Safe Bitstream

When XJTAG starts testing it puts the JTAG devices in the circuit into EXTEST mode (the main mode used during JTAG testing). This gives XJTAG control of the pins on the JTAG devices, and so at this point XJTAG has to choose values to set to all of the pins which it is taking control of. XJTAG calls this stream of data containing the pin values the safe bitstream.

The BSDL file for a JTAG device specifies 'safe' values to scan into the device - generally these are the values which disable the JTAG-controlled output pins. XJTAG's safe bitstream is based on these values but also takes into account the Constant Pins and Disable Values defined in the XJTAG setup.

In addition to this, logic devices will be driven to a known state (typically one which disables the logic device outputs) and external hardware equipment will also be set not to drive where possible.

Using the safe bitstream

As well as at the start of testing, the XJTAG system will use the safe bitstream to automatically set the circuit back to its initial state at the beginning and just after finishing the Connection Test. The safe bitstream is applied after RUNSVF and RUNSTAPL calls with their SVF_RESET and STAPL_RESET arguments enabled.

The user can also force XJTAG to apply the safe bitstream to reset the circuit to its initial state by using the SAFE statement during XJEase testing.