PICMG has published a COM-HPC Carrier Board Design Guide for building carrier boards for COM-HPC modules, including tips on PCIe Gen 5, USB4, and Ethernet KR and KR4 backplane signaling for 10GbE to 100GbE ports.

When PICMG launched the COM-HPC (Computer-On-Modules High Performance Computing) edge server-on-module standard in early 2020, it included specifications for developing carrier boards for the modules. Yet due to the complexity of the high-end spec, as well as the numerous variants and the support for novel technologies such as PCIe Gen 5, USB4, and up to 100GbE Ethernet connections, the standards organization decided developers needed some more help. PICMG has now published a 160-page COM-HPC Carrier Board Design Guide.

The guide is especially useful for its detailed discussion of the challenging module to carrier board Ethernet KR and KR4 backplane signaling, says PICMG. “To save pins on COM-HPC modules, the sideband signals for the 10G / 25G / 40G / 100G Ethernet KR interfaces are serialized and must then be deserialized on the carrier board,” explains the group. The design guide provides instructions for handling KR/KR4 in seven diagrams.

COM-HPC specs (left) and dimensions
(click images to enlarge)
Source: Congatec

Also of particular interest are the sections of USB4, as well as PCI Express up to Gen 5, which introduce some new concepts compared to earlier USB and PCIe specs. The guide provides schematics and block diagrams for these interfaces and all other COM-HPC I/Os, including SATA, Boot SPI, eSPI, eDP, MIPI-CSI, SoundWire, asynchronous serial, I2C/I3C, GPIO, and SMBus.

There are also sections on thermal protection and module type detection and tips on mechanical considerations. These include heat spreader/module attachment, alternative board stack assemblies, and board stiffeners for carrier boards.

 
COM-HPC basics

The COM-HPC/Client spec supports up to 65W TDP processors while COM-HPC/Server supports up to 125W CPUs. While Type 7 supports up to 32x PCIe Gen 3 lanes, COM-HPC/Client goes up to 49x PCIe Gen 4/5 lanes and COM-HPC/Server enables up to 65x Gen 4/5 lanes. Compared to Type 7’s support for up to 4x 10GbE ports, COM-HPC/Client supports dual 10GbE and dual 25GbE KR ports. COM-HPC/Server has only a single 10 GbE interface but offers 8x 25GbE connections.

COM-HPC’s dual 400-pin Samtec COM-HPC connectors can drive up to 4096 Gbps data rates with a data rate density of 2088 Gbps per square inch. The connectors are available in with 5mm and 10mm stack heights.

Adlink COM-HPC-cADP

Like the similarly edge server focused COM Express Type 7 (COM Express 3.0) spec, the higher-end COM-HPC/Server variant is headless (no graphics interfaces). This variant is adopted by Adlink’s COM-HPC Ampere Altra, which based an Arm v8.2 based Ampere Altra. The more embedded-oriented COM-HPC/Client spec, which has been more frequently seen on LinuxGizmos, supports up to 4x DDI video interfaces with 4K support, as well as dual MIPI-CSI links.

We have seen a variety of modules based on the 120 x 95mm COM HPC Client Size A, including Congatec’s 12th Gen Alder Lake-H based Conga-HPC/cALP and Eurotech’s Tiger Lake-U based CPU-180. There have also been some 120 x 120mm Client Size B form factor modules such as Adlink’s COM-HPC-cAD. Avnet/MSC’s MSC HCC-CFLS adopts the 160 x 120mm Size C format.

 
Further information

The COM-HPC Carrier Board Design Guide is available for free download. More information may be found in PICMG’s announcement, and the document itself is here.