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JTAG named after the Joint Test Action Group which codified it is an industry standard for verifying designs and testing printed circuit boards after manufacture.
JTAG implements standards for on-chip instrumentation in electronic design automation EDA as a complementary tool to digital simulation. The interface connects to an on-chip test access port TAP that implements a stateful protocol to access a set of test registers that present chip logic levels and device capabilities of various parts. The Joint Test Action Group formed in to develop a method of verifying designs and testing printed circuit boards after manufacture.
The JTAG standards have been extended by many semiconductor chip manufacturers with specialized variants to provide vendor-specific features. In the s, multi-layer circuit boards and non-lead-frame integrated circuits ICs were becoming standard and connections were being made between ICs that were not available to probes. The majority of manufacturing and field faults in circuit boards were due to poor solder joints on the boards, imperfections among board connections, or the bonds and bond wires from IC pads to pin lead frames.
In the same year, Intel released their first processor with JTAG the which led to quicker industry adoption by all manufacturers. In , a supplement that contains a description of the boundary scan description language BSDL was added. Although JTAG's early applications targeted board level testing, here the JTAG standard was designed to assist with device, board, and system testing, diagnosis, and fault isolation. Today JTAG is used as the primary means of accessing sub-blocks of integrated circuits , making it an essential mechanism for debugging embedded systems which may not have any other debug-capable communications channel.
Those modules let software developers debug the software of an embedded system directly at the machine instruction level when needed, or more typically in terms of high level language source code. System software debug support is for many software developers the main reason to be interested in JTAG. Frequently individual silicon vendors however only implement parts of these extensions.
There are many other such silicon vendor-specific extensions that may not be documented except under NDA. Processors can normally be halted, single stepped, or let run freely. Data breakpoints are often available, as is bulk data download to RAM. Some toolchains can use ARM Embedded Trace Macrocell ETM modules, or equivalent implementations in other architectures to trigger debugger or tracing activity on complex hardware events, like a logic analyzer programmed to ignore the first seven accesses to a register from one particular subroutine.
For example, custom JTAG instructions can be provided to allow reading registers built from arbitrary sets of signals inside the FPGA, providing visibility for behaviors which are invisible to boundary scan operations.
Similarly, writing such registers could provide controllability which is not otherwise available. JTAG allows device programmer hardware to transfer data into internal non-volatile device memory e.
Some device programmers serve a double purpose for programming as well as debugging the device. In addition, internal monitoring capabilities temperature, voltage and current may be accessible via the JTAG port. JTAG programmers are also used to write software and data into flash memory.
Some modern debug architectures provide internal and external bus master access without needing to halt and take over a CPU. In the worst case, it is usually possible to drive external bus signals using the boundary scan facility. As a practical matter, when developing an embedded system, emulating the instruction store is the fastest way to implement the "debug cycle" edit, compile, download, test, and debug.
JTAG boundary scan technology provides access to many logic signals of a complex integrated circuit, including the device pins. This permits testing as well as controlling the states of the signals for testing and debugging. Therefore, both software and hardware manufacturing faults may be located and an operating device may be monitored. The scan chain mechanism does not generally help diagnose or test for timing, temperature or other dynamic operational errors that may occur.
The ability to perform such testing on finished boards is an essential part of Design For Test in today's products, increasing the number of faults that can be found before products ship to customers.
A JTAG interface is a special interface added to a chip. Depending on the version of JTAG, two, four, or five pins are added. The four and five pin interfaces are designed so that multiple chips on a board can have their JTAG lines daisy-chained together if specific conditions are met. In either case a test probe need only connect to a single "JTAG port" to have access to all chips on a circuit board.
The TRST pin is an optional active-low reset to the test logic, usually asynchronous, but sometimes synchronous, depending on the chip. If the pin is not available, the test logic can be reset by switching to the reset state synchronously, using TCK and TMS. Note that resetting test logic doesn't necessarily imply resetting anything else. There are generally some processor-specific JTAG operations which can reset all or part of the chip being debugged. Since only one data line is available, the protocol is serial.
The clock input is at the TCK pin. Different instructions can be loaded. Instructions for typical ICs might read the chip ID, sample input pins, drive or float output pins, manipulate chip functions, or bypass pipe TDI to TDO to logically shorten chains of multiple chips.
As with any clocked signal, data presented to TDI must be valid for some chip-specific Setup time before and Hold time after the relevant here, rising clock edge. Faster TCK frequencies are most useful when JTAG is used to transfer lots of data, such as when storing a program executable into flash memory.
The JTAG state machine can reset, access an instruction register, or access data selected by the instruction register. Sometimes there are event signals used to trigger activity by the host or by the device being monitored through JTAG; or, perhaps, additional control lines.
When exploited, these connections often provide the most viable means for reverse engineering. Reduced pin count JTAG uses only two wires, a clock wire and a data wire. This is defined as part of the IEEE The two wire interface reduced pressure on the number of pins, and devices can be connected in a star topology.
Other two-wire interfaces exist, such as Serial Wire Debug. The picture above shows three TAPs, which might be individual chips or might be modules inside one chip. A daisy chain of TAPs is called a scan chain , or loosely a target. Scan chains can be arbitrarily long, but in practice twenty TAPs is unusually long.
So at a basic level, using JTAG involves reading and writing instructions and their associated data registers; and sometimes involves running a number of test cycles. Most JTAG hosts use the shortest path between two states, perhaps constrained by quirks of the adapter. For example, one adapter [ which? Some layers built on top of JTAG monitor the state transitions, and use uncommon paths to trigger higher level operations.
Instruction register sizes tend to be small, perhaps four or seven bits wide. Two key instructions are:. This allows JTAG hosts to identify the size and, at least partially, contents of the scan chain to which they are connected. So the bits not written by the host can easily be mapped to TAPs. It could for example identify an ARM Cortex-M3 based microcontroller, without specifying the microcontroller vendor or model; or a particular FPGA, but not how it has been programmed.
That way all TAPs except one expose a single bit data register, and values can be selectively shifted into or out of that one TAP's data register without affecting any other TAP. Some of these instructions are "mandatory", but TAPs used for debug instead of boundary scan testing sometimes provide minimal or no support for these instructions.
Devices may define more instructions, and those definitions should be part of a BSDL file provided by the manufacturer. Devices communicate to the world via a set of input and output pins. By themselves, these pins provide limited visibility into the workings of the device. However, devices that support boundary scan contain a shift-register cell for each signal pin of the device.
These registers are connected in a dedicated path around the device's boundary hence the name. The path creates a virtual access capability that circumvents the normal inputs and outputs, providing direct control of the device and detailed visibility for signals.
Commercial test systems often cost several thousand dollars for a complete system, and include diagnostic options to pinpoint faults such as open circuits and shorts.
They may also offer schematic or layout viewers to depict the fault in a graphical manner. To enable boundary scanning, IC vendors add logic to each of their devices, including scan cells for each of the signal pins. These cells are then connected together to form the boundary scan shift register BSR , which is connected to a TAP controller.
Overhead for this additional logic is minimal, and generally is well worth the price to enable efficient testing at the board level. An example helps show the operation of JTAG in real systems. This is a non-trivial example, which is representative of a significant cross section of JTAG-enabled systems. Licensees of this core integrate it into chips, usually combining it with other TAPs as well as numerous peripherals and memory. Examples of such chips include:. Those processors are both intended for use in wireless handsets such as cell phones, which is part of the reason they include TAP controllers which modify the JTAG scan chain: Debugging low power operation requires accessing chips when they are largely powered off, and thus when not all TAPs are operational.
That scan chain modification is one subject of a forthcoming IEEE This debug TAP exposes several standard instructions, and a few specifically designed for hardware-assisted debugging , where a software tool the "debugger" uses JTAG to communicate with a system being debugged:. That model resembles the model used in other ARM cores. Also, the newer cores have updated trace support. One basic way to debug software is to present a single threaded model, where the debugger periodically stops execution of the program and examines its state as exposed by register contents and memory including peripheral controller registers.
When interesting program events approach, a person may want to single step instructions or lines of source code to watch how a particular misbehavior happens. After saving processor state, it could write those registers with whatever values it needs, then execute arbitrary algorithms on the CPU, accessing memory and peripherals to help characterize the system state. Debug mode is also entered asynchronously by the debug module triggering a watchpoint or breakpoint, or by issuing a BKPT breakpoint instruction from the software being debugged.
When it is not being used for instruction tracing, the ETM can also trigger entry to debug mode; it supports complex triggers sensitive to state and history, as well as the simple address comparisons exposed by the debug module. Asynchronous transitions to debug mode are detected by polling the DSCR register. This is how single stepping is implemented: Modern software is often too complex to work well with such a single threaded model.
ARM processors support an alternative debug mode, called Monitor Mode , to work with such situations. This is distinct from the Secure Monitor Mode implemented as part of security extensions on newer ARM cores; it manages debug operations, not security transitions. In those cases, breakpoints and watchpoints trigger a special kind of hardware exception, transferring control to a "debug monitor" running as part of the system software.