Miniaturisation and power efficiency: the growing importance of precision timing in wearable design
By Joe Salvador, senior director of marketing, SiTime
The first wearable step counters which emerged in the early 2000s bear little resemblance to today’s sophisticated AI-enabled devices. From humble clip-ons and bands such as FitBit and Jawbone, we’ve arrived in an era that considers motion analysis and biometric monitoring standard features in fitness trackers, smartwatches, rings and smart glasses.
Sensor-rich compute devices aren’t restricted to the fitness world. Wearable and screenless IoT designs are becoming prevalent across the consumer space, as well as in industrial and medical; from audio devices to safety equipment, health wearables and personal assistant solutions.
As wearable devices grow smaller, smarter and more functionally dense, timing components – often overlooked in system-level design – are emerging as a critical variable in achieving the performance, size, power, and reliability targets that modern wearable platforms demand.
Features of a constrained environment
Wearable device designers face some unique challenges some of which are not always immediately obvious. Of course, the size and weight of every component needs to be carefully evaluated and optimized along with power consumption design tradeoffs due to limited battery sizes. Even more important can be performance and reliability tradeoffs which are not as critical in other electronic devices. For example, rapidly turning powerful AI-capable processors and wireless subsystems on and off within small enclosed designs can generate rapid thermal gradients which can impact timing subsystems. Tight routing requirements across flexible PCBs can insert jitter, phase noise and EMC-related interference.
Wearables and other IoT endpoints like sensors, smart labels, and asset trackers typically operate in variable-to-harsh environments. These devices often face vibration, physical strain and repeated high impact events that go well beyond those faced even by portable electronics such as mobile phones. Similarly, medical wearables such as continuous glucose monitors (CGMs), wearable electrocardiograms (ECGs) and hearing aids must assure consistent, long-term dependability and power efficiency to fulfil their role of improving patient outcomes.
Timing’s role in wearable design
Wearables depend on input from many sensors, such as health, motion, video and voice input. Timing components provide accurate synchronisation and time stamping between sensors, allowing data from multiple sensors to be properly interpreted in a process known as sensor fusion. Operating within a tight space, timing solutions must maintain frequency accuracy, support stable wireless links and provide a backbone for reliable system performance even under harsh conditions such as rapidly changing temperatures, physical strain and high shock and vibration.
Precision timing devices such as resonators and oscillators based on Microelectromechanical System (MEMS) provide a number of advantages for wearable devices including rapid startup, improved stability under harsh conditions, lower system power, and significantly smaller footprints. For example, the lower startup time for MEMS-based oscillators provide predictably lower latency for familiar wearable features such as motion tracking, wake-word voice activation and biometric signalling.
In activity-tracking and other AI-based wearable applications, MEMS timing devices are seeing increased adoption due to their inherent advantages in size, stability across temperature and under both high temperature transitions and physical stress. During motion, MEMS oscillators offer resistance to mechanical displacement while maintaining frequency accuracy. They contain MEMS resonators, which are built on silicon using semiconductor manufacturing techniques. These components offer significantly greater resistance to mechanical stresses than traditional alternatives, maintaining frequency stability under the conditions that wearables are specifically designed to monitor. Wearables also face the challenge of rapid internal temperature changes due to operating powerful AI processors and multiple radios within a very confined space. Precision MEMS devices are designed to maintain high stability even under these harsh conditions.
There is also the battery dimension. High-current operations such as GNSS acquisition and Bluetooth pairing require the radio to remain active until the operation completes. The more accurate clock of MEMS – designed for high stability even in harsh environments – shortens that window, allowing the system to finish its handshake in fewer cycles and return to idle sooner. In an always-on device with a constrained battery, this efficiency compounds significantly across a product’s operational lifetime.
Where every millimetre counts
PCB area is the most visible constraint facing wearable designers. Timing components are competing for space against neighbouring components such as sensors, antennas, batteries, and processors.
Designers have the task of balancing architecture footprints with power consumption and frequency stability. Every precious millimetre recovered from the timing component footprint can be reallocated for purposes such as battery capacity or antenna clearance.
Component count amplifies the footprint problem. A design that deploys separate timing components for each functional domain not only consumes more board area in aggregate and increases BOM, but also means more landing pads, more routing and more potential for interference.
Programmable MEMS oscillators – in chip-scale packages as small as 1.5 × 0.8 mm – are serving multiple frequency requirements from a single device to directly reduce this burden. They eliminate the external load capacitors traditionally required by the oscillator circuit. These miniaturised architectures are a direct response to current design trends, which are seeing wearables transition into ever more compact and discreet formats.
Trends in ultra-miniaturised timing components
The clear direction of travel towards miniaturisation presents a number of challenging form factors for designers of consumer electronics.
Advanced finger-worn wearables such as smart rings offer a screen-free, discreet smartwatch alternative which excels at everything from providing highly accurate sleep tracking to body temperature sensing and daytime stress monitoring. In these devices, the entire PCB — including processor, sensors, wireless radio, battery, and timing components — must fit within a ring band typically 6-8 mm wide and no more than 2-3 mm in height. Even in the absence of a display to accommodate, the remaining circuitry is packed into a curved band with a total internal volume that may be less than 0.5 cm³. At these dimensions, they are significantly larger than most jewelry rings, so future miniaturization is still required.
Disposable and reusable health patches monitor and treat conditions, helping to avoid invasive procedures or bulky equipment. They are used for applications such as CGM, ECG recording, respiratory rate tracking and drug delivery monitoring. These printed electronics have space constraints due to their requirement of flexibility, with patches requiring placement on a thin substrate to conform to skin curvature. Timing accuracy in health patches is clinically significant: for example, CGM sensors sample glucose concentration at fixed intervals over periods of 7-14 days, and any drift in the sampling clock introduces measurement error.
There is an abundance of other examples. In the most highly constrained wearable applications, MEMS-based precision timing is invariably an enabler, providing the smallest footprints. Other trends the technology supports include clothing-mounted sensors, which present the challenges of wash cycles, mechanical flexing and humidity; and in-ear wearables, where timing requirements are driven by audio latency and left-right channel synchronisation, with an available PCB area measured in single-digit square millimetres.
Future integration: co-packaging timing
Looking ahead, we see the most significant short-term integration trend as the movement of the timing reference off the PCB entirely, into the semiconductor package itself. MEMS resonators available in known-good-die format can now be co-packaged directly inside SoC and MCU packages during assembly. This eliminates the discrete timing component from the board; freeing two pins on the host device, reducing parasitics, and removing the assembly cost of a separate component.
In wearables, every decision that extends battery life, reduces board area, or improves measurement accuracy contributes to product viability. Timing components are now contributing meaningfully to all three.
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