JTAGulator: Introduction

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


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
  • Debugging of hardware
  • Debugging of software
  • Program / Modify internal memory
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)

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


JTAGulator Overview

Since JTAG pins are not always labelled, 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.


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 following output:


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 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/op amp hardware on the JTAGulator that's used to create the target system voltage and drive the on-board level translators.)

Used Hardware