The JTAG (IEEE Standard 1149.1-1990 Standard-Test-Access Port and Boundary Scan Architecture) interface is used for BSDL testing and emulation of the device. The trstn pin only needs to be released when it is necessary to use a JTAG controller to debug the device or exercise the device's boundary scan functionality. For maximum reliability, the device includes an internal Pulldown (IPD) on the trstn pin to ensure that trstn is always asserted upon power up and the device's internal emulation logic is always properly initialized. JTAG controllers from Texas Instruments actively drive trstn high. However, some third-party JTAG controllers may not drive trstn high but expect the use of a Pullup resistor on trstn. When using this type of JTAG controller, assert trstn to initialize the device after powerup and externally drive trstn high before attempting any emulation or boundary-scan operations.
The main JTAG features include: