Feature: Embedded
COM-HPC carrier board designs: when planning meets perfection
By Zeljko Loncaric, Marketing Engineer, Congatec C
OM-HPC is the latest in the category of modular high-end edge servers, offering nearly twice as many interfaces as COM Express, and much faster. The first COMs (computer-on-modules) came
out in the early 1990s, to make the office PC technology suitable for industrial use. Named
ModulAT, the modules were based on the popular AT/ISA96 bus, and included a 9.54MHz Intel CPU 80C88 with 640kbytes DRAM memory. At the end of 2019, the technical subcommittee PICMG
COM-HPC agreed on the pinout for the current COM specification. Although system developers are already designing carrier boards, the large number of high- speed interfaces located on the connectors is a hindrance, specifically concerning signal compliance. Te two 400-pin connectors include PCIe Gen 4 and 5 and
interfaces with clock rates to 25Gb/s. PCle Gen 4 can achieve 16 gigatransfers per second (16GT/s) – double the PCle Gen 3 at 8GT/s. PCle Gen 5 can complete 32GT/s. Two bits/clock are transmitted rather than the usual one bit/clock by means of a four-step pulse amplitude modulation (PAM4). Upgrading from PCle3 to 4 generates a 100% increase in performance, whereas using USB 4.0 (40Gb/s) instead of USB 3.2 Gen 2 (previously USB 3.1 Gen 2 or SuperSpeed+) results in a 400% performance increase; replacing 10GbE with a 25GbE achieves 150%.
26 May 2021
www.electronicsworld.co.uk
Dealing with errors Developers must focus on early compliance testing to guarantee their solutions’ efficient operation during testing and in the field; see Figure 1. Tis is because the application could show proven RF, dimensioning and layout rules for optimum signal quality and still end up with errors, which can only be detected during rigorous compliance testing. Whenever a system functions without compliance, problems can be expected. Although systems may run smoothly, external components can malfunction, causing the entire system to stop working through errors that aren’t easy to anticipate or evaluate; that’s why they should be dealt with early on. Equally, transmitters and receivers of high-speed communication
interfaces should abide by specific signal-quality standards, and be subject to EMC compliance tests in order to certify that the set radiation levels remain within acceptable safe ranges. With the PCIe interfaces, for instance, compliance with the PCIe specification ensures effective communication between motherboard and peripherals, assuming that both ends comply efficiently with the specification. Even if the values are not exactly what the limits dictate, as long as they are not too far from the range, regular communication between board and devices can still occur. Nonetheless, transmission errors are always possible.
Moreover, because a communication error results in the re- transmission of the data packet, the achieved data transfer
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