The MicroTCA standards let designers combine hardware modules in a wide variety of configurations. However, MicroTCA integration is not simply a matter of plugging components together. System integrators must carefully consider factors such as thermal management, backplane connectivity, and mechanical incompatibility. This blog takes a look at some practical tips for assembling MicroTCA platforms and points out key issues designers should keep in mind.
The first step in developing a MicroTCA-based application is selecting the AdvancedMC (AMC) modules. Decisions made at this stage can have a significant impact on later integration efforts. It is important to answer questions such as:
- How are the ports used? The AMC specifications dedicate certain ports to interfaces such as PCI Express*, SAS and SATA, Ethernet, and Serial RapidIO*. (See slide 7 of the Intro to MicroTCA presentation, for example.) However, the function of most ports is undefined. If a module uses undefined ports for standard interfaces like PCI Express, it can be difficult to integrate the module with the rest of the system.
- Where is the storage controller? Most processing modules include SAS or SATA controllers. For example, this is the case with the Concurrent Technologies AM 210/x0x. However, you may want to use a processing module without SAS/SATA, or you may want additional controllers so you can expand your storage. For these situations you can turn to storage modules with on-board controllers (e.g., the SanBlaze SB-AMC51), or you can use standalone controller modules (e.g., the Astek A3803-AMC). Carefully consider the needs of your system when selecting your controller solutions, including the connectivity available in the chassis.
Once you have selected the AMC modules, it’s time to choose a chassis. Obviously you need enough slots to fit your selected modules, but there are other questions to answer such as:
- Which interfaces are supported? Are there enough PCI Express lanes? Enough Ethernet interfaces? Does the SAS/SATA configuration support redundancy?
- What is the power supply? This is a particularly important question if you need AC power. Some chassis do not support AC power at all. Other chassis support AC power by using additional hardware that takes up space and blocks access to some AMC slots. If you need AC power, it is a good idea to use a module like the systerra STCA-1U6S-01 that is designed specifically for this purpose.
With these questions resolved, we can get down to the main problem of where to place the modules. Modules fall into four main types: CPU, storage, peripheral, and filler. The following table summarizes the typical characteristics of each module type, using Kontron modules as examples. Note that the CPU module is a PCI Express Root Complex, which means that it is the endpoint for the PCI Express I/O hierarchy.
CPU (e.g., Kontron AM4010)
x4 (Root Complex)
Storage (e.g., Kontron AM4500)
Peripheral (e.g., Kontron AM4301)
Filler Module (w/ baffle)
Table 1. Typical AMC module characteristics.
Let’s suppose we have one of each type of module and a typical 4-slot chassis, as illustrated in Figure 1. Where should we put each module?
Figure 1. A four-slot “serial” chassis has a thermal rise of 2.5 C per slot.
Let’s start by looking at the thermal considerations. Per the MicroTCA specification, the maximum temperature rise is 2.5 C per module. The obvious conclusion is that modules with the highest heat tolerance should go near the exhaust, while modules with lowest tolerance should be placed near the air intake. Taking the tolerances listed in Table 1, we arrive at the solution in Figure 2:
Figure 2. A thermally-optimized solution.
Now let’s look at interface issues. The MicroTCA Carrier Hub (MCH) and chassis typically have interface limitations that constrain module placement. For example, low-cost MCH modules do not provide full switching of all PCI Express, Ethernet, and SATA/SAS signals. (For examples of MicroTCA Carrier Hubs, see Kontron’s MCH page.) Similarly, most backplanes do not provide all interfaces on all slots, but instead provide select point-to-point connections between specific slots. In addition, most MicroTCA systems only support the PCI Express Root Complex in a specific slot.
Let’s assume our chassis supports PCI Express in each slot, and that slot 1 is dedicated to the Root Complex. Let’s further assume that the chassis supports 2x SAS/SATA connections only between slots 1 and 3 and between slots 2 and 4. With these constraints, we would need to move the CPU to slot 1 and the storage AMC to slot 3, as shown in Figure 3.
Figure 3. A revised module placement that takes PCI Express and SAS/SATA interfaces into account.
In most MicroTCA solutions, the MCH provides one or more Ethernet links to each slot. Thus, Ethernet generally does not constrain module placement. However, you should make sure that the selected MCH supports any Ethernet features you need, such as QoS. In addition, you should make sure your MCH supports dual Ethernet links per slot if your system needs this feature. (For more on this and other interface issues, I recommend the Kontron white paper MicroTCA Integration.)
You should also look out for mechanical conflicts between modules. For example, conflicts can occur when different modules have RJ-45 connectors in different orientations (see Figure 4). Unusually large USB cables or devices (such as ruggedized flash drives) can also cause conflicts. Sometimes these issues can be solved by using different modules. When mechanical conflicts arise late in the design process, however, the only practical option may be to change the module placement. In this case, it may be better to put up with hard-to-remove cables than to compromise the thermal design by moving the modules.
Figure 4. A mechanical conflict can result when different modules have RJ-45 connectors in different orientations.
The principles outlined here are fairly simple, but these are many other details to consider. For an in-depth look at the issues, I recommend Advantech’s three-part series An Integrator's guide to MicroTCA. Part one looks at the MicroTCA Carrier Hub. Part two looks at AMC module start-up. Part three covers system management software.
Now it’s your turn. Did you find this overview helpful? What integration challenges have you encountered, and how did you overcome them?
Kontron and Advantech are Premier members of the Intel® Embedded Alliance. Concurrent Technologies is an Affiliate member of the Alliance.
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