What is IEEE Standard 1149.7?
IEEE Standard (Std) 1149.7 is a standard for reduced-pin and enhanced-functionality test access port (TAP) and boundary scan architecture. The IEEE Std 1149.7, also known as compact JTAG or cJTAG, is compatible with the traditional IEEE Std 1149.1 (JTAG) standard to provide an enhanced test and debug standard that meets the demands of modern systems.
One unique feature of IEEE Std 1149.7 is the reduced pin count interface for the TAP interface. The new Standard IEEE Std 1149.1 uses a two-wire interface compared to the IEEE Std 1149.1 which uses a mandatory four-wire TAP (TDI, TDO, TMS and TRST) interface. Since the IEEE Std 1149.7 is compatible with IEEE Std 1149.1, this new standard also permits four-wire or five-wire (optional TRST) implementation.
With the adoption of a two-wire interface on IEEE Std 1149.7, devices on the IEEE Std 1149.1 standard will benefit from this, as it makes it easier for boundary scan to be implemented on complicated new package technologies such as system-on-chip (SOC), system-on-package (SOP) and package on package (POP), which are not using 1149.1 TAP interface.
Figure 1: IEEE 1149.7 STD device connections