Identify JTAG

1. Reminder of the Basics of JTAG

JTAG is typically implemented using the following standard signals, which we need to find:

  • TDI: Test Data In

  • TDO: Test Data Out

  • TCK: Test Clock

  • TMS: Test Mode Select

  • TRST: Test Reset (optional)

2. Preliminary Examination

  • Inspect the PCB Layout

    • Look for pin headers or test points with 4-10 pins in a row or dual row configuration.

    • Check for labeled pads or silkscreen markings such as JTAG, TDI, TDO, etc.

    • Examine components for known JTAG-compatible chips (e.g., ARM Cortex processors, FPGAs).

  • Consult the Datasheet

    • Identify major ICs on the PCB and locate their datasheets online. Search for:

    • JTAG or boundary scan capabilities.

    • Pin numbers corresponding to JTAG signals.

  • Look for Clues

    • Use magnification to inspect nearby traces. JTAG pins often connect directly to the processor or debug interfaces.

    • Check for standardized pinouts like ARM’s 20-pin or 10-pin connectors.

3. Test for Common JTAG Pinouts

Here are a few common JTAG pinouts to reference:

4. Electrical Verification

  • Check Pin Voltage Levels

    • Using a multimeter, measure the voltage on suspected pins when the device is powered on:

      • TDI, TDO, TMS, TCK: Typically operate at 1.8V, 3.3V, or 5V.

      • GND: Should read 0V.

      • Vcc: Will match the system voltage (e.g., 3.3V or 5V).

  • Probe Continuity

    • Use a multimeter in continuity mode to trace suspected pins:

      • Test connections between suspected pins and the processor’s JTAG pins (refer to the datasheet).

      • Identify Vcc and GND connections to nearby capacitors or power lines.

5. Verification Using Test Tools

a) Use a JTAG Finder

Tools like JTAGulator or Bus Pirate can assist in identifying JTAG signals by probing the pins automatically.

b) Connect a Debugger

  1. Attach a known JTAG debugger (e.g., OpenOCD or Segger J-Link) to the suspected interface.

  2. Use debugging software to scan for a JTAG chain.

  3. Look for a valid response to confirm the JTAG interface.

6. Analyze Signal Activity

a) Use an Oscilloscope or Logic Analyzer

  1. Monitor pin activity during device operation or reset.

  2. Look for:

    • Clock signals on TCK.

    • Data transitions on TDI, TDO, or TMS.

b) Identify Pull-Up Resistors

Check if certain pins have pull-up resistors to Vcc, which is common for TMS or TDI.

Resources

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