Jtag example

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Jtag example

The debug and programming tools commonly associated with JTAG only make use of one aspect of the underlying technology — the four-wire JTAG communications protocol.

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This standard was developed to provide a technology for testing Printed Circuit Board Assemblies PCBAs without needing the level of physical access required for bed-of-nails testing or the amount of custom development needed for functional test. The TAP was designed to interact with new registers that were added to devices to implement this method of testing. Very quickly however silicon manufacturers recognised the benefits of using the TAP to access registers offering other functionalities such as debug and programming.

As its name suggests the individual bits, or cells, of this register are at the boundary of the device, between its functional core and the pins or balls by which it is connected to a board — very often JTAG testing is referred to as boundary scan. Boundary scan cells see above can operate in two modes. In their functional mode they have no effect on the operation of the device — this is the mode in which they operate when the board is running normally.

In their test mode they disconnect the functional core of the device from the pins. By putting boundary scan cells into test mode they can be used to control the values being driven from an enabled device onto a net and also be used to monitor the value of that net.

Disconnecting the control of the pins from the functionality of the enabled device makes boundary scan test development significantly easier than traditional functional test as no device configuration or booting is required to use the pins.

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By providing a mechanism to control and monitor all the enabled signals on a device from a four-pin TAP, JTAG significantly reduces the physical access required to test a board. There are two main ways that this boundary scan capability can be used to test a board. The first way, connection testing see next section gives good test coverage, particularly for short circuit faults.

Where two JTAG enabled pins are meant to be connected the test will make sure one pin can be controlled by the other. Where enabled pins are not meant to be connected they are tested for short circuit faults by driving one pin and checking that these values are not read on the other pins.

XJTAG will automatically generate the vectors required to run a connection test based on the netlist of a board and JTAG information for the enabled devices.

In order to add this open circuit coverage it is necessary to communicate with the peripheral device from boundary scan on the enabled device. If communication can be verified, there cannot be an open circuit fault. This type of testing can be very simple, for example lighting an LED and asking an operator to verify it has activated, or more complex, for example writing data into the memory array of a RAM and reading it back.

The library files contain models for all types of non-JTAG devices from simple resistors and buffers to complex memory devices such as DDR3. Because boundary scan disconnects the control of the pins on JTAG devices from their functionality the same model can be used irrespective of the JTAG device controlling a peripheral. Most boards already contain JTAG headers for programming or debug so there are no extra design requirements.

In order to run any boundary scan based testing it is necessary to have some information about the implementation of JTAG on the enabled devices on a board. Not at all. One of the key benefits to boundary scan testing is that the only test hardware required is a JTAG controller.

Using boundary scan during board bring-up can remove uncertainties — hardware engineers can test prototype boards for manufacturing defects before system testing, and even before firmware is complete. Test systems developed at this early stage of the product lifecycle can easily be reused, and extended for production. Each BGA device on a board imposes severe restrictions on the testing that can be done using traditional bed-of-nails or flying probe machines. The non-recurring engineering NRE costs of building test fixtures can be prohibitively high.

Starting with JTAG Hacking

For boards with low production volumes it has always been difficult to justify the cost of test fixture development. In these cases one alternative is flying probe testing; however the test cycle times tend to be high for this technology. This standard interface, which is the same for all JTAG enabled devices, means a generic set of test models can be used, and re-used, when building test systems. JTAG is often already used as one step in production: programming. By also using JTAG for boundary scan test it is possible to reduce the number of steps and handling operations in the production process.

Traditional test technologies require very large and expensive equipment. XJTAG also provides the capability to view both the physical location of a fault on the layout of the board and the logical design of the area of the circuit in which the fault exists on the schematic.

Traditional functional tests cannot be run if the board does not boot; simple faults on key peripherals, such as RAM or clocks, would be found using JTAG but would prevent functional tests from providing any diagnostic information. Download as PDF. Testing BGA Connections. This site tracks visits anonymously using cookies.Surface-mount technology rang the death knell for bed-of-nails testing.

That's why a consortium of companies called the Joint Test Access Group came together to define a standard for boundary-scan testing of ICs and boards. Here's a primer on the technology. One disadvantage of shrinking technology is that the testing of small devices gets exponentially more complex. When circuit boards were large, we tested them with techniques such as bed-of-nails, which employed small spring-loaded test probes to make connections with solder pads on the bottom of the board.

Such test fixtures were custom made, expensive, and inefficient, and much of the testing could not be performed until the design was complete. The problems with bed-of-nails testing were exacerbated as board dimensions got smaller and surface-mount packaging technology improved. If devices were mounted on both sides of a circuit board, no attachment points were left for the test equipment.

Boundary scan. The consortium devised a specification for performing boundary-scan hardware testing at the IC level. Inthat specification resulted in IEEE The specification JTAG devised uses boundary-scan technology, which enables engineers to perform extensive debugging and diagnostics on a system through a small number of dedicated test pins.

Today, boundary-scan technology is probably the most popular and widely used design-for-test technique in the industry. 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 nameas shown in Figure 1.

JTAG Tutorial

The path creates a virtual access capability that circumvents the normal inputs and provides direct control of the device and detailed visibility at its outputs. Figure 1: An integrated circuit with boundary scan. The boundary-scan cells can be configured to support external testing for interconnection between chips or internal testing for logic within the chip. To provide the boundary scan capability, IC vendors add additional logic to each of their devices, including scan registers for each of the signal pins, a dedicated scan path connecting these registers, four or five additional pins, and control circuitry.

The overhead for this additional logic is minimal and generally well worth the price to have efficient testing at the board level. Test Access Port. The boundary-scan control signals, collectively referred to as the Test Access Port TAPdefine a serial protocol for scan-based devices.

There are five pins:. The TAP controller manages the exchange of data and instructions. With the proper wiring, you can test multiple ICs or boards simultaneously. Test process. The standard test process for verifying a device or circuit board using boundary-scan technology is as follows:.

Simple tests can find manufacturing defects such as unconnected pins, a missing device, an incorrect or rotated device on a circuit board, and even a failed or dead device. The primary advantage of boundary-scan technology is the ability to observe data at the device inputs and control the data at the outputs independently of the application logic.

Another benefit is the ability to reduce the number of overall test points required for device access. With boundary scan there are no physical test points. This can help lower board fabrication costs and increase package density. Boundary scan provides a better set of diagnostics than other test techniques. Conventional techniques apply test vectors patterns to the inputs of the device and monitor the outputs. If there is a problem with the test, it can be time consuming to isolate the problem.

Additional tests have to be run to isolate the failure. With boundary scan, the boundary-scan cells observe device responses by monitoring the input pins of the device.There is a specific order in which commands must be executed. For example, you cannot program a part unless you first set the mode, issue the command to select the cable, and define a chain of at least one device.

Most users only need to perform six basic operations in their command. Set the operating mode Set up the cable port Define the chain and assign files Program the device s Verify the device s Exit from the programming software. In this section, nine examples are shown. In each example, command sequences are shown first, then each command description follows.

The number is attached to the left of the each command in the command sequences but this number is not part of the syntax. Refer to the number in the description section to see the detail of the command. Adds a device to the first position in the JTAG chain, assigns the bitstream1.

Extracting firmware from devices using JTAG

Adds a device to the second position in the JTAG chain, assigns the bitstream2. Programs and verifies the first device in the JTAG chain. Programs and verifies the second device in the JTAG chain.

Saves the project chain composition and assigned files to filename. Sets the operating mode for PROM file generation.

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Adds a single design set. The version and name need to be set to zero for a PROM file generation sequence, and only needs to be set once.

The index needs to be set to zero for PROM file generation sequence, and only needs to be set once. A Xilinx programming cable must be connected to your computer. Adds a third party device to the second position in the JTAG chain, assigns the thirdparty.

Erases, programs, and verifies the first device in the JTAG chain. Erases, programs, and verifies the third device in the JTAG chain.

Loads the filename. Programs the second device in the JTAG chain. This command is only valid for the PCIV cable. Erases, programs and verifies the first device in the JTAG chain. Adds a collection named collection1. Addresses are permitted. The design name assigned to this address is rev0a. Adds a JTAG device chain. The design name assigned to this address is rev1a. The design name assigned to this address is rev0b.This document provides you with interesting background information about the technology that underpins XJTAG.

Advances in silicon design such as increasing device density and, more recently, BGA packaging have reduced the efficacy of traditional testing methods. This standard has retained its link to the group and is commonly known by the acronym JTAG. The main advantage offered by utilising boundary scan technology is the ability to set and read the values on pins without direct physical access. The process of boundary scan can be most easily understood with reference to the schematic diagram shown in figure 1.

In normal operation these boundary scan cells are invisible. Not all boundary scan cells are the same — there are 10 types of cell in the There are two types of registers associated with boundary scan. Each compliant device has one instruction register and two or more data registers. Instruction Register — the instruction register holds the current instruction.

Its content is used by the TAP controller to decide what to do with signals that are received. Most commonly, the content of the instruction register will define to which of the data registers signals should be passed. Other data registers may be present, but they are not required as part of the JTAG standard.

What is JTAG and how can I make use of it?

Figure 2, below, shows the state-transition diagram. The two main paths allow for setting or retrieving information from either a data register or the instruction register of the device. The data register operated on e.

The IEEE These instructions are:. Introduction Advances in silicon design such as increasing device density and, more recently, BGA packaging have reduced the efficacy of traditional testing methods. Boundary Scan The main advantage offered by utilising boundary scan technology is the ability to set and read the values on pins without direct physical access.This approach is particularly useful for any field test that be required as the item under test can be accessed without the need for complete disassembly of the unit.

For most units there is no specific JTAG connector. Instead the connections to the JTAG interface are routed via the main connector to the assembly. There is a maximum of five lines that may be used for a JTAG interface, although one of them is optional and therefore may not always be present. This may be the case when the design becomes short of pins on a connector and the optional one can be sacrificed. In some instances there may be more than one JTAG connector. The JTAG interface is generally an integral part of any electronics assembly.

While some items of equipment may provide a specific JTAG port for field test, this is not always the case. In these cases the JTAG interface may accessible via the main assembly connector for which there are a few dedicated pins used for JTAG boundary scan testing.

jtag example

The pins that make up the JTAG interface would not be used under normal operational circumstances. Many chip manufacturers have their own proprietary additional connections that work together with the basic JTAG signals to provide a considerable degree of additional functionality. These additional lines are generally vendor specific, although a new standard known as IJTAG defined under IEEE provides standardisation to the additional lines and functionality. There is no standard for the connector type that should be used for the JTAG connection.

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Different vendors use different JTAG connector types, often in the form of headers. Different types may also be used between development and production, and in some cases multiple headers may be incorporated to enable different tool support. On some production boards, test points or connections within existing connectors may be used.

When connecting to the JTAG interface, care must be taken to keep leads and internal PCB links as short as possible to preserve the signal integrity and timing. JTAG interface signals There is a maximum of five lines that may be used for a JTAG interface, although one of them is optional and therefore may not always be present. On the falling edge test clock outputs the test data on the TDO pin.

jtag example

It is important that the clock line is properly terminated to prevent reflections that may give rise to false triggering and incorrect operation of the JTAG interface. It receives serial input data which is either feed to the test data registers or instruction register, dependent upon on the state of the TAP controller.

jtag example

The TDI line has an internal pull-up, and therefore the input is high with no input. It delivers serial data which comes from either the test data registers or instruction register, dependent upon on the state of the TAP controller.

Data applied to the TDI pin will appear at the TDO pin but may be shifted by a number of clock cycles, depending on the length of the internal register.

The TDO pin has a high-impedance.Create JTAG sequence object. The jtag sequence command creates a new sequence object. After creation the sequence is empty. The following sequence object commands are available:. Generate delay between sequence commands.

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No JTAG clocks will be generated during the delay. Supported pins is cable specific. Set or clear atomic sequences.

jtag example

This is useful to creating sequences that are guaranteed to run with precise timing or fail. Atomic sequences should be as short as possible to minimize the risk of failure. Using Xilinx SDK. Data is either given as the last argument or if -tdi option is given then data will be all zeros or all ones depending on the argument given to -tdi. This option is only supported for irshift. The least significant bit of data is shifted first. The first bit in the string is shifted first.

The least significant bit of the first byte in the string is shifted first. Run JTAG operations in sequence for the currently selected jtag target. The first bit shifted out is the least significant bit in the first byte returned. The first bit shifted out is the least significant bit of the integer. The first bit shifted out is the first character in the string.

The first bit shifted out is the least significant bit of the first byte of the in the string. Related reference jtag targets. UG v Select instruction register by name. Cature TDO data during shift and return from sequence run command. State to enter after shift is complete. Format return value s as binary.As part of our PIP Production Integration Packages family our National Instruments support options have enabled our customers to seamlessly and reliably integrate high-quality boundary-scan applications into their test and device programming systems for almost 20 years.

National Instruments software tools are commonly used within the electronics industry to create experimental research set-ups in design environments or functional test systems in production environments. Using the digital test power of boundary-scan with analog measuring or stimulus instruments allows engineers to cross the boundaries into mixed signal testing with ease.

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We have been able to solve thousands of board test problems by actively engaging with our customers. Once you become a JTAG Technologies customer you are an integral part of our business with free access to our world-wide support network. National Instruments Support. Download datasheet. Request a quote. Get advice. Request a free trial. Benefits Applications Features. Happy to serve you! Contact FAQ.


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