Pi with the PiFEX shield on the right, the SSD under test on the left with testpoints held by a jumper clip, jumper wires connecting the two together

JTAG Hacking An SSD With A Pi: A Primer

[Matthew “wrongbaud” Alt] is well known around these parts for his hardware hacking and reverse-engineering lessons, and today he’s bringing us a JTAG hacking primer that demoes some cool new hardware — the PiFEX (Pi Interface Explorer). Ever wondered about those testpoint arrays on mSATA and M.2 SSDs? This write-up lays bare the secrets of such an SSD, using a Pi 4, PiFEX, OpenOCD and a good few open-source tools for JTAG probing that you can easily use yourself.

The PiFEX hat gives you level-shifted bidirectional GPIO connectors for UART, SPI, I2C, JTAG, SWD and potentially way more, an OLED screen to show any debugging information you might need, and even a logic analyzer header so that you can check up on your reverse-engineering progress.

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Showing the ESP-Prog-Adapter board plugged into the ESP-Prog adapter, wired to a SOIC clip, that then attaches to a PCB under test

ESP-Prog-Adapter Makes Your ESP32 Tinkering Seamless

Did you ever struggle with an ESP32 board of yours, wishing you had exposed that UART, or seriously lacking the JTAG port access? If so, you should seriously check out [0xjmux]’s ESP-PROG-Adapter project, because [0xjmux] has put a lot of love and care into making your ESP32 hardware interfacing a breeze. This project shows you how to add JTAG and UART headers with extra low board footprint impact, gives you a KiCad library to do so super quickly, and shares a simple and helpful adapter PCB you can directly use with the exceptionally cheap Espressif’s ESP-Prog dongle you should have bought months ago.

The hardware is perfect for ZIF no-soldering interfacing – first of all, both UART and JTAG can be connected through a SOICBite connection, a solderless connector idea that lets you use SPI flashing clips on specially designed pads at the edge of your board. For the fancy toolkit hackers among us, there’s also a Tag Connect symbol suggested and a connector available, but it carries JTAG that you will already get with the SOICBite, so it’s maybe not worth spending extra money on.

Everything is fully open-source, as one could hope! If you’re doing ESP32 hacking, you simply have to order this board and a SOIC clip to go with it, given just how much trouble [0xjmux]’s board will save you when programming or debugging your ESP32 devices. Now, you don’t strictly need the ESP-Prog dongle – you could remix this into an adapter for the Pi Pico board instead. Oh, and if designing boards with ARM CPUs are your thing, you might benefit from being reminded about the Debug Edge standard!

A Pi Pico connected to a MYIR Z-turn board with a set of jumper wires

Need A JTAG Adapter? Use Your Pico!

JTAG is a powerful interface for low-level debugging and introspection of all kinds of devices — CPUs, FPGAs, MCUs and a whole lot of complex purpose-built chips like RF front-ends. JTAG adapters can be quite obscure, or cost a pretty penny, which is why we’re glad to see that [Adam Taylor] from [ADIUVO] made a tutorial on using your Pi Pico board as a JTAG adapter. This relies on a project called XVC-Pico by [Dhiru Kholia], and doesn’t require anything other than a Pi Pico board itself — the XVC-Pico provides both a RP2040 firmware implementing the XVC (Xilinx Virtual Cable) specification and a daemon that connects to the Pico board and interfaces to tools like Vivado.

First part of the write-up is dedicated to compiling the Pico firmware using a Linux VM. There’s a pre-built .uf2 binary available in the GitHub repo, however, so you don’t have to do that. Then, he compiles and runs a daemon on the PC where the Pico is connected, connects to that daemon through Vivado, and shows successful single-stepping through code on a MYIR Z-turn board with a Xilinx XC7Z020. It’s worth remembering that, if your FPGA’s (or any other target’s) JTAG logic levels are 1.8V or 2.5V-based, you will need a level shifter between it and the Pi Pico, which is a board firmly in the 3.3V realm.

You just cannot beat the $3 price and the ease of setup. Pi Pico is shaping up to be more and more of a hardware multi-tool. Just a month ago, we covered how the Pico can work as a logic analyzer. A lot of that, we have the PIO peripherals to thank for — an assembly of state machines that even let you “bitbang” high-speed interfaces like DVI. If you’re interested in how PIO functions, there are some good write-ups around here. Lacking a Pi Pico, you can use this board’s bigger sister to interface with JTAG, too.

Automatic JTAG Pinout Detection

Figuring out the JTAG pinout on a device turns out to be the most time consuming hardware portion of many hacks. [hunz] started a project called JTAG Finder to automatically detect the JTAG pinouts on arbitrary devices using an 8bit AVR ATmega16/32L microcontroller. Check out the slides (PDF) from the talk as they break down how one finds JTAG ports on an arbitrary device, with or without a pinout detection tool. [hunz] is looking for people to pick up the project where he left off.

Once you determine the correct pinout, you will need a JTAG cable: there are two main types, buffered and unbuffered, both of which I have soldered up and tested from these circuit diagrams (image of completed buffered cable here). The software most hardware people use today are the openwince JTAG Tools. To get the JTAG Tools to compile, grab the latest source directly from their CVS repository.

The last time we featured JTAG was with regards to Linksys devices, but the tools listed above can be applied to any device with JTAG.