SN54LVT8980A Datasheet, PDF (17/35 Pages) Texas Instruments – EMBEDDED TEST-BUS CONTROLLERS IEEE STD 1149.1 (JTAG) TAP MASTER WITH 8-BIT GENERIC HOST INTERFACES

SN54LVT8980A, SN74LVT8980A
EMBEDDED TESTĆBUS CONTROLLERS
IEEE STD 1149.1 (JTAG) TAP MASTERS WITH 8ĆBIT GENERIC HOST INTERFACES
SCBS755B − APRIL 2002 − REVISED MARCH 2004
command/control
The eTBC command-based architecture is structured around a set of comprehensive IEEE Std 1149.1 (JTAG)
test objectives, which include TAP state movement, scan operations, and run test (operation of test logic in
Run-Test/Idle state). The set of test operations, as decoded from the command register (bits 3−0, OPCOD) is
given in Table 5. Commands are initiated by writing the eTBC command register; upon command initiation, the
test-control logic is initialized and the TDO and TDI buffers are cleared. Command completion is indicated when
the operation code (OPCOD) field of the command register returns to the value of the null command.
The eTBC command operation is modified by the configurationA and configurationB registers, which should be
written prior to writing the command register, as the values in these registers cannot be modified while a
command is in progress. Also, commands are operated only in automatic test modes, as specified in the
configurationA register (bits 2−0, MODE); while in the discrete-control mode, commands are ignored.
All eTBC commands operate similarly to accomplish IEEE Std 1149.1 test objectives. First, the eTBC generates
a TMS sequence to move the target scan chain from its current TAP state to a working state that depends on
the test objective. Second, the command is operated (test run, bits scanned) in the working state for a number
of TCK cycles (or scan bits) determined by the value of the counter upon command initiation. Third, the eTBC
generates a TMS sequence to move the target scan chain from the working state to the end state specified in
the command register (bits 5−4, ENDST). For some commands, one or more of these steps are omitted.
TAP-state-movement commands
Two eTBC commands are provided to accomplish TAP state movement. The state-move command operates
to generate a TMS sequence to move the target scan chain directly from its current TAP state to the end state
specified in the command register. The state-jump command moves the eTBC’s stored value of the target TAP
state without generating any changes to the TMS output. The state-jump command can, therefore, be used to
switch between targets that share the same test bus, such as those in a multidrop backplane configuration
implemented with TI addressable scan ports, but that may be left in different TAP states.
run-test command
The run-test command allows the test logic of the target scan chain to execute autonomously in the
Run-Test/Idle TAP state. Such test logic is commonly used to implement chip- or board-level built-in self test.
The run-test command generates TMS sequences to move the target scan chain from its current TAP state to
the Run-Test/Idle TAP state where it remains for a number of TCK cycles determined by the value of the counter
upon command initiation. Upon the countdown of the counter to zero, the eTBC generates TMS sequences to
move the target scan chain to the end state specified in the command register.
scan commands
Eleven eTBC commands are provided to perform scan operations to target scan chains. These can be classified
by the destination of scan data in the target-addressable scan port (ASP), IEEE Std 1149.1 instruction register,
or IEEE Std 1149.1 data register—and by the nature/direction of the data transfer—full duplex (default), input
only, output only, or recirculate. The only combination of these two factors that is not implemented is recirculate
ASP scan.
addressable scan-port (ASP) scan commands
The ASP scan commands scan data to and/or from an addressable scan-port target. Since ASP devices require
that TMS remain fixed throughout their select and acknowledge protocols, the eTBC does not generate TMS
sequences or change its stored value of the target’s TAP state. Also, for the same reason, ASP scan commands
that target ASP devices should be operated in gated-TCK mode. The ASP scan commands do allow data written
to the TDO buffer to be driven serially onto the TDO pin and bits received serially at the TDI pin to be stored
into the TDI buffer for reading by the host. However, the ASP scan commands do not perform any bit-pair
encoding of ASP select protocols or decoding of ASP acknowledge protocols. Such encoding/decoding must
be performed in the host. The number of data bits transferred in and/or out is determined by the value of the
counter upon command initiation.
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