This project is aimed to find which pins exposed by the target device are JTAG pins. It does so by enumerating throughout the provided pins set and trying to abuse some JTAG features, such as BYPASS and IDCODE registers.

It is written in Go and supposed to be used under Linux (or any OS which Go supports) on the device with GPIO lines exported to userspace. Raspberry Pi 1,2,3 is the most famous example.

The tool drives GPIO either using go-rpio or libgpiod. The first one is designed for Raspberry Pi and uses gpiomem driver which makes it really fast. libgpiod works on any Linux system. Thus, this tool can be used on general-purpose laptops/desktops as well. However, the corresponding device is required that exposes /dev/gpiochipX pseudo-files.

Initially this project was a port of JTAGenum to Golang. Current version has implementation mostly ported from another great project JTAGulator.

For technical documentation refer to the original project. Also, consider comments in the source code that were taken from JTAGulator implementation.

The goal was to just port JTAGenum Arduino project to Go for the following reasons:

• shell version simply did not work in my case where this tool helped a lot;
• shell version is very slow due to echo 1 > /sys/...gpio interface;
• Arduino version requires... Arduino controller;
• Arduino version requires source code modification;
• to practice Golang a bit :-)

After porting was finished it became clear that logic behind is not perfect and produces unstable results. Thus, implementation of the core functions was taken from JTAGulator. Once features were tested the source code was adopted to Go coding style.

Install libgpiod development package, usually called as libgpiod-dev or libgpiod-devel. This is required even on Raspberry Pi where you will not use "gpiod" driver. However, this is very lightweight library which could be useful by itself (well, tools from this library).

Package installation is standard for Go packages:

$ go get github.com/gremwell/go-jtagenum

The result can be used as $GOPATH/bin/go-jtagenum.

Investigate your target and try to determine JTAG pins in hardware way. This will help to analyse this tool's output.

Do the required wiring to connect JTAG pins with GPIOs on your board (which runs this tool).

Write-down GPIO pin numbers (as OS understands them) and give each number unique identifier.

Again, for technical documentation refer to the original project.

Prepare pins configuration in JSON format, the following example is self-descriptive:

{ "pin1": 18, "pin2": 23, "pin3": 24, "pin4": 25, "pin5": 8, "pin6": 7, "pin7": 10, "pin8": 9, "pin9": 11 }`

Check for loops:

# go-jtagenum -pins '{ "pin1": 18, "pin2": 23, "pin3": 24, "pin4": 25, "pin5": 8, "pin6": 7, "pin7": 10, "pin8": 9, "pin9": 11 }' -command check_loopback
defined pins: map[24:pin3 25:pin4 8:pin5 11:pin9 18:pin1 23:pin2 10:pin7 9:pin8 7:pin6]
================================
Starting loopback check...
================================

Perform enumeration:

# go-jtagenum -pins '{ "pin1": 18, "pin2": 23, "pin3": 24, "pin4": 25, "pin5": 8, "pin6": 7, "pin7": 10, "pin8": 9, "pin9": 11 }' -command scan_bypass
defined pins: map[18:pin1 24:pin3 8:pin5 9:pin8 25:pin4 7:pin6 11:pin9 23:pin2 10:pin7]
================================
Starting scan for pattern 0110011101001101101000010111001001
FOUND!  TCK:pin4 TMS:pin3 TDO:pin2 TDI:pin1, possible nTRST: pin5 pin7 
================================

Dump IDCODE:

# go-jtagenum -pins '{ "pin1": 18, "pin2": 23, "pin3": 24, "pin4": 25, "pin5": 8, "pin6": 7, "pin7": 10, "pin8": 9, "pin9": 11 }' -command scan_idcode
defined pins: map[23:pin2 8:pin5 7:pin6 24:pin3 9:pin8 11:pin9 18:pin1 10:pin7 25:pin4]
================================
Starting scan for IDCODE...
FOUND!  TCK:pin4 TMS:pin3 TDO:pin2
     devices:
        0x0684617f (mfg: 0x0bf (Broadcom), part: 0x6846, ver: 0x0)
        0x5ba00477 (mfg: 0x23b (Solid State System Co., Ltd.), part: 0xba00, ver: 0x5)
        0x0684617f (mfg: 0x0bf (Broadcom), part: 0x6846, ver: 0x0)
     possible nTRST: pin6 pin8 pin9 pin1 pin5 pin7 
================================

Verify determined pins:

# go-jtagenum -known-pins '{ "tdi": 18, "tdo": 23, "tms": 24, "tck": 25, "trst": 8 }' -command test_bypass
================================
Starting BYPASS test for pattern 0110011101001101101000010111001001
sent pattern: 0110011101001101101000010111001001
recv pattern: 0110011101001101101000010111001001
match!
================================

# go-jtagenum -known-pins '{ "tdi": 18, "tdo": 23, "tms": 24, "tck": 25, "trst": 8 }' -command test_idcode
================================
Attempting to retreive IDCODE...
devices:
0x0684617f (mfg: 0x0bf (Broadcom), part: 0x6846, ver: 0x0)
0x5ba00477 (mfg: 0x23b (Solid State System Co., Ltd.), part: 0xba00, ver: 0x5)
0x0684617f (mfg: 0x0bf (Broadcom), part: 0x6846, ver: 0x0)
================================

Below are the real-world examples of running this tool under Raspberry Pi 3 to enumerate JTAG over five pins using both drivers:

# time ./go-jtagenum -pins '{ "pin1": 5, "pin2": 6, "pin3": 13, "pin4": 19, "pin5": 26 }' -command scan_bypass -driver rpio
defined pins: map[13:pin3 19:pin4 26:pin5 5:pin1 6:pin2]
================================
Starting scan for pattern 0110011101001101101000010111001001
FOUND!  TCK:pin4 TMS:pin3 TDO:pin2 TDI:pin1, possible nTRST: pin5
================================

real    0m25.291s
user    0m7.946s
sys     0m11.806s

# time ./go-jtagenum -pins '{ "pin1": 5, "pin2": 6, "pin3": 13, "pin4": 19, "pin5": 26 }' -command scan_bypass -driver gpiod
defined pins: map[26:pin5 5:pin1 6:pin2 13:pin3 19:pin4]
================================
Starting scan for pattern 0110011101001101101000010111001001
FOUND!  TCK:pin4 TMS:pin3 TDO:pin2 TDI:pin1, possible nTRST: pin5
================================

real    0m26.892s
user    0m9.544s
sys     0m13.344s

As can be seen, the tool itself is quite fast. go-rpio driver is faster than libgpiod as expected. Difference should become more noticeable when more pins used.

If tool's output is not clear or not expected, try the following:

• enable pull-up, toggle -pullup switch and run the same commands;
• increase toggle delay (-delay-tck) and run the same commands;
• increase reset delay (-delay-reset) and run the same commands;
• combine previous.

There is a room for improvements and several ideas already came to our minds:

• Special mode to adapt GPIO toggle delay;
• Support partially known JTAG pins configuration;