Embedded Systems Glossary: J
N. A piece of silicon, or IP, capable of executing Java bytecodes natively. When the Java programming language was created, there were no such processors. Therefore, the only way to execute Java code was with a Java virtual machine. Java processors, like those from aJile, make it possible to execute Java bytecodes without a JVM. An interesting side note: compilers exist to translate programs written in other languages, like C and C++, to Java bytecodes. So a Java processor—or a Java virtual machine—can execute code written in any igh-level language.
n. A dithering in time of a pulse or pulse train. Clock jitter is a tremendous problem in high-speed systems, where even 1 ns of jitter in a signal propagated all over a PCB can cause crashes.
(jay tag) abbr. A standard for providing external test access to integrated circuits serially, via a four- or five-pin external interface. Short for Joint Test Action Group, which developed the standard. The JTAG standard has been adopted as an IEEE standard (IEEE 1149 Standard Test Access Port and Boundary-Scan Architecture). JTAG ports have been widely embraced by processor manufacturers. Debug monitors and in-circuit emulators increasingly leverage the capabilities inherent in JTAG. [more]
n. An array of pointers to functions. A jump table is an efficient way to call one of several functions based on some input parameter. The input parameter is typically turned into an integer first, then used as an index into the array of function pointers. The address found there is the destination for the function call.
n. A small piece of metal, usually within a plastic sheath, that is placed over a pair of pins to connect them electronically. By closing or opening this electrical circuit, the jumper acts as a switch. Embedded software can make run-time decisions based on the user's attachment or removal of each jumper on a circuit board.
n. A transistor made by growing P- and N-type material together on a single substrate. The junctions are the P–N interfaces. Junction transistors are thus named to differentiate them from point-contact transistors, the first type of transistor ever made. See also bipolar transistor.