In addition to mandatory features, the IEEE Std. 1149.1 also specifies optional capabilities and resources.
Optional device features include the following:
- An IEEE-1149.1 compliant device may provide the optional Test Logic Reset pin (/TRST, low-active);
- An IEEE-1149.1 compliant device may feature additional data registers (such as Identification Register, internal self test registers, etc.) and test instructions (such as IDCODE, HIGHZ, CLAMP, RUNBIST, etc.);
- Private instructions may be implemented in a device, their opcode has to be identified in the components BSDL file;
- Having an ID register designed in is extremely helpful throughout the whole product life cycle.
- Built-In Self Test (BIST) is a very helpful feature especially for testing in production and field service.
- The HIGHZ instruction can be used to tri-state all Boundary Scan outputs (including Output2) while the Bypass register is put between TDI and TDO of the component.
- The CLAMP instruction should be supported because it allows the shortening of the scan path length without loss of external driven test values.
