Milliwatts per Gigahash: Crypto Miner ASICs Look to 22FDX

By: Dave Lammers

The last couple of blogs I’ve written have looked at using the 22FDX® technology process for Internet of Things and automotive radar applications, markets that call for a combination of performance at low power consumption. Cryptocurrency mining is another market where power consumption is a defining characteristic, one reason that miners are moving gradually away from GPUs to ASICs.

One of the interesting things about the semiconductor industry is that every application needs a different mix of performance, power consumption, cost, and other factors. The cryptocurrency mining applications are that way, even down to the major coins — Bitcoin, Litecoin, and Ethereum — and the ways they are mined.

Anshel Sag, an associate analyst at Moor Insights & Strategy, who tracks the coin mining market, said miners “don’t want to buy any extra logic on-chip. They want to minimize power. Everything is extremely lean because most of this boils down to power consumption.”

Each of the different algorithms, Sag said, presents “a different bottleneck, and the ASICs need to be architected in different ways to minimize the bottlenecks.” (Sag and Moor principal analyst Patrick Moorhead wrote a white paper on fabs and cryptocurrency miners that provides details on the subject.)

“With so much energy consumed daily, mining operations and manufacturers are always looking at the efficiency of their ASIC miners. Most mining devices ‘performance’ is measured in hashes per watt rather than total hashing capability.” Source: Moor Insights & Strategy White Paper, “The Importance Of Fabs In Crypto Mining”

Architectures Differ

Sanjay Charagulla, senior director of technology marketing and business development at GF, outlined the differences in the miner ASICs optimized for Bitcoin, Litecoin, and Ethereum. The Litecoin ASICs tend to have a relatively small fraction of logic transistors, while SRAM cells account for roughly two-thirds of the transistors. Charagulla argued that GLOBALFOUNDRIES’ 22FDX process has “one of the most efficient SRAM bit cells,” and cited that as a reason why GF has “already taped out multiple customer designs.”

Ethereum mining, which accounts for roughly 10 percent of the total mining IC market, thus far has been dominated by graphics processors (GPUs). The Ethereum algorithm requires a relatively large amount of external memory, and the die sizes are larger. Charagulla said he sees Ethereum mining growing to as much as a quarter of the market, because its overall commercial technology offers good transaction flexibility over Bitcoin.

With Bitcoin – still the dominant cryptocurrency despite a flood of new entries – the mining appliances typically have multiple PCB boards, with each board holding on the order of 50-100+ ASICs. These tiny ASICs are logic devices, with hundreds of multiply and accumulate (MAC) circuits on each die, requiring no external memories or co-processors. And if a couple of cores are not working, the ASIC is able to still grind away. “The Bitcoin ASICs are not that complex. The layout and back end design are key to efficiency,” he said.

With the cost of power so important to miners, efficiency is measured in milliWatts per gigahash, rather than total hashing capability. Bitmain, the dominant mining vendor, has described a 98 milliwatts per gigahash Bitcoin miner, and new competitors are seeking to either match or improve on that. “We have multiple customers engaged, and a handful already taped out, with good results,” Charagulla said.

Good Enough Performance

I asked Charagulla if miners could get to the next space on the blockchain faster by jacking up the ASIC’s frequency and paying for the extra power consumption. He replied that to keep the thermal flow within the miner at an optimum level and conserve power, the smart strategy is to run the ASICs at a “reasonable frequency of 400-500 MHz at the lowest power.”

Even though some Bitcoin ASICs are shifting to FinFET-based processes, Charagulla argued that the better strategy is to keep the manufacturing costs and power consumption down by using the FD-SOI-based FDX process, while maintaining sufficient performance. “The way to do this is to run thousands of cores in parallel, at a certain frequency, so they can still solve the puzzle. The cores are basically a bunch of XOR gates with a 16-bit-wide datapath, in a confined layout. Our belief is that 22FDX will meet the requirements here. Where FinFETs shine is at gigahertz clock speeds, with wider buses and bit-adder logic. In this case (Bitcoin ASICs) there is not any high-speed I/O, so if you can optimize the layout of the cores, FD-SOI can be as good as FinFETs, and at lower costs.”

Many customer designs using the FDX process operate at just 0.4V. Charagulla said one tier one customer is pushing down to a 0.3 Vdd to provide miners with a lower-power-consumption ASIC at 80 milliwatts per gigahash, while “still able to run the algorithm efficiently.” Back biasing and forward biasing can be used to meet the performance and power specs, he added.

Capacity Constraints

Sag, the Moor Insights analyst, said while some “premium” ASIC miners will continue to be made at leading-edge FinFET processes, other miners may take a different tack. “FinFETs on the more-expensive nodes provide higher performance, but at a cost. When the mining ASICs go to smaller design rules, the wafers are more expensive. Right now, people want to drive down the cost of miners, so they can sell more at a lower cost and get more profit. By being on a leading-edge node, such as 10nm or 7nm, the yields are not the greatest initially. The costs are high on a leading-edge node.”

Moreover, mining companies are “jockeying for fab capacity, another reason why the costs are higher,” Sag said.

With GF’s Malta, N.Y. fab running at nearly full capacity at 14nm and the upcoming 7nm processes based on FinFETs, Sag said the mining companies see available 22FDX capacity at Dresden as an opportunity. Moreover, with more than a half-dozen miner device manufacturers based in China, Sag said “22FDX could be used in China relatively soon.”

Sag noted that “GF is doing a good job of choosing the right processes for the right customers, for what is important to them. Not every chip needs billions of FinFET transistors. 22FDX makes sense in terms of price sensitivity, as well as the need for high efficiency.”

The Moor Insights white paper noted that the “GLOBALFOUNDRIES’ FDX roadmap will expand in 2019 and 2020 to include 12nm FDX, which should operate at even lower power and higher performance while also having a cost-friendly profile. We believe this product expansion could significantly benefit miners. The cost of manufacturing chips is becoming an increasingly important factor in their success, particularly as Bitcoin and other altcoin ASIC mining companies aim to turn as much volume as possible.”

Charagulla said the available capacity at Dresden is drawing new miner companies to 22FDX. “Malta is mostly full, and the Dresden fab is clearly positioned for 22FDX, and 12FDX going forward. We are getting design wins in millimeter wave RF, for base stations and mobile handsets and millimeter-wave radar. For the miner ASICs, FDX adds value, and that is why the new entrants are coming to us.”