AN4313
Application note
Guidelines for designing touch sensing applications
with projected sensors
Introduction
This application note describes the layout and mechanical design guidelines used for touch
sensing applications with projected sensors.
This document describes only what is specifically related to projected sensors, designed
using STM8L52/L53 microcontrollers. To have general tips and tricks for designing touch
application, please refer to AN4312.
Table 1
lists the microcontrollers concerned by this application note.
Table 1. Applicable products
Type
Microcontrollers
STM8TL52/L53 lines
Applicable products
November 2013
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Contents
AN4313
Contents
1
2
Technology Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Projected sensor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1
Touchkey sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1
2.1.2
2.1.3
Diamond type sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
H sensor single layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
H type sensor two layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2
2.3
2.4
Linear sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Rotary sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Specific recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4.1
PCB and layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3
4
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
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List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Electric field between 2 surface electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Diamond implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
H sensor (single layer) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Two-layer implementation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Normal linear sensor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Small linear sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Rotary sensor made of 5 parcels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Ground floods around Tx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Potential false key detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
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Technology Overview
AN4313
1
Technology Overview
STMicroelectronics offers capacitive sensing technology. This technology is based on:
The projected acquisition principle with STM8TL5x microcontrollers.
An overview of how it works: a capacitor is modified when the finger gets close to a sensor.
The finger changes the dielectric properties.
The sensor consists of two electrodes:
Tx driven by a port in output mode,
Rx in the return path to a dedicated port in read mode.
There is a sampling capacitor which stores the charges coming from the electrodes which
form a coupling capacitor with less capacitance than the sample one. When a finger
approaches, the dielectric (between the two electrodes) is modified and so the capacitance
decreases. As a consequence, the time taken to load the sample capacitor will increase and
this difference is used to detect if a finger is present or not.
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Projected sensor design
2
2.1
Projected sensor design
Touchkey sensor
The touchkey sensor can be of any shape, however it is recommended to use a square as
this shape is the simplest. The touch sensing library and ProxSense
TM
IP automatically
compensate for capacitance differences, but the acquisition time and processing
parameters can be optimized if the sensors have a similar capacitance. For this reason, it is
recommended to use the same shape for all sensors.
The physical principlefor a touchkey, linear or rotary sensor is that an electric field surrounds
the Tx and Rx electrodes (see
Figure 1).
This field is dependent on the permittivity
R
of
both the front panel and the PCB. It should not be dependent on air bubbles or moisture
which can be trapped between them, because they must be sufficiently well bonded by the
adhesive.
Figure 1. Electric field between 2 surface electrodes
Electric field
Electric field
Dielectric panel
Electrodes
Electrodes
MS18983V1
1. The above figure only shows a simplified representation of the sensor; for specification details, refer to
Figure 2
to
Figure 4.
Hence, the sensitivity is dependent on known materials and is optimized. This will ensure
that the disturbance caused by the user’s finger is detected and measured with accuracy.
It is recommended to define the sensor size in relation to the panel thickness (d) and vice-
versa. The following formula helps the designer to define the external size of touchkey
sensors (L), and also for linear touch sensor and rotary touch sensor parcels:
L (mm) = 3 + (1.75 * d)
For example, a typical 10*10 mm touchkey should be used with a 4 mm Plexiglas panel.
2.1.1
Diamond type sensor
In this “diamond type” implementation, the Tx and Rx electrodes form a diamond shape
where the two electrodes are on the same layer, using a bridge (see
Figure 2).
Note:
This symmetrical implementation allows swapping Rx & Tx.
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