The Bluetooth low energy (BLE) protocol has established itself as a formidable choice for wireless communication where multiple slave devices need to talk to a single master device. Factors that favor Bluetooth over competing communication protocols include:
Multi-vendor due very high rate of industry adoption. As per the Bluetooth SIG, by 2018, 90% of smartphones will support BLE. The adoption rate is high among other host devices such as PCs and smart TVs as well.
Published communication of 100m.
Ultra-low peak, average, and idle enabling most BLE slave devices to run for years on coin cell batteries.
Data transfer rates up to 1Mbps.
These advantages make BLE a best-fit choice for many Internet of Things (IoT) based devices, wearable electronics, wireless PC peripherals, remote controls, and other devices. In fact, the very advent of BLE has inspired innovators across the globe to create applications never fathomed before.
In a very simplistic sense, most BLE slave devices effectively capture an input and transfer the information using BLE to the client (i.e., a PC or smartphone). Thus, the key functionality of a BLE slave device can be identified as capturing the input, processing the input, and transmitting that processed input to the client (host) wirelessly using the BLE protocol.
When we consider the functionality of capturing an input we identify 2 segments:
Segment 1: Devices that capture input using sensors (i.e., sensor input devices or SIDs)
Segment 2: Device that capture input from a human user (i.e., human input devices or HIDs)
Consider a heart rate monitor, which uses a sensor to capture a person’s heart rate. The information can be processed and then transmitted to the client (PC or phone). In this case, the user just needs to wear the device and doesn’t need to manually enter an input.
Now consider the case of a wireless mouse that communicates to a PC wirelessly using BLE. In this case, the user manually provides the inputs (in the form or clicks and scrolls). The question that arises next: How will this difference impact the way the device is designed?
Human Input BLE Device Design Challenges
To capture a human user input, we can either use a button, slider, and/or rotor combination. This input can be mechanical or we can use capacitive sensing. In the case of the former we can use sensors to detect the user interaction with the mechanical components or connect the mechanical components to the controller directly. Once captured, the inputs are processed by an MCU and transmitted out to the client via the BLE stack.
There are numerous devices available in the market today that integrate the BLE stack with a microcontroller (MCU). This enables developers to build single-chip systems similar to that of a sensor input device:
However, using mechanical components lead to compromised reliability and ergonomics. Buttons are susceptible to wear and tear, which leads to premature death of the device. Due to these limitations, many industries are replacing mechanical user interfaces with capacitive sensing-based alternatives.
Using capacitive sensing-based user input introduces a different challenge altogether. For many architectures, two chips are required instead of one—one chip for the capacitive sensing implementation and the other for BLE implementation. This causes an increase in the PCB size that, in turn, increases the overall cost of manufacturing. There is also the problem of power management. The system would need additional clocks to coordinate the standby time of the two chips. In almost all cases, it can be expected that products utilizing BLE (remote control, mouse, etc.) will be battery operated. Thus, maximizing battery life is extremely important.
