TIDUE90 July   2018

 

  1.    Description
  2.    Resources
  3.    Features
  4.    Applications
  5.    Design Images
  6. 1System Description
    1. 1.1 Key System Specifications
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Classification of Scenarios With Liquid Present
      2. 2.2.2 Liquid Influence on Capacitive Touch Sensing
      3. 2.2.3 Self Capacitance and Mutual Capacitance
        1. 2.2.3.1 Self Capacitance
        2. 2.2.3.2 Mutual Capacitance
      4. 2.2.4 Other Considerations
    3. 2.3 Highlighted Products
      1. 2.3.1 MSP430FR2633
    4. 2.4 System Design Theory
      1. 2.4.1 Shield Sensor Electrodes
      2. 2.4.2 Mutual Capacitance Shielding
      3. 2.4.3 Design for Noise Immunity
      4. 2.4.4 Power Supply Grounding Effect
  8. 3Hardware, Software, Test Requirements, and Test Results
    1. 3.1 Required Hardware and Software
      1. 3.1.1 Hardware
      2. 3.1.2 Software
    2. 3.2 Test and Results
      1. 3.2.1 Liquid Test With Well Grounded Power Supply
        1. 3.2.1.1 Continuous Water Flow Test
        2. 3.2.1.2 Continuous Water Spray Test
      2. 3.2.2 Conductive Noise Immunity Test
      3. 3.2.3 Liquid Test With Battery-Powered Supply
        1. 3.2.3.1 Continuous Water Flow Test
        2. 3.2.3.2 Continuous Water Spray Test
      4. 3.2.4 Third Party Test Report
  9. 4Design Files
    1. 4.1 Schematics
    2. 4.2 Bill of Materials
    3. 4.3 PCB Layout Recommendations
      1. 4.3.1 Layout Prints
    4. 4.4 Altium Project
    5. 4.5 Gerber Files
    6. 4.6 Assembly Drawings
  10. 5Software Files
  11. 6Related Documentation
    1. 6.1 Trademarks
  12. 7About the Author

3.2.3.1 Continuous Water Flow Test

Figure 26 shows the sensor measurement count results for this test. The Y axis represents the sensor measurement count and the X axis represents number of samples. Each color represents the data for a specific button. The data shows that the sensors are calibrated at baseline count of 350, and the touch increases the count to approximately 450 in dry conditions. The increased delta is smaller than the well grounded power supply scenario.

When the flowing water is applied on the touch sensor area, the data shows the "negative touch" behavior (see Section 2.2.3.2). The flowing water causes the measurement result to go in the opposite direction of a touch event, so no false touch is detected. While the water is continuously flowing on the touch sensor area, each touch event still causes a count increase, and the touched button is still distinguishable. However, the amplitude of the count increase is significantly lower compare to well grounded power supply scenario. As the data shows, it is challenge to set the touch threshold for the system to work reliably under both dry conditions and flowing water conditions. If the threshold tracks with the "negative touch" behavior, there could be a false detection after the flowing water is removed, because the data shows that the count returns to the baseline when the flowing water stops.

This data is only intended to demonstrate the sensitivity reduction, and Section 2.4.4 describes methods to design a system with stronger coupling to earth ground, which improves the sensitivity in flowing water conditions.

Section 2.4.4 explains the sensitivity reduction in battery powered system for both dry conditions and flowing water conditions. More water on the touch sensor area causes more significant sensitivity reduction. Section 3.2.3.2 describes the spray water test, which has less water applied to the touch surface, compared to the flowing water test.

TIDM-1021 D013_TIDUE90.gifFigure 26. Sensor Count Result With Continuous Water Flow (Battery Power)