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 Ā°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 Ā°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 | ā |
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.
