Antminer S21

The Antminer S21 is Bitmain’s air-cooled, fifth-generation SHA-256 Bitcoin miner: 200 TH/s at a 3,500 W wall draw, or 17.5 J/TH. Built on the TSMC 5 nm BM1368 ASIC across three hashboards, it set the air-cooled efficiency bar when it launched in late 2023 and remains a workhorse today.

Chip and hashboard architecture

The S21 is driven by Bitmain’s BM1368, a TSMC 5 nm SHA-256 ASIC carrying roughly 894 hashing cores per die. Three identical air-cooled hashboards — Bitmain part BHB68603 — each carry 108 BM1368 chips, for 324 ASICs in a complete unit. We confirmed the board layout directly against Bitmain’s own AMTC single-board test fixture, so these are factory figures rather than estimates.

Each board organizes its 108 chips into 12 voltage domains of 9 chips wired in series. This matters for anyone tuning or repairing the machine: voltage is regulated per domain, not per chip. The nine chips in a domain share one regulated rail at roughly 1.2 V, so the full series string sits near 14.4 V. Domains 1–10 are fed by clusters of three LDOs each (one stepping the ~25 V boost rail down to 1.2 V for signaling, two more producing the 0.8 V core voltage); the top two domains use MP2019 buck converters. An added bank of 11 level shifters cleans up the longer signal chains versus earlier generations.

The biggest architectural change in this generation is what is not on the board. The BM1368 generation is a “no-PIC” design — Bitmain removed the dedicated PIC microcontroller that policed voltage and heartbeat on S9- and S19-era boards. The S21 also abandons the Xilinx Zynq FPGA control board of the S19 line. Instead it runs an Amlogic A113D (AXG) SoC with four ARM Cortex-A53 cores and 256 MB of RAM, booting from NAND — there is no FPGA in the data path at all. All three hashboards talk to the SoC over a single UART (chain selection is handled by GPIO), and an audio-class DAC (TAS5782M) handles voltage setpoints in place of the old PIC. A 25 MHz crystal (Y1), an onboard EEPROM (U6), and inlet/outlet temperature sensors (U5/U7) round out each board.

Real-world power and efficiency

Bitmain rates the air-cooled S21 at 200 TH/s drawing 3,500 W at the wall, which works out to 17.5 J/TH at 25 °C ambient. Real numbers drift with conditions: warmer intake air, a marginal PSU, or a single weak domain will nudge wall draw up and sustained hashrate down. Plan your circuit around the nameplate plus headroom — at 200–240 V the S21 pulls roughly 15–16 A, so a dedicated 240 V branch is the sane choice.

The S21 silicon has genuine tuning range. Across the wider S21 family, performance-mode profiles land around 16.1–16.9 J/TH while normal-mode tuning holds a remarkably flat ~17.9–18.0 J/TH, and low-power curves push efficiency lower still at the cost of raw terahash. Those operating points are not fixed presets baked into the chip — a firmware autotuner calculates per-domain frequency and voltage at runtime to hit a target, then trims continuously as the boards warm up. If you want to see concrete wattage/hashrate/efficiency steps for this class of hardware, our ASIC power profiles database lays out the curves so you can match the machine to your power budget rather than your ego.

Firmware compatibility

The S21 ships with Bitmain’s stock firmware (bmminer on the Amlogic Buildroot stack), which is stable, carries no developer fee, and is the right default for most operators. The chip is also well supported by the major third-party firmwares — we have run a live S21 on BraiinsOS+ 25.11, and the BM1368 is covered by the mainstream tuning firmwares as well. Be honest with yourself about the trade-offs: aftermarket firmware buys finer autotuning and curtailment control, but most options carry a developer fee in the ~2–3% range and replace Bitmain’s stock toolchain. If you require Stratum V2 — the encrypted, job-negotiating successor to V1 — note that BraiinsOS+ is the only firmware that natively speaks it today; stock and most aftermarket builds remain Stratum V1.

For operators who want sovereignty over their fleet, DCENT_OS — D-Central’s open, GPL-3.0 firmware — is in active development for the BM1368 S21 platform (it is in active closed beta on the Antminer S9 today; S21-class support is planned for a later stage, with a 0% mandatory dev-fee target you could optionally redirect to us as a donation). We build on the shoulders of the teams who proved this ground first; DCENT_OS is simply one more layer of decentralization for people who would rather own their stack than rent it. Whatever you flash, keep a verified copy of the stock NAND image before you start.

Common faults and troubleshooting

The S21’s series-domain wiring is efficient but unforgiving: because nine chips share a domain and twelve domains sit in series on a board, a single dead ASIC can drop an entire chain. The faults we see most on the bench are:

• “0 ASIC found” / chain count short of 108 — an open in the series string, usually one failed chip, a cracked solder joint, or a damaged level shifter.
• Hashrate that drops under load — thermal throttling from clogged fins, a tired fan, or a failing inlet/outlet temp sensor feeding the SoC bad data.
• PSU/under-power faults — the APW-class supply sagging, or a circuit that cannot hold the full ~3,500 W under warm conditions.
• No boot / control-board faults — corrupt NAND or a flash gone wrong on the Amlogic SoC.

Work the problem methodically: confirm all three boards enumerate their full 108 chips, check intake/exhaust temperatures, and reseat the signal and power connectors before condemning a board. Our ASIC fault finder walks the symptom-to-cause path for these error states, and one caution specific to this generation — because the S21 is a no-PIC design, do not apply old S9/S19 GPIO-reset habits to the chains; the control sequence is different.

Repair and longevity

An S21 is a long-lived asset when it is maintained, and a chip-level fault does not mean the board is scrap. D-Central has run an in-house ASIC repair lab in Laval since 2016, and we repair the Antminer S21 to the component level — diagnosing dead domains, reflowing or replacing individual BM1368 chips, rebuilding power stages, and servicing the Amlogic control board and PSU. Domain-level diagnosis is exactly where a series-string board rewards a real bench over a parts-swap, since the failure is often a single chip out of 324. If your S21 has dropped a board or won’t enumerate, see our ASIC repair service rather than retiring a machine that has years of useful life left.

Who the S21 is for

The S21 is a serious mining machine, not a living-room appliance. At ~75 dB and 3,500 W it is loud and hot — think dedicated mining space, garage, shed, or a purpose-built enclosure with real airflow, on a proper 240 V circuit. Its ~11,940 BTU/h of waste heat is genuinely useful if you duct it into a space that needs warming, turning a heating bill into hashrate, but it is too much machine to sit beside a desk. If you want Bitcoin hashing in a quiet home on a standard outlet, a small open-source device like a low-wattage Bitaxe-class miner is the saner entry point; the S21 is for operators ready to commit a circuit and a room. To compare it against other current hardware on hashrate, efficiency, and price, browse our ASIC miner catalog and database.

Generational context

The S21 is the anchor of Bitmain’s Gen-5 air-cooled lineup, succeeding the S19 series and its BM1398/BM1366 silicon. Credit where it is due: the jump from the S19’s ~29.5 J/TH-class chips to the S21’s 17.5 J/TH was one of the larger single-generation efficiency gains in SHA-256 mining, and Bitmain earned it with the move to the BM1368 and a leaner control board. The base S21 sits at the entry point of a family that scales well beyond it, all sharing the same Amlogic control platform:

The Pro and XP variants move to the BM1370 chip for better efficiency, while the hydro and immersion SKUs push well past 300 TH/s. The base air-cooled S21 remains the most accessible point of entry to this generation — straightforward to power, simple to cool, and supported by a deep firmware and repair ecosystem.

Antminer S21 specifications at a glance

D-Central’s in-house technicians repair, tune, and service the Antminer S21. View full specifications, live profitability, and side-by-side comparison data for the S21 in our comprehensive ASIC miner database.