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This section presents a simple introduction to JTAG technology.
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In today's complex systems, testability is an increasing concern in almost every application and in every area of application development. Manufacturers that thoroughly address the issue of testability at the device, board, and system levels deliver more consistently reliable and cost-effective products to the marketplace.

This means building in test capabilities in every phase of development and deployment, including design verification, hardware and software integration, manufacturing, and in the field.

In the 1980s, the Joint Test-Action Group (JTAG) formed by representatives from makers and users of components and boards, recognised that only a cooperative effort could address the mounting testability problems in a coordinated way. Its mandate was to propose design structures that semiconductor makers would incorporate into device designs to aid in testing boards and systems. In 1990 the IEEE adopted the proposal as IEEE Standard 1149.1. Its stated purpose was to test interconnections between Integrated Circuits (ICs) installed on boards, modules, hybrids, and other substrates. Manufacturers adopting the standard could also test the IC itself.

Architecture IEEE Standard 1149.1 is a testing standard. However it is described as a collection of design rules applied principally at the IC level that allow software to alleviate the growing cost of designing and producing digital systems. The primary benefit of the standard is its ability to transform extremely difficult printed circuit board testing problems that could be attacked with ad-hoc testing methods into well-structured problems that software can easily and swiftly deal with. To conform to the boundary-scan standard IEEE 1149.1, a device must contain the following: Test Access Port (TAP), Scannable Instruction Register, Scannable Data Registers, TAP Controller.

The Boundary Scan Register and other test features of the device are accessed through a standard interface - the JTAG Test Access Port (TAP). According to the standard, the TAP must contain four signals, each available through a dedicated device pin and they may not be shared with any other function:
- Test Data Input (TDI): it is used to shift data and instruction tests into the Boundary Scan register.
- Test Data Output (TDO): this pin provides data from the Boundary Scan register or other internal register.
- Test Clock (TCK): this input controls test-logic timing independent of clocks that normal system operations employ. The TDI shifts values into the appropriate register on the rising edge of TCK. Selected register contents shift out onto TDO during the TCK's falling edge.
- Test-Mode select (TMS): this input, which also clocks through on the rising edge of TCK, determines the state of the TAP controller.

An optional active-low Test-Reset Pin (TRST#) permits an asynchronous TAP controller initialisation without affecting other device or system logic. Asserting this pin inactivates the boundary-scan register and places the device in normal operating mode. These pins are used with a simple protocol to communicate with on-chip Boundary-Scan logic.

Several different data registers can be built into boundary-scan components. All Boundary-Scan instructions set operational modes that place a selected data register between TDI and TDO. This register is referred to as the target register. This preserves a fundamental notion of Boundary-Scan: TDI and TDO always form the ends of a shift register. The function of this register is dictated by the effective TAP instruction.

Two Data Registers are always required to be present on a 1149.1 component: the Boundary-Scan Register and the Bypass Register. Several others are described by the standard, such as an Identification Register, but are optional.

The Test Data Register loads the data in parallel on the rising edge of TCK in the TAP Controller state CAPTURE-DR (triggering test results), puts the data on the output parallel latch on the falling edge of TCK in the TAP Controller state UPDATE-DR (new test pattern generation), and shifts the serial data through TDI to TDO during the TAP Controller state SHIFT-DR.

The TAP Controller is a 16-state finite state machine added on the IC die itself. It recognises the communication protocol and generates internal control signals used by the remainder of the Boundary Scan logic. The TAP controller is driven by TCK and TMS only; no other signals affect TAP controller. They programme the TAP Controller as a 16-state machine, generating clock and control signals for the instruction and data registers. Only three events can trigger a change of controller state: a test-clock rising edge, and system power-up.

Movement through the state machine is controlled by the value of TMS, a set-up time prior to the rising edge of TCK. The 1s and 0s adjacent to each state transition arc show the value that must be present on TMS at the time of the next rising edge of TCK.

The different state can be divided in 4 groups: Reset, BIST, Data Register Update, Instruction Register Update.

The Boundary-Scan Standard allows for ICs to be linked into chains by linking the TDO pin of one IC with the TDI pin of the next. For example, the 1149.1 ICs on a board may all be linked together by their TDO-TDI pins in succession. Several distinct chains may exist on a board if they do not share any TAP signals.

The 1149.1 Standard allows us to exploit the Boundary-Scan to test a board composed of Boundary-Scan chips.

Interconnect tests look for shorts between boundary-scan nodes and conventional nodes with bed-of-nails or edge-connector access, as well as opens between tester nails and boundary-scan input pins. The EXTEST instruction latches boundary-scan nodes to the state that permits easier in-circuit backdrive (a logic 1 to TTL). The tester then looks for node movement when it forces non-boundary-scan nodes to their opposite states.

Applying this technique to a single conventional node places a 1 on that node and scans out boundary-scan-input states, then injects a 0 onto the test node and scans again. A short between the test node and a boundary-scan node will show up as a failure. An open connection will cause both scanning operations to produce exactly the same output pattern.

Some shorts other than the one between test and boundary-scan nodes can cause this operation to fail. Only rarely, however, will such a faulty node follow the test node at both the latched-1 and latched-0 states. Changing test-node states several times and declaring a short only when the suspect boundary-scan node exactly follows these transitions further improves the likelihood of a correct diagnosis.

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