JTAG: Writing Chips with Joint Test Action Group Protocol

JTAG (Joint Test Action Group) is not only used for reading memory and debugging hardware but also for writing data to chips. This includes flashing firmware, programming memory, and configuring FPGAs. This tutorial will guide you through the process of writing to chips using JTAG, including setup, tools, and best practices.

What is JTAG Writing?

JTAG writing refers to programming or flashing data onto an integrated circuit (IC) through its JTAG interface. This process is commonly used for:

Flashing firmware or bootloaders
Writing configuration files to FPGAs
Programming non-volatile memory (e.g., EEPROM, Flash)
Updating software or firmware in embedded systems

The JTAG interface provides direct access to the chip, enabling precise control over memory and configuration.

What You Will Need

JTAG Adapter: A hardware interface for JTAG communication (e.g., SEGGER J-Link, Xilinx Platform Cable, OpenOCD-supported adapters).
Target Device: The chip or system you want to program.
Software Tools: Tools like OpenOCD, urJTAG, Vivado (for Xilinx), or SEGGER’s J-Link software.
Firmware/Configuration File: The file to be written to the chip (e.g., .bin, .hex, .svf).
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 Pins

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

2. Connect the JTAG Adapter

Wire the JTAG adapter to the target device, ensuring correct pin connections:
- TDI (Test Data In) to TDI
- TDO (Test Data Out) to TDO
- TCK (Test Clock) to TCK
- TMS (Test Mode Select) to TMS
- Ground (GND) must be connected.

3. Verify Voltage Levels

Check that the JTAG adapter matches the target device’s voltage levels (e.g., 3.3V, 1.8V).
Some adapters allow configurable voltage settings.

Step 2: Installing JTAG Software Tools

1. OpenOCD

OpenOCD (Open On-Chip Debugger) is a popular open-source tool for programming and debugging.

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

2. Vendor-Specific Tools

SEGGER J-Link: Use the SEGGER J-Link software for JTAG operations.
Xilinx Vivado: Ideal for programming Xilinx FPGAs and CPLDs.
Intel Quartus: For programming Intel/Altera devices.

3. urJTAG

A lightweight tool for basic JTAG operations like writing firmware.

Install from urJTAG’s website.

Step 3: Writing Data to a Chip

1. Configure the JTAG Software

Set up the JTAG configuration file to specify the adapter and target device.

Example OpenOCD configuration for an STM32 chip:

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 ensure the target device is detected.

For OpenOCD:

openocd -f interface/jlink.cfg -f target/stm32f4x.cfg

Check the output log for detected devices.

3. Flash Firmware

Write the firmware file to the chip using JTAG commands.

Example OpenOCD command:
```
program firmware.bin verify reset exit 0x08000000
```
- firmware.bin: The binary file to write.
- verify: Ensures the data was written correctly.
- reset: Resets the device after programming.
- 0x08000000: Start address in memory.

4. Write Configuration to an FPGA

For FPGAs, use .bit or .svf files.

Example urJTAG commands:
```
cable jtagkey
detect
svf configure.svf
```

5. Erase Memory Before Writing

Some devices require erasing memory before flashing.

Example OpenOCD command:
```
flash erase_address 0x08000000 0x10000
```

Step 4: Best Practices for JTAG Writing

Backup Existing Firmware: Always read and save the existing firmware before writing new data.
Verify Data Integrity: Use the verify command to ensure successful programming.
Use Reliable Connections: Secure all wires to prevent data corruption during writing.
Match Voltage Levels: Ensure the JTAG adapter and target device use compatible voltage levels.
Check Write Protection: Disable write protection (if applicable) before programming.

Applications of JTAG Writing

Flashing firmware or bootloaders
Programming FPGAs or CPLDs
Updating embedded system software
Writing configuration data to memory
Developing and testing firmware in real-time

Troubleshooting

Device Not Detected:
- Verify wiring and pinout.
- Ensure the device is powered and properly grounded.
Write Errors:
- Ensure the memory region is not write-protected.
- Double-check the start address and file format.
Verification Fails:
- Check for reliable connections.
- Use a slower JTAG clock speed to improve stability.

Conclusion

JTAG is a versatile tool for writing firmware, configuring memory, and programming devices at a low level. By following this guide, you can confidently use JTAG to flash firmware, update software, and configure hardware. Mastering JTAG writing enables efficient development and maintenance of embedded systems!