Leveraging industry standards for FPGA PXI modules
01 May 2019
Today’s test engineers are choosing to incorporate user-programmable FPGA PXI modules as part of their functional test systems. There are a variety of user-programmable FPGA-based instruments available to test engineers, which can be used to support a wide range of applications & interfaces, and these instruments employ a range of programming tools & architectures.
This article was originally featured in EPDT's 2019 PXI for T&M supplement, included in the May 2019 issue of EPDT magazine [read the digital issue version of this article]. Sign up to receive your own copy of EPDT each month.
However, as Michael Dewey, Director of Marketing at aerospace T&M experts, Marvin Test Solutions explains, by employing industry standard design tools and interfaces, test engineers can simplify the overall design and implementation process – as well as facilitating long-term, organic support for test systems and applications.
In this article, we will explore how adopting industry standard design tools and standardised FPGA interfaces, such as the ANSI/VITA 57 standard, can help mitigate the complexity and effort required for test engineers to design, deploy and maintain these user-programmable modules as part of a complex functional test system.
FPGAs were an outgrowth of the programmable array logic (PAL) business in the late 1970s. These devices enabled design engineers to replace discrete logic chips with one programmable chip. FPGAs emerged in the 1980s as a common logic design component, with Xilinx and Altera becoming the market leaders.
Today’s FPGAs offer multiple processing cores, which can run in parallel, allowing different tasks to be executed within separate ‘blocks’ of the FPGA. Within the FPGA are a number of predefined resources, which include logic blocks, both configurable and fixed-function, as well as internal memory. In addition, modern FPGAs have evolved into SoC (system on chip) devices – offering higher levels of functionality, including embedded processors, high-speed serial I/O, video codecs and graphical processing units (GPU).
With a user-programmable, FPGA-based test instrument, such as a card modular PXI module (Fig 1), test engineers can implement solutions using commercially available IP (intellectual property) cores or in-house developed, custom IP – essentially creating their own custom instrument, by programming the module for a specific application. Applications for these FPGA-based modules include support for specialised custom interfaces and hardware-in-the-loop (HiL) implementations that incorporate an embedded, real-time controller, with a specific physical interface, as part of an overall control loop that interfaces to a UUT or unit-under-test (Figure 2).
FPGA design tools & methods
Suppliers of FPGAs and FPGA modules offer toolsets that are optimised for supporting their own specific product offerings. However, depending on the vendor, and as shown in Figure 3, the FPGA design and deployment process can employ either vendor-specific or industry standard tools.
For the design block, the use of industry standard tools, specifically HDL (either VHDL or Verilog) is highly desirable. VHDL and Verilog are open tools and are widely accepted standards. And since they are vendor-neutral tools, design portability can be maximised, minimising vendor ‘lock-in’ to a specific type of FPGA or module, as well as mitigating dependence on one supplier or vendor. In addition, these tools are supported by IEEE standards for common HDL languages (1076-2008 – IEEE Standard VHDL Language Reference Manual and 1364-2005 – IEEE Standard for Verilog Hardware Description Language). And as an added benefit, access and incorporation of third party IP as part of an overall FPGA design is greatly simplified, since almost all commercially available IP is supplied via HDL.
Leveraging a standardised FPGA interface
Virtually all user-programmable FPGA modules require external hardware that interfaces the FPGA to the external world or UUT. Table 1 lists examples of physical interfaces (PHY) that might be supported by an external FPGA interface. Implementation of these interfaces requires external interface boards that are custom to a specific vendor’s FPGA module. Figure 4 details an example of an FPGA module which employs a daughter board that is installed on the main board’s assembly. Other implementations involve the use of an external interface module, which must be attached to the FPGA module’s front panel interface.
The VITA 57 FMC standard
An alternative to vendor-specific PHY interface solutions is the VITA 57 FPGA Mezzanine Card (FMC) standard, which standardises both the physical and electrical interfaces to FPGAs, resulting in the availability of a portfolio of commercial, off-the-shelf mezzanine boards. The FMC standard is defined by the VITA 57.1 and 57.4 specifications, and specifies PCB dimensions, connector locations and standardised interface connections (Figure 5).
By choosing to adopt user-programmable FPGA modules that incorporate the VITA 57 interface, test engineers now have available to them a portfolio of off-the-shelf interface modules that address a wide range of applications, including:
• Analogue I/O – ADCs & DACs
• High speed digital parallel I/O – such as Camera Link or LVDS
• High speed serial digital I/O – such as fibre optics, serial front panel data port (FPDP), gigabit Ethernet, and so on
A PXIe user-programmable FPGA module, such as the one shown in Figure 6, incorporates the FMC interface and includes full PXIe functionality:
• On-board programmable clock source, for use as an FPGA timing reference or for FMC module support
• Access to the 10 MHz PXI clock and the 100 MHz PXIe clock
• Access to all PXI trigger and local bus signals
And by employing an off-the-shelf FMC module, test engineers can realise shorter development and verification time for designs, since both the interface and mezzanine card are known entities. In addition, most FMC modules are supplied with HDL code, further simplifying the overall FPGA integration and verification process.
Today’s test engineers can choose from a wide selection of user-programmable FPGA modules for ATE systems, with many of these modules employing proprietary design tools and methods. However, test engineers should look to leverage industry standards wherever possible when employing user-programmable FPGA instrumentation. By building on established standards and standard interface modules, such as VITA 57, the effort associated with the design, deployment and long-term maintenance of test systems that employ these user-programmable FPGA modules can be positively impacted.
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