 MicroTCA embeds boundary scan into standard
By Adam Ley
Chief Technologist
ASSET InterTech
A subcommittee of the PCI Industrial Computer Manufacturers Group (PICMG) is finalizing the MicroTCA standard and the specification will include a defined system-level boundary-scan architecture. (For a previous Connect article on MicroTCA from last year, click here.) Having JTAG as an option in the standard will give manufacturers and system integrators the core capabilities they need for a number of useful applications, such as remote management, testing and firmware updates. Effectively, a system-level JTAG scan path can be implemented as a secondary maintenance bus in MicroTCA systems.
The emergence of the MicroTCA standard coincides with the growing adoption rate of boundary scan in the telecommunications industry. JTAG has long been embedded into central office switches, core routers and other types of large systems, but the MicroTCA standard is squarely targeted at small and mid-range systems on the edge of the public switched telephone network (PSTN). In addition, MicroTCA-based systems could end up in non-telecom applications, including small office/home office (SOHO) networking, modular servers, storage systems, medical equipment and military/aerospace systems.
What is MicroTCA?
MicroTCA has evolved out of another PICMG standard, AdvancedTCA (ATCA), which was approved in 2001. ATCA is more typical of large central office systems in the telecommunications industry. It is based on a 19-in., 23-in. or 600-mm rack with large 8U-sized cards supporting multi-protocol interfaces up to 40 gigabits per second (Gbps). ATCA supports all of the prerequisites of core packet-based telecom systems, such as hot-swappable circuit boards, high-availability and high-density.
Soon after ATCA was specified, another PICMG subcommittee began developing a complementary standard for mezzanine cards suitable for ATCA cards and chassis. The Advanced Mezzanine Card (AdvancedMC or AMC) standard was intended to add flexibility to an ATCA system by extending its high-bandwidth, multi-protocol capabilities to individual hot-swappable mezzanine modules which could feature specialized functionality or simply additional resources. AMCs extend the ATCA fabric so that equipment manufacturers or system integrators can implement complex functions or combine a large number of channels in a single compact module.
The large form-factor of ATCA cards and chassis limits their application to core telecommunications applications. The smaller form-factor of the MicroTCA standard extends the horsepower of the ATCA fabric further to edge networking applications.
MicroTCA leverages the AMC as originally defined, but eliminates the need for a carrier card. Instead, AMCs can be mounted directly into a MicroTCA chassis in a number of variations such as half-height/single-width, half-height/double-width, full-height/single-width and full-height/double-width.
Reducing size and other factors have cut the cost target for MicroTCA systems to the $500 range, within striking distance of edge telecom gear and home or office computing systems. Like ATCA, the MicroTCA passive backplane is not based on the typical point-to-point multidrop architecture. Rather, it is a fabric architecture that supports topologies including a star, dual-star and a full mesh. Bandwidth over the backplane is scaleable from one to 40 Gbps.
For an introduction to MicroTCA from PICMG click here.
Fitting Boundary Scan In
From the first conception of the AMC standard, a JTAG interface has been specified as an optional feature on AMC cards. Spearheaded by several large telecommunications equipment companies, including Lucent and Motorola, and with the assistance of ASSET, a system-level boundary-scan architecture is defined as an optional feature on the MicroTCA backplane. Although JTAG is an optional feature, it does have support in all of the pertinent MicroTCA specifications. As an option, it will be up to the equipment manufacturers and system integrators to determine the extent to which system-level JTAG operations are deployed.
System-level boundary-scan applications recently have received considerable attention. Companies such as Lucent, Motorola, ASSET and Firecron have been at the forefront in developing mechanisms to accomplish system-level JTAG operations. In addition, an ad hoc industry initiative, the System JTAG (SJTAG) working group, has formed to study the topic.
In a MicroTCA system, a MicroTCA Carrier Hub (MCH) provides virtual carrier manager (VCM) services to all of the AMCs in the chassis. The MCH is essentially the hub or central controller for the fabric topologies the system may be configured into. The MCH includes a JTAG interface to the backplane as an option.
An MCH can act either as a JTAG slave or a master. It can function as a slave to an external JTAG tester or to a second MCH in the chassis when redundancy is required in the system. As a JTAG master, the MCH controls system-level JTAG operations. AMCs in the chassis are supported strictly as JTAG slaves.
A MicroTCA backplane that supports boundary-scan will provide full star routing for all MCH and AMC slots. Additionally, boundary-scan interfaces to power and/or cooling modules can also be provisioned.
If boundary scan is to be deployed in the system, a JTAG switch module (JSM) is also installed on the backplane, typically as a plug-in module. The JSM plays the role of the interconnecting hub for the entire boundary-scan infrastructure in the system. In addition, the JSM provides an interface to external JTAG test systems.
The Boundary-Scan Benefit
Although JTAG remains an option in MicroTCA systems, the benefits derived from both current and newly emerging applications, such as system-level JTAG, will certainly win over many system integrators and users. No other technology brings to MicroTCA computing and telecommunications systems the capabilities that boundary scan does, including embedded remote testing, diagnostics, in-system programming, system-level management functionality and many others.
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