While AMD's top-of-the-range X670E chipset for AM5 motherboards offers expandability by allowing daisy-chaining of the chipsets, it is rather hard to build a Mini-ITX motherboard featuring this dual-chip core logic. But it looks like Asus has found a solution: its ROG Strix X670E-I motherboard for AMD Ryzen 7000-series processors places the second chipset chip (Promontory 21) on an add-on PCIe card, as shown in an image published by Uniko's Hardware.
One of the peculiarities of AMD's latest 600-series chipsets is that B650E and B650 rely on a single Promontory 21 chip, whereas the I/O-rich X670E and X670 daisy chains another Promontory 21 chip using a PCIe 4.0 x4 interface to add more PCIe lanes, USB ports, and other input/output interfaces. But the second Promontory 21 chip doesn't have to be installed on the motherboard itself. Asus, for example, installs it on a special PCIe 4.0 x4 add-on card that enables an additional M.2 PCIe 4.0 x4 slot and I/O controllers.
By using the dual-chip X670E chipset and an additional Promontory 21 chip, Asus frees up PCIe lanes of the primary X670E platform controller hub (PCH) for Intel's JHL8540 controller that enables two USB 4 Type-C ports with a 40 Gbps throughput as well as DisplayPort support (the chip connects to PCH using a PCIe 3.0 x4 interface, so the maximum raw bandwidth is about 3.938 GB/s in both directions), a Wi-Fi 6E + Bluetooth 5.2 adapter, a 2.5GbE controller, and multiple USB ports.
Since AMD's Promontory 21 chip fully supports I/O virtualization features like IOMMU, using it to connect a PCIe 4.0 x4 SSD to the host promises predictable performance for both the drive and peripherals connected to the second Promontory 21 PCH.
In total, the Asus ROG Strix X670E-I supports two M.2 slots (one with a PCIe 5.0 x4, another with a PCIe 4.0 x4 interface), two SATA ports (using another bundled add-on card), two USB4 Type-C connectors, one USB 3.2 Gen2 Type-C port, six USB 3.2 Gen 1 Type-C connectors, and five USB 2.0 ports. Also, the unit has a 2.5 GbE (enabled by the Intel I225-V controller), and an advanced audio subsystem powered by the Realtek ALC4050 codec and ESS Sabre 9260Q DAC.
Speaking of the audio subsystem, it is necessary to note that to fully take advantage of it one will need to use the company's bundled ROG Strix Hive adapter which also has two USB ports.
Interestingly, Asus is not the only company that decided to place a Promontory 21 on an add-in card. Recently, Level1Techs demonstrated ASRock's X670 Xpansion Kit card for the company's B650 motherboards, transforming them into X670 mainboards. The card has a PCIe 4.0 x4 interface that carries the Promontory 21 chip to add two M.2 slots, one USB 3.2 Gen2x2 Type-C port, three USB 3.2 Gen1 Type-A connectors, two SATA connectors, and a 10GbE port (using the Marvell AQtion AQC113C controller).
The expansion board requires UEFI support, so it cannot be installed on platforms whose firmware does not support it. Level21Techs installed it into a custom ASRock B650 LiveMixer motherboard with an appropriate UEFI, so the X670 Xpansion Kit card worked fine. While the card's PCIe 4.0 x4 interface will be a performance bottleneck when the board is fully populated with a couple of SSDs, connected to a 10GbE network, and with several high-performance USB devices plugged into it, I/O virtualization support should help to ensure flawless operation and predictable performance.
