JTAGulator: Introduction

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JTAG explained

The JTAGulator is a tool to assist in identifying JTAG and USART pins.

JTAGulator.png

JTAG (named after the Joint Test Action Group) is an industry-standard for verifying designs and testing printed circuit boards after manufacture.

JTAG allows for many applications
  • Boundary Scan (read and set values of a pin)
  • Debugging of hardware/software
  • Program / Modify memory
  • Chips can be daisy-chained, so you can access multiple chips through one interface
JTAG consist of 4 essential signals
  • TCK ... Test Clock
  • TMS ... Test Mode Select (Steuerung)
  • TDI ... Test Data In
  • TDO ... Test Data Out
additional signals
  • Vcc ... 3.3V
  • GND ... ground
  • TRST .. test reset (low active)
  • RST ... system reset (low active)

Good video about the JTAG interface: EEVblog #499 - What is JTAG and Boundary Scan?

Manufactures have different Pin Layout, for Cortex Connectors, it looks like this:

source:http://www.keil.com/support/man/docs/ulink2/ulink2_connector_20_16_14_10pin.png

JTAGulator Overview

Since JTAG pins are not always labeled, they can be hard to find. This is when the JTAGulator comes into play.

You connect the channels of the JTAGulator to different test pins on the hardware you want to analyze.

The JTAGulator test each individual pins and tries to find JTAG connections. If found it will output the correct JTAG pins.

Setup

Make sure you have the latest firmware
JTAGulator: Update firmware
Connect the JTAGulator via serial to the computer
115200 bps, 8 data bits, no parity, 1 stop bit
  • Linux: you can use minicom
  • Windows: you can use Putty
You should see the following output:
JTAGulator welcomeScreen.png
Connect the JTAGulator to the test-device
  1. Connect the JTAGulator Ground (GND) to the GND of the hardware you want to analyze.
  2. Connect CHx from the JTAGulator to some pins (make sure via multimeter that the pins do not have a voltage level over 3.3V) on the hardware which may be JTAG pins (start from CH0). Do not use the VADJ pin on the JTAGualtor (VADJ is the output from the PWM/up-amp hardware on the JTAGulator that's used to create the target system voltage and drive the on-board level translators.)

JTAGulator commands

This tutorial is based on the firmware version 1.6

  • Display all commands: H
  • Display firmware version: I
First you need to set the voltage level
Press V and set the voltage level accordingly (check via multimeter or hardware specification, usually 3.3)

Finding JTAG

  • Type J to get to the JTAG commands
  • Then type H for useful help-information
IDCODE scan I
This is a quick scan, which checks the output of every possible combination and presents possible combination + device IDs
You can filter out incorrect JTAG identification by looking at the ID (device IDs are eye-catching, they are very non-uniform eg. (0x0D780237) and not like (0xFFFFFF7F) or (0x55555555) )
BYPASS scan B
After you used the IDcode scan, you want to determine the TDI line
Enter x for the unknown TDI line and the numbers of the possible JTAG combination
Test BYPASS T
After the TDI pin was found, test the communication using the echo command of the JTAG interface. A random string gets sent to the TDI line of the JTAG interface and the JTAG controller should now send the same string back on the TDO line.
With this you can make sure, that is really is a JTAG interface
Go into the Test BYPASS menu, enter the number of the pins and see if the string matches
If it does match it is very likely to be a JTAG interface, if not you will have to try out other pins.

For a real-world example look into the post: JTAGulator: Find a Smartphone's JTAG interface


Finding UART

The JTAGulator can also find UART pins by brute-forcing the pinout configuration as well as the baud-rate

Find UART pins
  • To enter the UART commands enter: U
(You can get help via H)
  • Set the voltage level with V
  • Enter the brute-force mode: U
  • Set parameters (make sure you do not ignore non-printable characters)
Analyze output
If the JTAGulator received a message it will output it to the screen
  • The JTAGulator will display the TX and RX pin, the baud rate it was using and the message it received
In the default configuration the JTAGualtor will send <CR> (0x0D) commands to the device
Usually UART interfaces answer on <CR> commands with a response and append a <LF> (0x0A) or <CR> ( 0x0D) to it. Receiving such bytes are a good indicator that the correct baud rate was used.
Connect to UART
The JTAGualtor allows for passthrough of UART commands
  • Enter P for UART passthrough
Set the RX, TX and the baud rate you found before.

For an example visit: JTAGulator: Find IoT-Device's UART interface


Used Hardware

JTAGulator

References