Consumer electronics have evolved over decades into a wide range of devices, from professional instruments to personal gadgets. Despite differences in performance and function, these products commonly trend toward greater capability, smaller size, and lower power. Wearables exemplify this trend: they are portable, battery-powered, highly integrated devices responsible for everything from high-precision analog measurements to intuitive user interfaces. Wearable designers must match device requirements across multiple integrated circuits (ICs) while often balancing competing priorities.
Example product concept
Consider a single-function wearable: a screenless pedometer powered by a coin cell. It notifies the user to move when required and tracks steps throughout the day. A simple capacitive touch interface provides user input, and a tricolor LED delivers expressive feedback. This design shows how capable ICs in very small packages can enable compact, differentiated wearable products.
Product requirements
Define the basic product requirements first, then select components to fulfill them. The example device is minimized to basic functionality with no display, buzzer, or smartphone app. Its user interface is similarly simple.
Key design requirements include:
- Minimized overall form factor: With enclosure, the product should be as close as possible in footprint to a CR2032 coin cell so users can carry it in a pocket or attach it to a keychain.
- User input: On one side of the coin-cell-shaped enclosure, provide a capacitive touch interface that recognizes:
- Swipe: dismiss a "time-to-move" alert
- Tap-and-hold: start a new day (reset step counter)
- Tap: check steps for the day
- Simple output: a single exposed LED provides all feedback:
- Red short flash: indicates the user has been idle too long
- Green double flash: triggered when the user starts a new day via tap-and-hold
- 1-second red/yellow/green output: indicates 33%, 66%, and 100% of daily step goal after a tap, lasting a few seconds
How to minimize size
A CR2032 battery is 20 mm in diameter and 3 mm thick. The system must be only slightly larger. Assume the plastic enclosure can be made very thin, adding no more than 5 mm to the diameter to allow battery replacement. For thickness, vertical stack height is determined by four elements: the battery, the printed circuit board, components on the PCB, and the plastic enclosure. A four-layer PCB is about 0.5 mm thick. Minimizing the height of components soldered to the PCB requires careful part selection. Selecting high-performance chip-scale packages is critical to this design.
Benefits of chip-scale packages
Wafer-level chip-scale packaging (WLCSP) reflects many years of progress in fabrication and assembly. In WLCSP, silicon is connected directly to solder bumps on one side of the package. Unlike older wire-bond designs that connect die pads to package pins, WLCSP lets the package dimensions approach the die size in both width and height.
IC vendors have introduced WLCSP options for many devices to enable ultra-small packages. A challenge is that some vendors use relatively large die sizes that limit the smallest achievable package. The EFM8SB1 microcontroller from Silicon Labs is well suited for CSP-style packaging because it delivers high functional density while supporting small packages (for example, a 3 mm × 3 mm QFN). The EFM8SB1 WLCSP option measures only 1.78 mm × 1.66 mm.
The EFM8SB1 MCU is a good fit for this design and other wearables. Key features include:
- An 8-bit MCU with ultra-low power and high-sensitivity capacitive touch input.
- An on-chip real-time clock that can periodically wake the system from very low-power (~300 nA) states. In this design, the clock measures time since the last motion and triggers activity reminders.
- 2–8 kB of flash and 512 bytes of RAM available across low-power cycles, combined with a 25 MHz 8051 core for control logic and system responses.
For step sensing, to exploit the thin profile made possible by CSP MCUs, the accelerometer should also support small package options. The Bosch BMA355 provides a highly integrated sensor with on-chip three-axis event detection and an SPI interface for communication with the MCU.
Because the MCU, accelerometer, and a few passive parts can use ultra-thin packages, the enclosure can be made very thin and positioned close to the capacitive touch surface to optimize sensitivity. The enclosure can even include a slight taper near the touch pad region to minimize the air gap between the PCB and the housing.
Figure 1: Vertical stack of a wearable board with a CSP MCU
PCB layout
Using CSP parts to maximize board space allows a capacitive touch interface on the PCB. The MCU and accelerometer should be clustered near the edge of the PCB in an approximately circular layout, together with the exposed LED, which may require a housing opening.
To detect finger swipe, the PCB needs two capacitive sensors of similar size, slightly staggered along the same edge. These two sensors should occupy much of the MCU side of the board but should be surrounded by a third, smaller sensor that encircles them. The third sensor provides the key reference information the MCU needs for touch and swipe detection during user interaction.
Figure 2: Wearable board layout showing capacitive sensors
Touch detection
Wearables are often carried on the body or held in a hand. Constant or near-constant contact with the human body can complicate capacitive touch sensing. The chosen MCU and accelerometer help overcome these challenges.
Although the system uses three capacitive sensors, it effectively provides four touch inputs. The accelerometer offers an interrupt-driven tap detector that the firmware can use in combination with capacitive sensing. With the accelerometer tap detector, touch detection by the EFM8SB1 proceeds as follows:
- A positive increase is detected at one of the boundary sensors on the device edge, indicating an input use case such as the user holding or gripping the device around the edge.
- The accelerometer signals a tap detection event, aligned with the following event.
- One or all of the central capacitive sensors register a significant positive increase.
All capacitive touch detection and filtering can be implemented using Silicon Labs' Simplicity Studio capacitive sensing firmware libraries.
Low-power operation
Both the accelerometer and MCU can be configured for low-power operation. The capacitive sensing library allows the EFM8SB1 MCU to enter a ~300 nA sleep mode and periodically wake to check the capacitive sensors. If the accelerometer signals that an event has been detected and data are ready, the MCU can be asynchronously woken via a port-match wake event.
The EFM8SB1 MCU remains in low power and consumes under 1 μA except when one of the following occurs:
- Capacitive sensing requires more active monitoring
- Accelerometer activity events (tap or step interrupts) require MCU service
- Motion notification events cause the device to drive the LED to encourage the user to stand and move
The accelerometer is configured for minimal power and only signals on tap events or axis changes. On-chip buffered data reduce MCU-accelerometer interactions, further optimizing battery life.
After the MCU reads buffered data from the accelerometer, it performs checks and analysis to determine subsequent actions. By comparing new three-axis data with historical data stored on the MCU, the firmware can update the step counter and quickly return to low-power mode.
Next steps
This example demonstrates a single-function wearable endpoint. The density, accuracy, and energy efficiency enabled by CSP-sized ICs also illustrate how such ICs can be used and controlled in larger systems. In larger products, chip-scale MCUs can act as low-power sensor hubs managing touch interfaces and accelerometers. As silicon vendors continue to integrate more features into smaller packages, system developers should leverage these innovations during design to create compact, efficient solutions.
