JTAG: Reading Chips with Joint Test Action Group Protocol

JTAG (Joint Test Action Group) is a widely used protocol for debugging, programming, and testing integrated circuits (ICs). It allows direct communication with a chip to read its memory, perform boundary scans, or load firmware. This tutorial will guide you through the basics of using JTAG to read chips, including setup, tools, and best practices.

What is JTAG?

JTAG is a standardized interface (IEEE 1149.1) that provides a way to test and debug hardware at the chip level. It is commonly used in:

Firmware development and debugging
Testing PCB connections
Flashing firmware onto devices
Extracting memory from ICs

JTAG uses a 4 or 5-pin interface:

Pin	Description
TDI	Test Data In
TDO	Test Data Out
TCK	Test Clock
TMS	Test Mode Select
TRST	Test Reset (optional)

What You Will Need

JTAG Adapter: A hardware device to interface with the chip (e.g., SEGGER J-Link, OpenOCD-supported adapters).
Target Device: The IC or system you want to read.
Software Tools: Applications for JTAG debugging, such as OpenOCD, urJTAG, or proprietary tools.
JTAG Pinout Documentation: Pinout diagram for the target device.
Connection Accessories: Wires, headers, and a breadboard (if necessary).

Step 1: Setting Up the JTAG Hardware

1. Identify the JTAG Interface on the Target Device

Refer to the chip’s datasheet or PCB schematics to locate the JTAG pins.
Common labels for JTAG pins include TDI, TDO, TCK, and TMS.
Some boards have labeled JTAG headers (e.g., 10-pin or 20-pin connectors).

2. Connect the JTAG Adapter

Wire the adapter to the target device, matching the JTAG pinout:
- TDI on the adapter connects to TDI on the device, and so on.
- Ensure GND is connected between the adapter and the target.
- Provide power to the target device if necessary.

3. Verify Voltage Levels

Ensure the JTAG adapter supports the voltage levels of the target device (e.g., 3.3V or 1.8V).
Some adapters have configurable voltage settings.

Step 2: Installing JTAG Software Tools

1. OpenOCD (Open On-Chip Debugger)

OpenOCD is an open-source tool widely used for JTAG debugging.

Install it using your package manager (e.g., apt install openocd on Linux).
Ensure your JTAG adapter is supported by OpenOCD.

2. urJTAG

A lightweight tool for boundary scan and basic JTAG operations.

Install via urJTAG’s website.

3. Vendor-Specific Tools

For proprietary adapters like SEGGER J-Link or Xilinx, use their official software:

SEGGER J-Link: Download the J-Link tools from SEGGER’s website.
Xilinx Tools: Use Vivado or iMPACT for FPGA debugging.

Step 3: Reading Data from a Chip

1. Configure the JTAG Software

Create or load a configuration file specifying the JTAG adapter and target device.
- Example for OpenOCD:
```
source [find interface/jlink.cfg]
transport select jtag
source [find target/stm32f4x.cfg]
init
halt
```

2. Detect the Target Device

Run a command to scan the JTAG chain and identify connected devices.

For OpenOCD:
```
openocd -f interface/jlink.cfg -f target/stm32f4x.cfg
```
Look for detected devices in the output log.

3. Dump Memory Contents

Use JTAG commands to read memory regions:

For OpenOCD:
```
dump_image memory.bin 0x08000000 0x10000
```
- memory.bin: File to save the memory dump.
- 0x08000000: Start address.
- 0x10000: Number of bytes to read.

4. Perform Boundary Scans

Boundary scans check for connectivity and functionality of the IC pins.

Use urJTAG or OpenOCD to perform boundary scans.

Example in urJTAG:

cable jtagkey
detect
bsdl path/to/bsdl/file.bsd
svf boundary_scan.svf

Step 4: Best Practices for JTAG Debugging

Secure the Connection: Use short, reliable wires to minimize noise.
Verify Pinout: Double-check the JTAG pin connections to avoid damaging the device.
Backup Firmware: Always back up the original firmware before making changes.
Use Correct Software: Ensure the tool you’re using supports the target device.
Monitor Voltage: Ensure the target device operates at the correct voltage level.

Applications of JTAG

Debugging firmware and software
Flashing new firmware or bootloaders
Extracting data from memory for analysis
Diagnosing PCB manufacturing defects
Reverse engineering hardware

Troubleshooting

Device Not Detected:
- Verify the wiring and pinout.
- Ensure the target device is powered on.
- Check voltage levels for compatibility.
JTAG Errors:
- Ensure the correct configuration file is used.
- Try lowering the JTAG clock speed if communication fails.
Permission Issues:
- On Linux, ensure you have proper permissions to access USB devices (use sudo or configure udev rules).

Conclusion

JTAG is an invaluable tool for debugging and programming chips at a low level. By understanding the basics of JTAG hardware and software, you can read chip memory, debug firmware, and perform boundary scans. Experiment with different tools and configurations to fully utilize JTAG in your projects!