DEV Community: Matt Howard

Driving an Undocumented 3nm ASIC with an ESP32: Inside the First Sub-10 J/TH Open-Source Bitcoin Miner

Matt Howard — Tue, 25 Aug 2026 19:45:22 +0000

Next week we ship the Bitaxe Naja Duo, and the spec that matters is 9.5 joules per terahash at stock. First open-source home miner under 10. But this is dev.to, so instead of a product pitch, here's the part that's actually interesting to build: what it takes to make a $3 microcontroller drive silicon that Bitmain designed for industrial hashboards and documented for nobody.

The problem: a peripheral with no datasheet

The BM1373 is Bitmain's 3nm SHA-256 ASIC from the Antminer S23 generation. Roughly 2.5 TH/s per chip. No public datasheet, no reference firmware, no SDK. Out of the box it's an inert slab of silicon speaking an undocumented serial protocol.

To make it hash, something has to handle:

Init sequences and chain enumeration: discovering what's on the wire and waking it up
Baud negotiation: the chips start slow and get bumped to working speed
Register writes for frequency and core voltage: the two knobs that decide everything about performance and efficiency
Work distribution: splitting block headers across chips and hashing domains
Nonce collection: catching results and mapping them back to the work that produced them

Every line of that, for every chip generation from the BM1397 forward, is community reverse-engineering. The esp-miner firmware (GPL-3.0, maintained by the Open Source Miners United community) packages it into something an ESP32-S3 can run: chip drivers, a stratum client, a self-hosted web UI (AxeOS), and a REST API for telemetry.

What's new on this board, from a firmware perspective

The Naja Duo is the first Bitaxe on the BM1373, and two chips at 3nm brought real changes to the stack:

Per-chip temperature monitoring. Previous boards throttled off a single reading. With two dies that can bin very differently (silicon lottery is real: one chip holds 600 MHz at lower voltage than its twin), the firmware now tracks each ASIC and drives throttling and the PWM fan curve off the hotter one. On a single-sensor board, one die can run ~10 °C above the reported number and you'd never know.

A 2-phase VRM worth respecting. Power delivery is a 2-phase TI TPS546024A. Split current, distributed heat, tighter ripple. That's what keeps 1,120 mV stable at the 600 MHz overclock profile (8 TH/s at ~92 W) instead of brownout resets.

Defaults as an engineering statement. Stock is 327 MHz / 1,000 mV: 4.4 TH/s at ~42 W, the efficient point on the curve. Pushed to the far end, a BM1373 board degrades right back to ~15 J/TH, which is last-generation territory. Some manufacturers ship BM1373 boards factory-overclocked for the spec sheet anyway. We expose frequency and voltage as ordinary dashboard settings and let the person with the actual board and live telemetry find its ceiling. On our bench, with an AIO cold plate on the pair, one board held 11.6 TH/s at 46 °C. Three 12V fan headers on the back of the board are there for exactly those builds.

The API surface

Everything the board knows is queryable. AxeOS exposes REST endpoints for hashrate, per-chip temps, voltage, power, fan RPM, shares, and best difficulty, which is why third-party monitoring tools (HashWatcher et al.) work across the whole Bitaxe family with zero vendor integration. Point GET /api/system/info at any board on your LAN and build whatever you want on top: Grafana dashboards, alerting bots, a fleet view for a shelf of them. Firmware updates go over the web UI or USB-C webflash, and the ESP32-S3's Boot/Reset buttons mean recovery never requires opening anything.

The license, because it matters

esp-miner is GPL-3.0. Use it, fork it, ship it, but shipped derivatives must publish source. Several vendors currently sell rebranded closed-source forks. That's not a business model, it's a license violation against volunteer maintainers with no legal department, and it orphans buyers from mainline updates. If you're evaluating open hardware in any category, "does it run mainline" is the best single question you can ask. The Naja Duo does.

The numbers, for the curious

	Stock	Overclocked	Hydro (bench)
Hashrate	4.4 TH/s	8 TH/s	11.6 TH/s
Power	~42 W	~92 W	~159 W
Efficiency	9.5 J/TH	11.5 J/TH	13.7 J/TH
Frequency	327 MHz	600 MHz	—

Full teardown and comparison against the rest of the lineup is in the release article. Board is $389.99, releasing next week, schematics at github.com/bitaxeorg.

If you build something against the API or the fan headers, post it. The best mod for this board will come from someone we've never met, and the hardware was designed assuming that's true.

Inside the Bitaxe: How an Open-Source ASIC Miner Goes from Schematic to Solo Block

Matt Howard — Thu, 19 Feb 2026 03:04:22 +0000

If you have spent any time in Bitcoin circles over the past year, you have probably seen the photos. A tiny circuit board, roughly the size of a credit card, with a single ASIC chip, a small fan, and an OLED screen showing a live hashrate. It sits on someone's desk next to a monitor, pulling 18 watts from a USB-C power supply.

That device is a Bitaxe. And it is one of the most interesting open-source hardware projects to emerge in the Bitcoin ecosystem.

What makes Bitaxe worth paying attention to is not the mining output. At 1.2 TH/s, a single Bitaxe Gamma 602 represents a vanishingly small fraction of Bitcoin's 800+ EH/s network hashrate. What makes it interesting is the engineering: a fully open-source Bitcoin ASIC miner built around a reverse-engineered Bitmain chip, an ESP32-S3 microcontroller, and a firmware stack that anyone can audit, modify, and flash.

For developers and hardware tinkerers, there is a lot to unpack here.

The Architecture

Every Bitaxe model shares the same core architecture. An application-specific integrated circuit (ASIC) handles the SHA-256 hashing. An ESP32-S3 microcontroller manages Wi-Fi connectivity, pool communication, and the web-based configuration interface called AxeOS. A small OLED display outputs real-time stats: hashrate, temperature, best difficulty share, and uptime.

The current flagship, the Bitaxe Gamma 602, uses Bitmain's BM1370 ASIC. This is the same chip found in the industrial Antminer S21 Pro, which runs thousands of these chips in parallel at rack scale. The Bitaxe puts one on a palm-sized PCB and drives it at 15 J/TH efficiency.

The dual-chip Bitaxe GT 801 runs two BM1370s on a single board at 2.15 TH/s and 18 J/TH. The Bitaxe Duo 650 takes a different approach: two BM1370s at the same 15 J/TH efficiency as the Gamma, pushing 1.63 TH/s in a slightly larger form factor.

All schematics, PCB layouts, Gerber files, and bill of materials are published at bitaxe.org and on Skot's GitHub repository. The firmware, ESP-Miner, is also fully open source. This is not "open-source-in-name-only" where a company publishes a README and keeps the interesting parts private. Every copper trace on the board is documented.

The Firmware: AxeOS

AxeOS is where the Bitaxe becomes genuinely useful as a developer platform. The firmware runs on the ESP32-S3 and exposes a full web interface accessible from any browser on the local network. From AxeOS you can configure:

Stratum pool URL and port
Bitcoin wallet address (stratum user)
ASIC frequency and core voltage (overclocking)
Fan speed and thermal thresholds
Autotune mode (automatic frequency/voltage optimization)
OTA firmware updates

The autotune feature is particularly well implemented. It incrementally adjusts ASIC frequency and voltage, monitors temperature and hashrate stability, and settles on an optimal operating point for your specific chip. Because of silicon lottery (natural variance in chip manufacturing), two identical Bitaxe units will often stabilize at slightly different frequencies. Autotune handles this automatically.

The entire AxeOS codebase lives at github.com/skot/ESP-Miner. It is written in C using the ESP-IDF framework. Pull requests are active. If you want to contribute, the ASIC driver layer and the Stratum V1 implementation are the two most impactful areas.

What Actually Happens When It Mines

When the Bitaxe boots, AxeOS connects to your configured Wi-Fi network, opens a TCP connection to the stratum pool, and begins requesting work. The pool sends a block template containing a set of transaction data, a previous block hash, and a target difficulty.

The ESP32-S3 passes this work to the BM1370 via a serial interface. The ASIC then iterates through nonce values at approximately 1.2 trillion attempts per second (for the Gamma 602), computing a double SHA-256 hash for each. If the resulting hash is below the target difficulty, the Bitaxe submits it back to the pool as a valid share.

For pool mining, any share meeting the pool's difficulty threshold earns proportional credit toward the next block reward. For solo mining (the mode most Bitaxe owners use), only a share meeting the full network difficulty counts. That means your device needs to find a hash below a target set by a difficulty exceeding 114 trillion as of February 2026.

The probability math is straightforward. A 1.2 TH/s device against a network of 800+ EH/s has roughly a 1-in-850,000 chance of solving a block in any given day. Each hash is an independent Bernoulli trial. There is no "building toward" a solution. Every hash is equally likely to be the winning one.

When the Improbable Happens

Despite those odds, Bitaxe devices have found confirmed Bitcoin blocks. In March 2025, a miner running a Bitaxe at roughly 480 GH/s found Block #887,212 and earned 3.125 BTC, worth approximately $200,000. In November 2025, a cluster of six Bitaxe Gamma 602 units with a combined 6.6 TH/s found Block #924,569 at a network difficulty of 221.39 trillion, earning over $310,000.

The most notable find came in October 2025. A cluster of NerdQaxe++ devices (a quad-chip variant running four BM1370s at 6+ TH/s) found Block #920,440 while connected to a self-hosted Public Pool instance running on Umbrel. The reward was 3.141 BTC, roughly $347,000. The miner was running a fully sovereign stack: his own hardware, his own node, his own pool software. No third party was involved at any point in the process.

In total, open-source mining hardware has found at least five confirmed blocks since July 2024, with combined rewards exceeding $1 million in BTC. The interval between finds has been compressing: 229 days, 179 days, 52 days, 25 days. More devices in the field means more aggregate hashrate, which means more statistically expected wins.

The Protocol Layer: DATUM and Stratum V2

The Bitaxe currently communicates with mining pools using Stratum V1, the protocol that has been the industry standard since 2012. In the V1 model, the pool constructs the block template and sends it to the miner. The miner hashes it. The miner has no say in which transactions are included.

Two newer protocols are changing this dynamic.

DATUM, developed by Ocean, allows miners to build their own block templates using their own Bitcoin node's mempool data. The miner selects which transactions to include, constructs the coinbase transaction, and submits the completed work to the pool. The pool handles coordination and payout, but the miner retains editorial control over block contents.

Stratum V2 pursues a similar goal with a different architecture. It introduces encrypted communication between miner and pool, reduces bandwidth requirements, and supports a "job declaration" mode where the miner can propose its own block templates.

For Bitaxe owners running a home node (via Umbrel, Start9, or a bare metal setup), these protocols transform the device from a passive hash generator into an active participant in Bitcoin's consensus process. You are not just submitting hashes. You are deciding what goes in the block.

Neither protocol is fully integrated into AxeOS at the time of writing, but development is active and the OSMU (Open Source Miners United) community on Discord is the primary coordination point.

Building or Buying

The Bitaxe is designed to be buildable from scratch. Order the PCB from JLCPCB or PCBWay using the published Gerbers. Source the BM1370 from NBTC on AliExpress (approximately $15 per chip). Order the remaining BOM from DigiKey. Solder it yourself.

This is, realistically, an advanced build. The BM1370 is a BGA package that requires reflow soldering. The passives are 0402 and 0201 size. You need a stencil, solder paste, and either a reflow oven or a hot air station with steady hands.

For most people, buying a pre-assembled, tested unit is the practical path. Retailers like Solo Satoshi assemble and test every unit in the USA, ship same day, and include a 90-day warranty. The Bitaxe Gamma 602 runs approximately $98. A fully assembled NerdQaxe++ Rev 6.1 (four BM1370 chips, 6+ TH/s) is roughly $382.

The open-source nature of the project means you are not locked into any vendor. If you buy a Bitaxe from any seller, you can flash it with the latest AxeOS firmware, point it at any pool, and configure it however you want. The hardware is yours. The firmware is yours. The keys are yours.

Why Developers Should Care

Bitaxe sits at the intersection of embedded systems, cryptographic protocols, network engineering, and economic incentive design. It is a real-world system that combines:

Low-level ASIC driver development (C, ESP-IDF)
Networking protocol implementation (Stratum V1/V2, TCP sockets)
Cryptographic hashing (SHA-256 double hash, nonce iteration)
Hardware design (4-layer PCB, BGA soldering, thermal management)
Distributed systems (pool coordination, block propagation, difficulty adjustment)

If you are looking for an open-source project where your contribution has tangible, measurable impact on a live production network processing billions of dollars in value daily, this is one of the few that qualifies.

The ESP-Miner repo accepts pull requests. The OSMU Discord has active hardware and firmware channels. The BM1370 register map is partially documented but still has gaps, which is one of the highest-impact reverse engineering opportunities available in the Bitcoin hardware space right now.

Eighteen watts. One chip. A live connection to the most valuable computational network on earth. The code is open. The schematics are public. The blocks keep getting found.

Matt Howard is the Founder and CEO of Solo Satoshi, a Bitcoin home mining hardware retailer based in Houston, Texas that has shipped over 40,000 open-source mining devices to 70+ countries.