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'''JTAG''' (named after the '''Joint Test Action Group''' which codified it) is an [[Technical standard|industry standard]] for verifying designs and testing [[printed circuit board]]s after manufacture.

JTAG implements standards for on-chip instrumentation in [[electronic design automation]] (EDA) as a complementary tool to [[logic simulation|digital simulation]].<ref name=stollon>{{cite book|author=Neal Stollon|title=On-Chip Instrumentation|publisher=Springer|year=2011}}</ref> It specifies the use of a dedicated debug port implementing a [[serial communication]]s interface for low-overhead access without requiring direct external access to the system address and data buses. The interface connects to an on-chip test access port (TAP) that implements a [[State (computer science)|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 1985 to develop a method of verifying designs and testing [[printed circuit board]]s after manufacture. In 1990 the [[Institute of Electrical and Electronics Engineers]] codified the results of the effort in IEEE Standard 1149.1-1990, entitled ''Standard Test Access Port and Boundary-Scan Architecture''.

The JTAG standards have been extended by many semiconductor chip manufacturers with specialized variants to provide vendor-specific features.<ref>[http://www.intel.com/content/dam/www/public/us/en/documents/white-papers/jtag-101-ieee-1149x-paper.pdf Randy Johnson, Steward Christie (Intel Corporation, 2009), ''JTAG 101—IEEE 1149.x and Software Debug'']</ref>

== History ==

In the 1980s, multi-layer circuit boards and non-lead-frame [[integrated circuit]]s (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. The Joint Test Action Group (JTAG) was formed in 1985 to provide a pins-out view from one IC pad to another so these faults could be discovered.

The industry standard became an [[IEEE]] standard in 1990 as IEEE Std. 1149.1-1990<ref name="ieee1149.1-1990">Copies of [http://standards.ieee.org/reading/ieee/std_public/description/testtech/1149.1-1990_desc.html IEEE 1149.1-1990] or its 2001 update may be bought from the IEEE.</ref> after many years of initial use. In the same year, [[Intel]] released their first [[Central processing unit|processor]] with JTAG (the [[80486]]) which led to quicker industry adoption by all manufacturers.  In 1994, a supplement that contains a description of the [[boundary scan description language]] (BSDL) was added. Further refinements regarding the use of all-zeros for EXTEST, separating the use of SAMPLE from PRELOAD and better implementation for OBSERVE_ONLY cells were made and released in 2001.<ref name="ieee1149.1-2001">[http://standards.ieee.org/reading/ieee/std_public/description/testtech/1149.1-2001_desc.html IEEE 1149.1-2001]</ref>   Since 1990, this standard has been adopted by [[electronics]] companies around the world. [[Boundary scan]] is now mostly synonymous with JTAG, but JTAG has essential uses beyond such manufacturing applications.

=== Debugging ===

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 circuit]]s, making it an essential mechanism for [[debugging]] [[embedded system]]s which may not have any other debug-capable communications channel.{{citation needed|reason=This statement claims that JTAG is of critical use for a certain application but doesn't provide any source that supports the statement.|date=October 2017}}  On most systems, JTAG-based debugging is available from the very first instruction after CPU reset, letting it assist with development of early boot software which runs before anything is set up. An [[in-circuit emulator]] (or, more correctly, a "JTAG adapter") uses JTAG as the transport mechanism to access on-chip [[debug]] modules inside the target [[Central processing unit|CPU]]. 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. Many silicon architectures such as PowerPC, MIPS, ARM, x86  built an entire software debug, instruction tracing, and data tracing infrastructure around the basic JTAG protocol. Frequently individual silicon vendors however only implement parts of these extensions. Some examples are ARM [[CoreSight]] and [[Nexus (standard)|Nexus]] as well as Intel's BTS (Branch Trace Storage), LBR (Last Branch Record), and IPT (Intel Processor Trace) implementations. There are many other such silicon vendor-specific extensions that may not be documented except under [[Non-disclosure agreement|NDA]]. The adoption of the JTAG standard helped move JTAG-centric debugging environments away from early processor-specific designs.  Processors can normally be halted, single stepped, or let run freely.  One can set code breakpoints, both for code in RAM (often using a special machine instruction) and in ROM/flash.  Data breakpoints are often available, as is bulk data download to RAM.  Most designs have “halt mode debugging”, but some allow debuggers to access registers and data buses without needing to halt the core being debugged.  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.

Sometimes [[FPGA]] developers also use JTAG to develop debugging tools.<ref name="fpga">[http://www.embeddeddesignindia.co.in/ART_8800568419_2800006_TA_d2d96055.HTM Select the right FPGA debug method] presents one of the models for such tools.</ref>  The same JTAG techniques used to debug software running inside a [[CPU]] can help debug other digital design blocks inside an FPGA.  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.

=== Storing firmware ===

JTAG allows [[Programmer (hardware)|device programmer hardware]] to transfer data into internal non-volatile device memory (e.g. [[Complex programmable logic device|CPLDs]]). Some device programmers serve a double purpose for programming as well as debugging the device. In the case of FPGAs, volatile memory devices can also be programmed via the JTAG port, normally during development work. 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]].  This is usually done using data bus access like the CPU would use, and is sometimes actually handled by a CPU, but in other cases memory chips have JTAG interfaces themselves.  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).{{citation needed|date=June 2015}}  This is because the in-circuit emulator simulating an instruction store can be updated very quickly from the development host via, say, USB. Using a serial UART port and bootloader to upload firmware to Flash makes this debug cycle quite slow and possibly expensive in terms of tools; installing firmware into Flash (or SRAM instead of Flash) via JTAG is an intermediate solution between these extremes.

=== Boundary scan testing ===

JTAG [[boundary scan]] technology provides access to many logic signals of a complex integrated circuit, including the device pins. The signals are represented in the boundary scan register (BSR) accessible via the TAP. 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.

When combined with built-in self-test ([[Built-in self-test|BIST]]), the JTAG scan chain enables a low overhead, embedded solution to testing an IC for certain static faults (shorts, opens, and logic errors).  The scan chain mechanism does not generally help diagnose or test for timing, temperature or other dynamic operational errors that may occur.  [[Test case]]s are often provided in standardized formats such as [[Serial Vector Format|SVF]], or its binary sibling XSVF, and used in production tests.  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.

== Electrical characteristics ==

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 chain (electrical engineering)|daisy-chained]] together if specific conditions are met.<ref>[http://www.jtagtest.com/faq/jtag-ieee-1149-1/under-what-conditions-can-i-daisy-chain-jtag FAQ: Under what conditions can I daisy-chain JTAG?]</ref> The two pin interface is designed so that multiple chips can be connected in a [[Star Network|star topology]]. In either case a [[test probe]] need only connect to a single "JTAG port" to have access to all chips on a [[circuit board]].

=== Daisy-chained JTAG (IEEE 1149.1) ===
[[Image:Jtag chain.svg|center|Example of JTAG chain. Test reset signal is not shown]]

The connector pins are:

#'''TDI''' (Test Data In)
#'''TDO''' (Test Data Out)
#'''TCK''' (Test Clock)
#'''TMS''' (Test Mode Select)
#'''TRST''' (Test Reset) optional.

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 communications|serial]]. The clock input is at the TCK pin. One bit of data is transferred in from TDI, and out to TDO per TCK rising clock edge. 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. TDO data is valid for some chip-specific time after the falling edge of TCK.

The maximum operating frequency of TCK varies depending on all chips in the chain (the lowest speed must be used), but it is typically 10-100&nbsp;MHz (100-10 ns per bit).  Also TCK frequencies depend on board layout and JTAG adapter capabilities and state.  One chip might have a 40&nbsp;MHz JTAG clock, but only if it is using a 200&nbsp;MHz clock for non-JTAG operations; and it might need to use a much slower clock when it is in a low power mode.  Accordingly, some JTAG adapters have ''adaptive clocking'' using an RTCK (Return TCK) signal.  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]].

Clocking changes on TMS steps through a standardized JTAG [[state machine]].  The JTAG state machine can reset, access an instruction register, or access data selected by the instruction register.

JTAG platforms often add signals to the handful defined by the IEEE 1149.1 specification.  A System Reset (SRST) signal is quite common, letting debuggers reset the whole system, not just the parts with JTAG support.  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.

Even though few consumer products provide an explicit JTAG port connector, the connections are often available on the [[printed circuit board]] as a remnant from development [[prototype|prototyping]] and/or production. When exploited, these connections often provide the most viable means for [[reverse engineering]].

=== Reduced pin count JTAG (IEEE 1149.7) ===
[[File:Example of reduced pin count JTAG interface.svg|thumb|Example of JTAG with reduced pin count]]

Reduced pin count JTAG uses only two wires, a clock wire and a data wire. This is defined as part of the IEEE 1149.7 standard.<ref name="ieee-1149.7"/> The connector pins are:

#'''TMSC''' (Test Serial Data)
#'''TCKC''' (Test Clock)

It is called cJTAG for compact JTAG.

The two wire interface reduced pressure on the number of pins, and devices can be connected in a [[star topology]].<ref>[http://www.corelis.com/education/Major-Benefits-of-IEEE-1149.7.htm]</ref> The star topology enables some parts of the system to be powered down, while others can still be accessed over JTAG; a daisy chain requires all JTAG interfaces to be powered. Other two-wire interfaces exist, such as [[#Similar interface standards|Serial Wire Debug]].

== Communications model ==

In JTAG, devices expose one or more ''test access ports'' (TAPs).  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.{{Citation needed|date=June 2010}}

To use JTAG, a host is connected to the target's JTAG signals (TMS, TCK, TDI, TDO, etc.) through some kind of ''JTAG adapter'', which may need to handle issues like level shifting and [[galvanic isolation]].  The adapter connects to the host using some interface such as USB, PCI, Ethernet, and so forth.

=== Primitives ===

The host communicates with the TAPs by manipulating TMS and TDI in conjunction with TCK, and reading results through TDO (which is the only standard host-side input). TMS/TDI/TCK output transitions create the basic JTAG communication primitive on which higher layer protocols build:

* ''State switching'' ... All TAPs are in the same state, and that state changes on TCK transitions.  This JTAG state machine is part of the JTAG spec, and includes sixteen states.  There are six “stable states” where keeping TMS stable prevents the state from changing.  In all other states, TCK always changes that state.  In addition, asserting TRST forces entry to one of those stable states (Test_Logic_Reset), in a slightly quicker way than the alternative of holding TMS high and cycling TCK five times.
* ''Shifting'' ... Most parts of the JTAG state machine support two stable states used to transfer data.  Each TAP has an ''instruction register'' (IR) and a ''data register'' (DR).  The size of those registers varies between TAPs, and those registers are combined through TDI and TDO to form a large shift register.  (The size of the DR is a function of the value in that TAP's current IR, and possibly of the value specified by a SCAN_N instruction.) There are three operations defined on that shift register:
** ''Capturing'' a temporary value
*** Entry to the ''Shift_IR'' stable state goes via the Capture_IR state, loading the shift register with a partially fixed value (not the current instruction)
*** Entry to the ''Shift_DR'' stable state goes via the Capture_DR state, loading the value of the Data Register specified by the TAP's current IR.
** ''Shifting'' that value bit-by-bit, in either the Shift_IR or Shift_DR stable state; TCK transitions shift the shift register one bit, from TDI towards TDO, exactly like a [[Serial Peripheral Interface Bus|SPI]] mode 1 data transfer through a daisy chain of devices (with TMS=0 acting like the chip select signal, TDI as MOSI, etc.).
** ''Updating'' IR or DR from the temporary value shifted in, on transition through the Update_IR or Update_DR state.  Note that it is not possible to read (capture) a register without writing (updating) it, and vice versa.  A common idiom adds flag bits to say whether the update should have side effects, or whether the hardware is ready to execute such side effects.
* ''Running'' ... One stable state is called Run_Test/Idle.  The distinction is TAP-specific.  Clocking TCK in the Idle state has no particular side effects, but clocking it in the Run_Test state may change system state.  For example, some [[ARM9]] cores support a debugging mode where TCK cycles in the Run_Test state drive the instruction pipeline.

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.  Behind those registers is hardware that is not specified by JTAG, and which has its own states that is affected by JTAG activities.

Most JTAG hosts use the shortest path between two states, perhaps constrained by quirks of the adapter.  (For example, one adapter{{Which|date=March 2010}} only handles paths whose lengths are multiples of seven bits.)  Some layers built on top of JTAG monitor the state transitions, and use uncommon paths to trigger higher level operations.  Some ARM cores use such sequences to enter and exit a two-wire (non-JTAG) [[Serial Wire Debug|SWD]] mode.  A Zero Bit Scan (ZBS) sequence is used in IEEE 1149.7<ref name="ieee-1149.7">Texas Instruments is one adopter behind this standard, and has an [http://tiexpressdsp.com/wiki/index.php?title=IEEE_1149.7 IEEE 1149.7 wiki page] with more information.</ref> to access advanced functionality such as switching TAPs into and out of scan chains, power management, and a different two-wire mode.

=== JTAG IEEE Std 1149.1 (boundary scan) instructions ===

Instruction register sizes tend to be small, perhaps four or seven bits wide.  Except for BYPASS and EXTEST, all instruction opcodes are defined by the TAP implementor, as are their associated data registers; undefined instruction codes should not be used.  Two key instructions are:

* The BYPASS instruction, an opcode of all ones regardless of the TAP's instruction register size, must be supported by all TAPs.  The instruction selects a single bit data register (also called BYPASS).  The instruction allows this device to be bypassed (do nothing) while other devices in the scan path are exercised.<ref name="ieee1149.1-2001" />
* The optional IDCODE instruction, with an implementor-defined opcode.  IDCODE is associated with a 32-bit register (IDCODE).  Its data uses a standardized format that includes a manufacturer code (derived from the [[JEDEC]] ''Standard Manufacturer's Identification Code'' standard, JEP-106), a part number assigned by the manufacturer, and a part version code.  IDCODE is widely, but not universally, supported.

On exit from the RESET state, the instruction register is preloaded with either BYPASS or IDCODE.  This allows JTAG hosts to identify the size and, at least partially, contents of the scan chain to which they are connected.  (They can enter the RESET state then scan the Data Register until they read back the data they wrote.  A BYPASS register has only a zero bit; while an IDCODE register is 32-bits and starts with a one.  So the bits not written by the host can easily be mapped to TAPs.)  Such identification is often used to sanity check manual configuration, since IDCODE is often unspecific.  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.

A common idiom involves shifting BYPASS into the instruction registers of all TAPs except one, which receives some other instruction.  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.

The IEEE 1149.1 (JTAG) standard describes a number of instructions to support boundary scan applications.  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.  Those "mandatory" instructions operate on the Boundary Scan Register (BSR) defined in the [[boundary scan description language|BSDL]] file, and include:

* EXTEST for external testing, such as using pins to probe board-level behaviors
* PRELOAD loading pin output values before EXTEST (sometimes combined with SAMPLE)
* SAMPLE reading pin values into the boundary scan register

IEEE-defined "Optional" instructions include:
* CLAMP a variant of BYPASS which drives the output pins using the PRELOADed values
* HIGHZ deactivates the outputs of all pins
* INTEST for internal testing, such as using pins to probe on-chip behaviors
* RUNBIST places the chip in a self-test mode
* USERCODE returns a user-defined code, for example to identify which FPGA image is active

Devices may define more instructions, and those definitions should be part of a BSDL file provided by the manufacturer.  They are often only marked as PRIVATE.

=== Boundary scan register ===

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.<ref name="oshana">{{cite news
 | first = Rob
 | last = Oshana
 | url = http://www.embedded.com/electronics-blogs/beginner-s-corner/4024466/Introduction-to-JTAG
 | title = Introduction to JTAG
 | work = Embedded Systems Design
 | date = 29 October 2002
 | accessdate =5 April 2007
}}</ref>

The contents of the boundary scan register, including signal I/O capabilities, are usually described by the manufacturer using a part-specific [[boundary scan description language|BSDL]] file.  These are used with design 'netlists' from CAD/EDA systems to develop tests used in board manufacturing.  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. These designs are parts of most Verilog or VHDL libraries.  Overhead for this additional logic is minimal, and generally is well worth the price to enable efficient testing at the board level.

== Example: ARM11 debug TAP ==

An example helps show the operation of JTAG in real systems.  The example here is the debug TAP of an [[ARM11]] processor, the ARM1136<ref name="ARM1136">[http://infocenter.arm.com/help/topic/com.arm.doc.ddi0211k/DDI0211K_arm1136_r1p5_trm.pdf ARM1136JF-S and ARM1136J-S Technical Reference Manual] revision r1p5, ARM DDI 0211K.  Chapter 14 presents the Debug TAP.  Other ARM11 cores present the same model through their Debug TAPs.</ref> core.  The processor itself has extensive JTAG capability, similar to what is found in other CPU cores, and it is integrated into chips with even more extensive capabilities accessed through JTAG.

This is a non-trivial example, which is representative of a significant cross section of JTAG-enabled systems.  In addition, it shows how control mechanisms are built using JTAG's register read/write primitives, and how those combine to facilitate testing and debugging complex logic elements; [[CPU]]s are common, but [[FPGA]]s and [[Application-specific integrated circuit|ASIC]]s include other complex elements which need to be debugged.

Licensees of this core integrate it into chips, usually combining it with other TAPs as well as numerous peripherals and memory.  One of those other TAPs handles boundary scan testing for the whole chip; it is not supported by the debug TAP.  Examples of such chips include:

* The [[Texas Instruments OMAP|OMAP2420]], which includes a boundary scan TAP, the ARM1136 Debug TAP, an ETB11 trace buffer TAP, a [[Texas Instruments TMS320|C55x DSP]], and a TAP for an [[ARM7]] TDMI-based imaging engine, with the boundary scan TAP ("ICEpick-B") having the ability to splice TAPs into and out of the JTAG scan chain.<ref name="omap2420">Documentation for the OMAP2420 is not publicly available.  However, a [[Texas Instruments]] document [http://wiki.davincidsp.com/images/9/90/Dbjtag_users_guide.pdf The User's Guide to DBGJTAG] discussing a JTAG diagnostic tool presents this OMAP2420 scan chain example (and others).</ref>
* The [[i.MX31]] processor, which is similar, although its "System JTAG" boundary scan TAP,<ref name="iMX31">See "i.MX35 (MCIMX35) Multimedia Applications Processor Reference Manual" from the [[Freescale]] web site.  Chapter 44 presents its "Secure JTAG Controller" (SJC).</ref> which is very different from ICEpick, and it includes a TAP for its DMA engine instead of a DSP and imaging engine.

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 1149.7<ref name="ieee-1149.7"/> standard.

=== JTAG facilities ===

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:

* <code>BYPASS</code> and <code>IDCODE</code>, standard instructions as described above
* <code>EXTEST</code>, <code>INTEST</code>, standard instructions, but operating on the core instead of an external boundary scan chain.  <code>EXTEST</code> is nominally for writing data to the core, <code>INTEST</code> is nominally for reading it; but two scan chains are exceptions to that rule.
* <code>SCAN_N</code> ARM instruction to select the numbered scan chain used with <code>EXTEST</code> or <code>INTEST</code>.  There are six scan chains:
** <code>0</code> - Device ID Register, 40 bits of read-only identification data
** <code>1</code> - Debug Status and Control Register (DSCR), 32 bits used to operate the debug facilities
** <code>4</code> - Instruction Transfer Register (ITR), 33 bits (32 instruction plus one status bit) used to execute processor instructions while in a special "Debug Mode" (see below)
** <code>5</code> - Debug Communications Channel (DCC), 34 bits (one long data word plus two status bits) used for bidirectional data transfer to the core.  This is used both in debug mode, and possibly at runtime when talking to debugger-aware software.
** <code>6</code> - Embedded Trace Module (ETM), 40 bits (7 bit address, one 32-bit long data word, and a R/W bit) used to control the operation of a passive instruction and data trace mechanism.  This feeds either an on-chip Embedded Trace Buffer (ETB), or an external high speed trace data collection pod.  Tracing supports passive debugging (examining execution history) and profiling for performance tuning.
** <code>7</code> - debug module, 40 bits (7 bit address, one 32-bit long data word, and a R/W bit) used to access hardware breakpoints, watchpoints, and more.  These can be written while the processor is running; it does not need to be in Debug Mode.
* <code>HALT</code> and <code>RESTART</code>, ARM11-specific instructions to halt and restart the CPU.  Halting it puts the core into the "Debug Mode", where the ITR can be used to execute instructions, including using the DCC to transfer data between the debug (JTAG) host and the CPU.
* <code>ITRSEL</code>, ARM11-specific instruction to accelerate some operations with ITR.
That model resembles the model used in other ARM cores.  Non-ARM systems generally have similar capabilities, perhaps implemented using the [[Nexus (standard)|Nexus]] protocols on top of JTAG, or other vendor-specific schemes.

Older [[ARM7]] and [[ARM9]] cores include an ''EmbeddedICE'' module<ref name="arm9ejs">[http://infocenter.arm.com/help/topic/com.arm.doc.ddi0222b/DDI0222.pdf ARM9EJ-S Technical Reference Manual] revision r1p2.  Appendix B "Debug in Depth" presents the EmbeddedICE-RT module, as seen in the popular ARM926ejs core.</ref> which combines most of those facilities, but has an awkward mechanism for instruction execution:  the debugger must drive the CPU instruction pipeline, clock by clock, and directly access the data buses to read and write data to the CPU.  The ARM11 uses the same model for trace support (ETM, ETB) as those older cores.

Newer ARM Cortex cores closely resemble this debug model, but build on a ''Debug Access Port'' (DAP) instead of direct CPU access.  In this architecture (named ''CoreSight Technology''), core and JTAG module is completely independent.  They are also decoupled from JTAG so they can be hosted over ARM's two-wire '''SWD''' interface (see below) instead of just the six-wire JTAG interface.  (ARM takes the four standard JTAG signals and adds the optional TRST, plus the RTCK signal used for adaptive clocking.) The CoreSight JTAG-DP is asynchronous to the core clocks, and does not implement RTCK<ref>http://infocenter.arm.com/help/topic/com.arm.doc.ddi0314h/Chdhbbjd.html#Chdihede</ref>.  Also, the newer cores have updated trace support.

=== Halt mode debugging ===

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.

So for example a JTAG host might HALT the core, entering Debug Mode, and then read CPU registers using ITR and DCC.  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.  After the debugger performs those operations, the state may be restored and execution continued using the RESTART instruction.

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:  HALT the core, set a temporary breakpoint at the next instruction or next high-level statement, RESTART, poll DSCR until you detect  asynchronous entry to debug state, remove that temporary breakpoint, repeat.

=== Monitor mode debugging ===

Modern software is often too complex to work well with such a single threaded model.  For example, a processor used to control a motor (perhaps one driving a saw blade) may not be able to safely enter halt mode; it may need to continue handling interrupts to ensure physical safety of people and/or machinery.  Issuing a HALT instruction using JTAG might be dangerous.

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.  This monitor communicates with the debugger using the DCC, and could arrange for example to single step only a single process while other processes (and interrupt handlers) continue running.

== Common extensions ==

Microprocessor vendors have often defined their own core-specific debugging extensions.  Such vendors include [[Infineon]], MIPS with EJTAG, and more.  If the vendor does not adopt a standard (such as the ones used by ARM processors; or Nexus), they need to define their own solution.  If they support boundary scan, they generally build debugging over JTAG.

[[Freescale]] has COP and OnCE (On-Chip Emulation). OnCE includes a JTAG command which makes a TAP enter a special mode where the IR holds OnCE debugging commands<ref name="OnCE">AN1817/D, "MMC20xx M•CORE OnCE Port Communication and Control Sequences"; Freescale Semiconductor, Inc.; 2004.  Not all processors support the same OnCE module.</ref> for operations such as single stepping, breakpointing, and accessing registers or memory.  It also defines EOnCE (Enhanced On-Chip Emulation)<ref name="EOnCE">AN2073 "Differences Between the EOnCE and OnCE Ports"; Freescale Semiconductor, Inc.; 2005.</ref> presented as addressing real time concerns.

[[ARM architecture|ARM]] has an extensive processor core debug architecture (CoreSight) that started with EmbeddedICE (a debug facility available on most ARM cores), and now includes many additional components such as an ETM (Embedded Trace Macrocell), with a high speed trace port, supporting multi-core and multithread tracing.  Note that tracing is non-invasive; systems do not need to stop operating to be traced.  (However, trace data is too voluminous to use JTAG as more than a trace control channel.)

[[Nexus (standard)|Nexus]] defines a processor debug infrastructure which is largely vendor-independent.  One of its hardware interfaces is JTAG.  It also defines a high speed auxiliary port interface, used for tracing and more.  Nexus is used with some newer platforms, such as the [[Atmel]] AVR32 and Freescale MPC5500 series processors.

== Uses ==

* Except for some of the very lowest end systems, essentially all [[embedded systems]] platforms have a JTAG port to support in-circuit debugging and firmware programming as well as for boundary scan testing:
** [[ARM architecture]] processors come with JTAG support, sometimes supporting a two-wire "SWD" variant or high speed tracing of traffic on instruction or data busses.
** Modern 8-bit and 16-bit [[microcontroller]] chips, such as [[Atmel AVR]] and [[TI MSP430]] chips, support JTAG programming and debugging.  However, the very smallest chips may not have enough pins to spare (and thus tend to rely on proprietary single-wire programming interfaces); if the pin count is over 32, there is probably a JTAG option.
** Almost all [[FPGA]]s and [[CPLD]]s used today can be programmed via a JTAG port. A [[Standard Test and Programming Language]] is defined by JEDEC standard JESD-71 for JTAG programming of PLD's.
** Many [[MIPS architecture|MIPS]] and [[PowerPC]] processors have JTAG support
** Intel Core, Xeon, Atom, and Quark processors all support JTAG probe mode with Intel specific extensions of JTAG using the so-called 60-pin eXtended Debug Port [XDP]. Additionally the Quark processor supports more traditional 10-pin connectors.
** Consumer products such as networking appliances and [[satellite television]] [[integrated receiver/decoder]]s often use microprocessors which support JTAG, providing an alternate means to reload [[firmware]] if the existing [[bootloader]] has been corrupted in some manner.
*The [[Peripheral Component Interconnect|PCI]] bus connector standard contains optional JTAG signals on pins 1-5;<ref>[http://www.techfest.com/hardware/bus/pci.htm#4.10 PCI Local Bus Technical Summary, 4.10 JTAG/Boundary Scan Pins]</ref> [[PCI Express]] contains JTAG signals on pins 5-9.<ref>[http://www.interfacebus.com/Design_PCI_Express_16x_PinOut.html PCI-Express 16x Connector Pin Out]</ref> A special JTAG card can be used to reflash a corrupt [[BIOS]].
* Boundary scan testing and in-system (device) programming applications are sometimes programmed using the [[Serial Vector Format]], a textual representation of JTAG operations using a simple syntax. Other programming formats include 'JAM' and STAPL plus more recently the IEEE Std. 1532 defined format 'ISC' (short for In-System Configuration). ISC format is used in conjunction with enhanced BSDL models for programmable logic devices (i.e. FPGAs and CPLDs) that include addition ISC_<operation> instructions in addition to the basic bare minimum IEEE 1149.1 instructions. FPGA programming tools from [[Xilinx]], Altera, Lattice, Cypress, Actel, etc. typically are able to export such files.
* As mentioned, many boards include JTAG connectors, or just pads, to support manufacturing operations, where boundary scan testing helps verify board quality (identifying bad solder joints, etc.) and to initialize [[flash memory]] or FPGAs.
* JTAG can also support field updates and troubleshooting.

== Client support ==

The target's JTAG interface is accessed using some JTAG-enabled application and some JTAG adapter hardware.  There is a wide range of such hardware, optimized for purposes such as production testing, debugging high speed systems, low cost microcontroller development, and so on.  In the same way, the software used to drive such hardware can be quite varied.  Software developers mostly use JTAG for debugging and updating firmware.

=== Connectors ===
[[Image:Netgear ProSafe Dual WAN VPN Gigabit Firewall FVS336G JTAG interface.jpeg|thumb|right|A [[Netgear]] FVS336G [[firewall (computing)|firewall]] with a 14 pin JTAG header at lower left.]]
[[Image:ADSL modem router internals labeled.jpg|thumb|right|A Netgear DG632 [[ADSL modem]] with an 8 pin JTAG header at location "5".]]

There are no official standards for JTAG adapter physical connectors.  Development boards usually include a header to support preferred development tools; in some cases they include multiple such headers, because they need to support multiple such tools.  For example, a microcontroller, FPGA, and ARM application processor rarely shares tools, so a development board using all of those components might have three or more headers.  Production boards may omit the headers, or when space is limited may provide JTAG signal access using test points.

Some common pinouts<ref>[http://www.jtagtest.com/pinouts/ JTAG Pinouts] lists a few JTAG-only header layouts that have widespread tool support.</ref> for {{convert|2.54|mm|in|abbr=on}} [[pin header]]s are:

* ARM 2×10 pin (or sometimes the older 2×7), used by almost all ARM based systems
* MIPS EJTAG (2×7 pin) used for [[MIPS Technologies|MIPS]] based systems
* 2×5 pin [[Altera]] ByteBlaster-compatible JTAG extended by many vendors
* 2×5 pin [[Atmel AVR|AVR]] extends Altera JTAG with SRST (and in some cases TRST and an event output)
* 2×7 pin [[Texas Instruments]] used with [[Texas Instruments TMS320|DSPs]] and ARM-based products such as [[OMAP]]
* 8 pin (single row) generic PLD JTAG compatible with many Lattice ispDOWNLOAD cables
* [[MIPI Debug Architecture|MIPI]]10-/20-connectors (1.27&nbsp;mm 050") for JTAG, cJTAG and SWD

Those connectors tend to include more than just the four standardized signals (TMS, TCK, TDI, TDO).  Usually reset signals are provided, one or both of TRST (TAP reset) and SRST (system reset).  The connector usually provides the board-under-test's logic supply voltage so that the JTAG adapters use the appropriate logic levels<!-- eg 5, 3.3, ..., 1V, ... -->. The board voltage may also serve as a "board present" debugger input.  Other event input or output signals may be provided, or [[General Purpose Input/Output|general purpose I/O]] (GPIO) lines, to support more complex debugging architectures.

Higher end products frequently use dense connectors (frequently 38-pin [[MICTOR]] connectors) to support high-speed [[Tracing (software)|tracing]] in conjunction with JTAG operations.  A recent trend is to have development boards integrate a USB interface to JTAG, where a second channel is used for a serial port.  (Smaller boards can also be powered through USB.  Since modern PCs tend to omit serial ports, such integrated debug links can significantly reduce clutter for developers.)  Production boards often rely on [[Bed of nails tester|bed-of-nails]] connections to test points for testing and programming.

=== Adapter hardware ===

Adapter hardware varies widely.  When not integrated into a development board, it involves a short cable to attach to a JTAG connector on the target board; a connection to the debugging host, such as a USB, PCI, or Ethernet link; and enough electronics to adapt the two communications domains (and sometimes provide [[galvanic isolation]]).  A separate power supply may be needed. There are both "dumb" adapters, where the host decides and performs all JTAG operations; and "smart" ones, where some of that work is performed inside the adapter, often driven by a microcontroller.  The "smart" adapters eliminate link latencies for operation sequences that may involve polling for status changes between steps, and may accordingly offer faster throughput.

{{As of|2009}}, adapters with full speed USB links are probably the most common approach, and new products often include high speed USB support.  Higher end products often support Ethernet, with the advantage that the debug host can be quite remote.  Adapters which support high speed trace ports generally include several megabytes of trace buffer and provide high speed links (USB or Ethernet) to get that data to the host.

Personal computer [[parallel port]] adapters are simple and inexpensive, but they are relatively slow because they use the host CPU to change each bit ("[[bit banging]]"). They have declined in usefulness because newer computers do not have parallel port hardware.  <!-- logic level translation may also be a problem --> Driver support is also a problem, because the adapter electronics varied so widely.{{Citation needed|date=October 2010}}

Serial port adapters also exist, and are similarly declining in usefulness.  They generally involve either slower bit banging than a parallel port, or a microcontroller translating some command protocol to JTAG operations.  Such serial adapters are also not fast, but their command protocols could generally be reused on top of higher speed links.

With all JTAG adapters, software support is a basic concern.  Many vendors do not publish the protocols used by their JTAG adapter hardware, limiting their customers to the tool chains supported by those vendors.  This is a particular issue for "smart" adapters, some of which embed significant amounts of knowledge about how to interact with specific CPUs.

=== Software development ===

Most development environments for embedded software include JTAG support.  There are, broadly speaking, three sources of such software:

* ''Chip Vendors'' may provide the tools, usually requiring a JTAG adapter they supply.  Examples include FPGA vendors such as [[Xilinx]] and [[Altera]], [[Atmel]] for its AVR8 and AVR32 product lines, and [[Texas Instruments]] for most of its DSP and micro products.  Such tools tend to be highly featured, and may be the only real option for highly specialized chips like FPGAs and DSPs.  Lower end software tools may be provided free of charge.  The JTAG adapters themselves are not free, although sometimes they are bundled with development boards.
* ''Tool Vendors'' may supply them, usually in conjunction with multiple chip vendors to provide cross-platform development support.  [[ARM architecture|ARM]]-based products have a particularly rich third party market, and a number of those vendors have expanded to non-ARM platforms like [[MIPS architecture|MIPS]] and [[PowerPC]].  Tool vendors sometimes build products around free software like [[GNU Compiler Collection|GCC]] and [[GNU Debugger|GDB]], with GUI support frequently using [[Eclipse (software)|Eclipse]].  JTAG adapters are sometimes sold along with support bundles.
* ''Open Source'' tools exist.  As noted above, GCC and GDB form the core of a good toolchain, and there are GUI environments to support them.

All such software tends to include basic debugger support:  stopping, halting, single stepping, breakpoints, data structure browsing, and so on.
Commercial tools tend to provide tools like very accurate simulators and trace analysis, which are not currently available as open source.

== Similar interface standards ==

''Serial Wire Debug'' (SWD) is an alternative 2-pin electrical interface that uses the same protocol. It uses the existing GND connection.  SWD uses an ARM CPU standard bi-directional wire protocol, defined in the ARM Debug Interface v5.<ref>{{Cite web|url=http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ihi0031c/index.html|title=ARM Information Center|website=infocenter.arm.com|access-date=2017-08-10}}</ref>  This enables the debugger to become another [[Advanced Microcontroller Bus Architecture|AMBA]] bus master for access to system memory and peripheral or debug registers. Data rate is up to  {{nowrap|4 Mbytes/sec}} {{nowrap|at 50 MHz}}. SWD also has built-in error detection.  On JTAG devices with SWD capability, the TMS and TCK are used as SWDIO and SWCLK signals, providing for dual-mode programmers.

== See also ==
{{Portal|Electronics}}
* [[Automated optical inspection]]
* [[Automated x-ray inspection]]
* [[In-circuit test]]
* [[Acceptance testing]]

== References ==

{{refs|30em}}

== External links ==

* [http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5412866 IEEE Standard for Reduced-Pin and Enhanced-Functionality Test Access Port and Boundary-Scan Architecture] The official IEEE 1149.7 Standard.
* [http://www.intel.com/content/dam/www/public/us/en/documents/white-papers/jtag-101-ieee-1149x-paper.pdf JTAG 101 - IEEE 1149.x and Software Debug] Intel whitepaper on JTAG usage in system software debug across a wide range of architectures.
* [http://www.ti.com/lit/an/ssya002c/ssya002c.pdf IEEE Std 1149.1 (JTAG) Testability Primer] Includes a strong technical presentation about JTAG, with design-for-test chapters.
* [https://www.electronics-notes.com/articles/test-methods/boundary-scan-jtag-ieee1149/what-is-boundary-scan-jtag.php Boundary Scan / IEEE 1149 tutorial] including details of variants of the IEEE standard, BSDL, DFT, and other topics
* [http://www.corelis.com/education/JTAG_Tutorial.htm JTAG Tutorial] Useful tutorials and information on JTAG technology.
* [http://www.corelis.com/education/What-Is-JTAG.htm What is JTAG?] Useful information on JTAG, timeline, architecture and more.
* [http://www.corelis.com/education/JTAG-Applications.htm JTAG Applications] JTAG for product development, life cycle, testing, tools needed, and other topics.

{{IEEE standards}}
{{Microcontrollers}}

{{use dmy dates|date=March 2012}}

[[Category:IEEE standards]]
[[Category:Electronics manufacturing]]
[[Category:Embedded systems]]
[[Category:Hardware testing]]
[[Category:Printed circuit board manufacturing]]