JTAG (Joint Test Action Group) is an international standard test protocol, compliant with IEEE 1149.1. The standard JTAG interface uses four wires: TMS (Test Mode Select), TCK (Test Clock), TDI (Test Data Input), and TDO (Test Data Output). These lines are used for mode selection, clocking, data input, and data output respectively. JTAG is one of the fundamental communication protocols, similar in concept to RX/TX or USB, but it operates at a much lower level.
Unlike other communication methods, JTAG gives direct control over the CPU. In normal operation, the CPU manages the system, but when JTAG is active, it can override the CPU’s control. This makes JTAG a powerful tool for debugging and testing. While other protocols allow the CPU to read and write data from memory, JTAG can directly access and modify the CPU’s program and internal registers. Think of it as a "dragon knife" that allows full control over the device’s core functions.
JTAG is widely used in advanced devices such as DSPs (Digital Signal Processors) and FPGAs (Field-Programmable Gate Arrays). It provides a standardized way to test and debug chips, making it essential for development and maintenance. The JTAG interface is typically defined with the following pins:
- **TCK**: Test Clock Input – provides the timing signal.
- **TDI**: Test Data Input – sends data into the JTAG interface.
- **TDO**: Test Data Output – transmits data out of the JTAG interface.
- **TMS**: Test Mode Select – controls the state transitions of the JTAG state machine.
- **TRST**: Test Reset – resets the JTAG controller (not always required).
JTAG has two main functions: **chip testing** and **debugging**. It can detect electrical faults within a chip and also provide access to internal registers, memory, and peripherals. A CPU with a JTAG Debug interface can be fully controlled through the JTAG port, allowing developers to inspect and manipulate its internal state.
In addition to the CPU, JTAG can access on-chip memory (like L1, L2, L3 caches) and built-in modules such as the MMU (Memory Management Unit). This makes it a crucial tool for low-level debugging and firmware development.
### 1. JTAG Principle Analysis
At its core, JTAG works by defining a **Test Access Port (TAP)** inside the device. This TAP allows external tools to communicate with the device's internal components. Two key concepts are involved: **Boundary Scan** and the **TAP Controller**.
### 1.1 Boundary Scan
Boundary Scan technology adds a shift register (called the **Boundary-Scan Register**) to the I/O pins of a chip. When the chip is in debug mode, this register isolates the chip from its surroundings, allowing the external tool to observe and control the signals at the I/O pins.
For example, an input pin can receive data through the boundary scan register, while an output pin can send data back via the same register. In normal operation, the boundary scan register is transparent, so the chip functions normally. By connecting multiple boundary scan registers across different chips, a **Boundary-Scan Chain** can be formed, enabling serial communication and control of multiple devices in a system.
### 1.2 Test Access Port (TAP)
The TAP is a general-purpose interface that allows access to all data registers (DRs) and instruction registers (IRs) within a device. The TAP is controlled by a state machine, which transitions between various states based on the TMS (Test Mode Select) signal and driven by the TCK (Test Clock) signal.
Key signals of the TAP include:
- **TCK**: Clock signal for the TAP.
- **TMS**: Controls the state transitions of the TAP controller.
- **TDI**: Data input to the TAP.
- **TDO**: Data output from the TAP.
- **TRST**: Optional reset signal for the TAP controller.
The TAP goes through a sequence of states, starting from **Test-Logic Reset**, then moving to **Run-Test/Idle**, followed by **Select-DR-Scan**, **Capture-IR**, **Shift-IR**, and so on. During these transitions, instructions and data can be loaded into the device’s registers, allowing for deep inspection and control.
Today, JTAG interfaces are commonly available in 14-pin and 20-pin configurations. The 20-pin version is the most common, though some systems use the 14-pin version. With simple signal conversion, these interfaces can often be made compatible with different systems.
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