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The basic principle and debugging principle of jtag
JTAG, originally designed for chip testing, operates by embedding a Test Access Port (TAP) within the device. This TAP allows engineers to access and test internal nodes using a dedicated JTAG tool. One of its key features is the ability to connect multiple devices in a chain through the JTAG interface, enabling individual testing of each component in the system. Today, JTAG is also widely used for In-System Programming (ISP), particularly for devices like Flash memory, making it an essential tool in modern electronics development.
The JTAG programming method enables online programming, which streamlines the production process. Traditionally, chips were pre-programmed before being mounted on a board. With JTAG, however, devices can be fixed onto the circuit board first and then programmed, significantly accelerating project timelines. This method allows full programming of all components inside a PSD (Programmable System Device) chip via the JTAG interface.
In simple terms, the core principle of JTAG involves defining a TAP within the device, allowing engineers to test and debug internal signals using a specialized JTAG test tool. This capability makes JTAG a powerful diagnostic and programming interface.
**Boundary Scan**
Boundary scan technology enhances JTAG by adding shift register units—called Boundary-Scan Register Cells—to each input and output pin of a chip. These registers are positioned at the chip’s boundary, hence the name "boundary scan." During debugging, these registers isolate the chip from external circuits, enabling observation and control of its input and output signals.
For input pins, the boundary scan registers can load data into the pin. For output pins, the boundary scan registers can capture the signal before it leaves the chip. Importantly, during normal operation, these registers remain transparent, ensuring that the chip functions as intended without disruption.
Moreover, boundary scan registers on different pins can be connected to form a boundary scan chain around the chip. This chain allows serial communication through clock and control signals, enabling detailed monitoring and control of the chip’s I/O states during debugging.
The CPU typically includes hardware implementation of JTAG, providing an external interface with four main pins: TMS (Test Mode Select), TCK (Test Clock), TDI (Test Data In), and TDO (Test Data Out). These pins allow communication between the JTAG tool and the device under test.
It's important to clarify that the term "CPU" here refers to the arithmetic processing unit, consisting of basic components such as internal registers and arithmetic logic units. The term "processor," on the Other hand, usually refers to an expanded CPU chip rather than a System-on-Chip (SoC).
JTAG is widely used for chip testing, with the boundary scan chain being the most critical component. As its name suggests, this chain resides at the chip's boundary, making it ideal for monitoring signals that pass through the pins.
Since the CPU communicates with peripherals through its pins, all data must enter or exit via these points. JTAG leverages this by monitoring the pin signals, allowing for comprehensive testing and debugging of the chip's functionality.
As shown in the diagram, the boundary scan chain is integrated into the chip’s pins. When a signal is input, the boundary scan chain captures it. Similarly, when the CPU is about to output a signal, the chain can either capture it or send it directly out. This capability is crucial for both testing and programming.
To store and transfer these signals, the TDI (Test Data In) and TDO (Test Data Out) pins serve as the interface. The signal flow follows a path from TDI to the boundary scan chain and then to TDO.
Whether capturing or sending data, the boundary scan chain plays a central role in JTAG operations. It allows engineers to monitor command and data signals (including addresses and data) exchanged between the CPU and the outside world. By observing these signals, developers can effectively debug programs running on the chip.
Most chips feature multiple independent boundary scan chains, and their control is managed by the TAP (Test Access Port) controller. While the above explanation covers the fundamental principles of JTAG, more advanced debugging often requires controlling various components and combining multiple registers for greater flexibility and precision.