One Breath, One Milliwatt

To understand how challenging it is to successfully implement Always-on Technology, consider doing any kind of sport while holding your breath. Sounds crazy? There’s actually one sport in which participants do just that – freediving. So what does freediving have to do with always-on technology? Quite a few things apparently.

Harsh Environment
In freediving, there is one resource which is by far scarcer than others – oxygen. In always-on technology that would be power. The problem is that these two resources are the most important ones to the functionality of the two types of systems, oxygen to the biological one and power to the electrical one. Being short of such fundamental resources usually spells bad news to the system, unless you know what you are doing.

Optimal Resource Utilization
To maintain an operational level of such precious resources, activity level has to be kept to the minimum necessary. Furthermore, any activity that is taking place has to make optimal use of the limited resources, which mandates deviating from old habits. For example, any component that is always-on must not include wasteful elements such as power-hungry processors like the ones found in smartphones. On the other hand, the always-on processor has to be efficient enough to be able to execute its tasks (e.g. Bluetooth-enabled voice activation, gesture recognition) — which could be quite intensive — with minimal power consumption. Similarly, to succeed in freediving one has to let go of terrestrial habits that might be fine given unlimited oxygen credit but have no real justification when submerged. Consider, for instance, hand movement that automatically happens when we walk. Such movement has no effect underwater and it wastes oxygen in vain. Furthermore, neutralizing the hands also helps in keeping a streamlined position.

Cutting Edge Technology to the Rescue
Less than 100 years ago, medical doctors believed that freediving below 30 meters was biologically impossible as the lungs would crash under the pressure of the water column above. Around that depth the air in the lungs indeed compresses dramatically and reaches residual volume which is about a third of original volume on surface. What the doctors didn’t know is that two mechanisms called mammalian diving reflex and blood shift kick in and slow down the heart rate, optimize blood circulation and transfer blood to the lungs to prevent them from crushing upon reaching residual volume. The current world record is 281 meters, so don’t believe everything your doctor tells you. In always-on technology, breakthroughs are still ahead of us. First and foremost SoC vendors should let dedicated processors handle always-on tasks rather than running them on yesterday’s power-hungry application processors. For example, the CEVA-TeakLite-4 runs various always-on functions simultaneously in less than 150uW at a 28nm HPM process node: voice trigger, face trigger, sensor fusion and Bluetooth Low Energy (BLE). Running similar functions on the application processor (AP) would require at least two orders of magnitude more power, clearly surpassing the power consumption threshold required for a reasonable battery life. This gap in power consumption is not accidental – unlike an AP, the CEVA-TeakLite-4 DSP is well adept to running such functions, which often require intensive signal processing. Furthermore, with its power-optimized hand-crafted RTL, power scaling mechanism and 10-stage pipeline that easily fits low power memories, the CEVA-TeakLite-4 consumes ultra-low power by design. On top of that there are a few technologies that still sound a little like science fiction but will probably commercialize given enough time, such as subthreshold conduction and energy harvesting that can dramatically reduce power consumption or charge the device with scavenged energy.

Separate the Men from the Boys
Any inefficiency in always-on systems has immediate implications, namely poor battery life as witnessed with recent smartwatches, for instance. While poor battery life can be tolerated by users of other devices, users of wearable ones are less forgiving, demanding longer times between charging (weeks vs. hours), and many of those devices end up in the back of a drawer as a consequence. In freediving, any inefficiency immediately translates to poor bottom time. The challenge here goes far beyond neutralizing frenzied limbs, as out of all organs, the brain is the biggest oxygen consumer and there is only one natural way to keep brain oxygen consumption to the minimum – relaxation. So next time you go freediving keep that in mind (or even better don’t keep anything in mind) and next time you design an always-on application, think carefully about which processor is the best fit for always-on functions. In both cases it will help you reach a similar goal – less frequent recharging.