Driven Shield for Touch

Driven Shield is an essential feature to ensure robust moisture tolerance. Based on the technology used, Driven Shield can be classified into the following types:

• Passive Shield
• Hardware Shield
  • 3-Level Shield
  • 2-Level Shield
  • PIC^® microcontroller - 2-Level Shield
• Timer-based Driven Shield
  • 2-Level Shield

The How to Enable Driven Shield page is a step-by-step guide on how to enable Driven Shield in MPLAB^® Code Configurator (MCC) Harmony and Melody.

Passive Shield

Passive Shield is when the shielding electrode is connected to a static DC level (normally ground). More information on the Passive Shield can be found in the Capacitive Touch Sensor Design Guide application note.

Hardware Shield

When a device includes hardware shield support, it uses a dedicated pin to implement Driven Shield. Here, the hardware shield indicates that the shield electrode is driven with the same signal that the sensor produces. Based on the type of sensor signals, hardware shield is classified as follows.

3-Level Driven Shield

Three-level Shield is driven through a sequence of voltages matching the electrode potential at each stage in measurement. For example, a PIC32CMJH CPRO board is used and the following pins have been enabled:

The following image illustrates the waveforms of the shield signal in sequence with the sensor signal. 

2-Level Driven Shield

Two-level Driven Shield drives a signal between the shield and the sensor during self‑capacitance charge sharing.

For example, the PIC32CM PL10 device has been used to demonstrate this. The pins used are:

Sensor signal waveforms of the shield signal in sequence with the sensor signal look like the accompanying image. 

PIC® 2-Level Driven Shield

All Charge Voltage Discharge (CVD) devices include 2-Level Driven Shield. The shield pins are not touch pins as present in the above subdivision. The shield pins are those which have Analog-to-Digital Ground Reference (ADGRD) functionality mapped to them. 

For example, the PIC18F56Q71 CNANO board is used and the following pins have been enabled:

The following image shows the waveforms of the shield signal in sequence with the sensor signal.

Timer-Based Driven Shield

2-Level Driven Shield

The Peripheral Touch Controller (PTC) on certain devices does not support the Driven Shield feature. However, it is possible to implement a driven shield on those devices using Pulse Width Modulation (PWM) output from timers. More information on how to use timers to set up a Driven Shield and configure such a project using MCC Harmony/Melody can be found in Timer Based Driven Shield. 

Driven Shield Plus

Some devices are capable of driving the shield signal – two or three-level – not only to a dedicated shield electrode but also to other touch sensor electrodes on the user interface. This is used even when there is no dedicated shield pin available in the devices. More details can be found in Capacitive Touch Sensor Design Guide

Consider a PIC32CMJH device as an example, with a three-button project and a dedicated shield pin, along with Driven Shield Plus enabled.

When Button 2 and Button 3 are probed, all buttons can be observed being driven. 

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