JTAG or Joint Test Action Group was developed in the 1980s as a way to improve the testing and debugging of digital circuits. Before JTAG, the process of testing and debugging integrated circuits was time-consuming and expensive. JTAG simplified the process by providing a standardized way to access and control the internal components of an integrated circuit.
What is JTAG?
JTAG, or Joint Test Action Group, is a standard interface used to test and debug integrated circuits. It is widely used in the electronics industry and is an essential protocol for hardware engineers, developers, and testers. Boundary scan registers are inserted between the input/output pins of an integrated circuit to test the same.
The Architecture of JTAGÂ
The JTAG architecture allows for the testing of complex digital circuits, including digital signal processors, field-programmable gate arrays (FPGAs), and other integrated circuits. By providing a standardized protocol for testing, debugging, and programming, JTAG enables more accessible and more efficient development and debugging of digital circuits. The architecture of JTAG consists of several key components:
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- Test Access Port (TAP): The TAP is a set of pins that provides access to the JTAG circuitry. The TAP consists of five signals: Test Clock (TCK), Test Mode Select (TMS), Test Data Input (TDI), Test Data Output (TDO), and Test Reset (TRST).
- Instruction Register (IR): The IR is a shift register that stores instructions for the JTAG circuitry. The length of the IR is determined by the number of instructions that the JTAG circuitry supports.
- Data Register (DR): It contains a set of registers like design specific,Device ID registers, ID registers,Bypass registers, Boundary scan registers etc.
- Boundary-Scan Cells (BSC): The BSCs are registers that are inserted between IO pins and core logic.The BSCs provide a means to test the interconnections between devices in the circuit.
- Boundary Scan Register(BSR): The scan chain is a series of BSCs that are connected together to form a single chain.Â
Working of JTAG
The JTAG protocol provides a flexible and standardized method for testing and debugging digital circuits. It involves several key steps: initialization, instruction phase, data phase, response phase, scan chain, and debugging and programming. Initialization involves asserting the Test Reset (TRST) signal to reset all the JTAG components to a known state. The instruction phase sends an instruction to the Instruction Register (IR) through the Test Data Input (TDI) pin. The data phase sends the data to the Data Register (DR) through the TDI pin.
The response phase sends the response data out of the DR and into the TDO pin. Bypass Register (BYPASS) is a special register that allows the JTAG controller to bypass the JTAG circuitry and directly access the device
Conclusion
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