This document introduces the features and characteristics of the STM8TL5xxx microcontroller group, which is used for touch sensing applications. It uses a high-performance capacitive sensing engine based on the projected ProxsenseTM acquisition principle. The outstanding performance of STM8TL5xxx enables long-range proximity detection, which is crucial for many end applications such as personal navigation devices or stove tops with backlighting that turns on when a user is detected. Proximity detectors are also widely implemented in white goods, automotive devices, palm tops, and various kitchen or office appliances where the display needs to be turned on for user adjustment. One of the key features of capacitive sensors compared to conventional proximity detectors (such as infrared light sensors) is the low power scheme, in the microampere range. This low power scheme is fundamental for power-sensitive applications. The events described in this document are present in all capacitive sensing implementations. Due to the high sensitivity required for proximity detection, their impact seems amplified compared to touch sensors and therefore must be carefully considered. Each application has specific proximity detection requirements, and the design of the proximity sensing electrode is the result of these requirements.
The document outlines key factors influencing proximity sensitivity when designing systems with STM8TL5xxx devices, which feature a high-performance capacitive sensing engine. It details how parameters such as sensor settings (sampling capacitance, target reference, detection threshold), electrode design (spacing, length, width, shape), and environmental conditions affect detection distance and noise.
The note includes results from tests performed using specific hardware boards to evaluate the impact of various variables. Topics covered include the influence of ground coupling, front panel materials and thickness, temperature variations, humidity effects, and approach speed on the sensor's performance.
The sensors were connected to an STM8TL5xxx controller board identified as MCD10-016.
The electrode spacing for these specific test boards ranges from 1 mm up to 20 mm.
Increasing the distance between the ground backplane and the electrode increases the detection distance for FR4 material.