As boundary scan test was making its way to center stage for testing surface mounted devices, there was no agreed upon protocol to manage the hundreds of thousands of bits of IEEE 1149.1 test pattern information. Instead, many vendors had vendor specific protocols for managing the data. In the early 1990s, Texas Instruments and Teradyne jointly developed Serial Vector Format, most commonly known as SVF, a vendor independent format that could be used across a wide variety of test equipment and simulation software. For example, Xilinx’s iMPACT software generates SVF files. Those SVF files can then be used by third-party programming utilities, like onTAP, as input files to program Xilinx devices, such as FPGAs and Flash, in a JTAG chain.
How Does SVF Do It?
SVF can best be viewed as an organizational tool. Rather than scanning hundreds of thousands of bits of information, SVF condenses the information in a compact manner by representing JTAG test patterns in ACSII file format.
SVF uses control instructions to manage boundary scan Test Access Ports (TAPs) and uses data instructions to load boundary scan registers. An example of a control instruction is STATE RESET which equipment reading an SVF uses to manipulate TAP pins and place a TAP’s state machine in a reset state. Many types of boundary scan registers exist but the common registers for test are a TAP’s instruction, IDCODE, and boundary registers. Equipment reading an SVF uses Scan Instruction Register (SIR) instructions and data to load a TAP’s instruction register with OPCODE values, and uses the Scan Data Register (SDR) instruction to load data values into and read data values from a selected register.
Limitations of SVF
While the basic SVF met the goal of providing a protocol that was vendor independent and could be used on a variety of test platforms, such as ATE and boundary scan software, there were limitations. For example, after running SVF files an operator could be alerted that the returned result was not the expected result. But, SVF could not pinpoint the error. And, what about running multiple boundary scan chains? Or, what if a cluster test needs to be developed and run? SVF alone could not address these issues, yet these very issues are commonplace in boundary scan testing. So, what’s the solution?
onTAP and Extended SVF
Because the limitations of SVF were at a crossroads with our customer requirements, we extended the SVF by adding additional instructions and data that are embedded in the SVF file. These instructions allow onTAP to deliver pin-level diagnostics, run unlimited JTAG chains, and provide flow control to facilitate cluster test and Flash programming.
If you would like to learn more about JTAG and Boundary Scan Testing, please see some of our past blog posts. If you would like to learn more about the onTAP Boundary Scan JTAG Solution, feel free to call us at (603) 598-4444.