates, like switches and mux layouts, and trace lengths longer than six inches won't be able to carry the signal. Those restrictions would have limited the PCIe 4.0 connection to the first PCIe slot on older motherboards, but several vendors also told us the first slot wouldn't operate at full speeds due to the closer spacing of the PCIe 3.0-designed slots, which would introduce signal integrity issues.

That means, even under the best circumstances, the first slot on older motherboards would only operate at a portion of the PCIe 4.0 throughput (akin to a "PCIe 3.8" or "PCIe 3.7" connection). The increased data errors on the line would also trigger PCIe's error correction mechanisms more frequently, which could introduce data integrity issues.

AMD also told us at Computex 2019 that older motherboards would require PCI-SIG certification to support PCIe 4.0, a costly expense for motherboards that are already in production, and a few vendors that had planned to retroactively enable the feature told us that they would only do so unofficially and not market the faster transfer rates as a feature on older motherboards.

Even on newer motherboards with correct trace spacing, PCIe slots that extend beyond six inches (think the last slot on the motherboard) also require redrivers, one for each lane, that add even more cost to the new 500-series motherboards. Be prepared to whip out the wallet for these new motherboards. In fact, several vendors told us that they anticipate such high pricing for 500-series motherboards that they will keep the 400-series motherboard in extended production as the value option for third-gen Ryzen chips. That doesn't bode well for A-series motherboard pricing with the new motherboards.

AMD will disable the feature in the next AGESA update for the third-gen Ryzen processors. You can opt to use the few motherboard BIOS's that already exist to enable support for "PCIe 3.8," but that's a risky proposition.

It's easy to be disappointed that AMD isn't delivering on its promise of full backward compatibility for the AM4 socket until 2020, but the company has already been forced to drop support for some chips due to BIOS restrictions. The company also isn't supporting third-gen Ryzen on all previous-gen motherboards. That's all part of the challenge of undertaking the monumental task of supporting all Ryzen processors on a single socket. Perhaps AMD representative Robert Hallock's statement to us sums it up best:

"No one in the history of x86 has created an upgradeable socket quite like AM4. In a time where our competitor is breaking socket compatibility yearly, basically, we have three consecutive generations that all drop into the same socket, and that socket started with four cores years ago, and is now twelve cores, twenty-four threads and PCIe 4.0."

Given the circumstances, AMD is making the right choice here. Complicated support matrices, unofficial (and thus un-warrantied) "support," lowered throughput, and possible data loss are all untenable side effects of giving users what they want -- PCIe 4.0 support on motherboards that weren't designed for the task.

  • Dark Lord of Tech
    Thanks for the great info!
    Reply
  • jimmysmitty
    Not surprising. There is more than just traces to PCIe. Typically new standards can change those traces, how wide or thick they are or the distances they can travel to qualify for their rated speeds.

    The only downside is that AMD should have said something much sooner as a lot of people hyped that as a massive feature.

    However considering that PCIe 2.0 is still barely saturated for what this would have given people, it would have only given them the x16 slot and MAYBE the M.2 slot as 4.0, its not a major deal as 3.0 still has plenty of headroom for GPUs.
    Reply
  • Mandark
    current graphics cards are not going to benefit from pcie4 much--if at all. where you get the kick in the pants is with the new NVMe pcie 4.0 drives coming out. so it's still worth it to get a new mobo. glad I waited to build.
    Reply
  • rajag89
    taking a bit of the shine of AMD's vaunted backward compatibility with Ryzen processors on the AM4 socke
    Why do we expect a mb designed for PCIE3 to support PCIE4 ? The processor runs as promised with backward compatibility. The article tries to show AMD in bad lights because it can't allow PCIE3 ports of older generation to run on PCIE4 magically. Yay, as per the writer AMD broke the promise because the PCIE3 ports can't work as 4.
    Reply
  • Mandark
    exactly. i never bought the hype that pcie4 would be available on old motherboards. not possible, and if it is, it's gimped and full of bugs. not stable, not good, not usable. get a new motherboard
    Reply
  • Math Geek
    when reading about AM4 supporting the newer gen chips at no point did i even consider that pcie 4 would work on pcie3 slots, nor that usb 2/3 ports would all of a sudden become thunderbolt ports, that the DDR4 slots would support DDR 5, that the SATA 6 ports would all of a sudden become SATA 12 or anything else magically changing to include features that are not the cpu itself.

    not really sure why the author believes this was ever a promise AMD made. they promised the new cpu's would work on AM4 boards and for the most part it does. few of the super budget boards from 2 generations ago can't support it due to BIOS limitations, but overall, they have delivered a lot more than Intel ever has for a socket lasting longer than a year.

    if you want bleeding edge, be prepared to pay for it like always. ok with last year's fashions? then enjoy the cost savings the older chipsets will bring you. :)
    Reply
  • PaulAlcorn
    Math Geek said:
    when reading about AM4 supporting the newer gen chips at no point did i even consider that pcie 4 would work on pcie3 slots, nor that usb 2/3 ports would all of a sudden become thunderbolt ports, that the DDR4 slots would support DDR 5, that the SATA 6 ports would all of a sudden become SATA 12 or anything else magically changing to include features that are not the cpu itself.

    not really sure why the author believes this was ever a promise AMD made. they promised the new cpu's would work on AM4 boards and for the most part it does. few of the super budget boards from 2 generations ago can't support it due to BIOS limitations, but overall, they have delivered a lot more than Intel ever has for a socket lasting longer than a year.

    if you want bleeding edge, be prepared to pay for it like always. ok with last year's fashions? then enjoy the cost savings the older chipsets will bring you. :)


    AMD confirmed directly to us that it would support that feature on older boards. That's why the author feels that way :)
    Reply
  • rajag89
    PaulAlcorn Well, even then it isn't a part of backward compatibility. They hoped to deliver something extra which eventually didn't workout. But calling this as a failure to deliver backward compatibility is a bit too much from writer. Just MO.
    Reply
  • Math Geek
    really? i saw where they said it could work on some boards but did not see where they promised that.

    that would of course change things a bit. if they made a promise before testing it fully and now have to take it back, then i can see where the comments are justified. can you link that release so i can work on my appropriate level of disgust :)
    Reply
  • King_V
    I personally always saw it as a bonus rather than anything. I don't recall, for example, in the days of Socket 7, any new features that didn't exist on the board being promised.

    I can't imagine in general that customers would be disappointed. PCIe 4.0 was certainly never even thought of when people were buying, say, the B350 and B450 boards, etc. just that the socket would still be usable by new processors up to 2020.

    Maybe AMD was a little too hyped when they first said it, though. Still, I'd be shocked of people (outside of trolls, anyway) started screaming and bemoaning the inability to get PCIe 4.0 on older boards. I fully agree with the closing paragraph:
    Given the circumstances, AMD is making the right choice here. Complicated support matrices, unofficial (and thus un-warrantied) "support," lowered throughput, and possible data loss are all untenable side effects of giving users what they want -- PCIe 4.0 support on motherboards that weren't designed for the task.
    Reply