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

3.2.1.1 Continuous Water Flow Test

Test Setup

Figure 16 shows a tank filled with water and a 12-V pump to provide the continuous water flow for this test. The TIDM-1021 PCB is attached on the back of a 3-mm-thick plastic overlay with 467MP double-sided adhesive. The overlay is mounted on a PCB holder to hold the test unit on a support frame. This setup allows a continuous water flow directly on the touch sensor area. Figure 17 shows a photograph of the test setup. The water used in all the liquid test cases is tap water from the local piped supply.

TIDM-1021 tida-1021-liquid-testing-setup-drawing-block-diagram.gifFigure 16. Liquid Test Setup
TIDM-1021 tidm-1021-continuously-flow-water-testing-photograph.pngFigure 17. Continuous Water Flow Test Setup

The TIDM-1021 PCB is connected to the CapTIvate communications isolation board (CAPTIVATE-ISO) and the MSP430 CapTIvate MCU programmer board (CAPTIVATE-PGMR) (see Figure 18). This setup collects the data with a laptop through the USB cable and provides the 3.3-V power to the system using an external DC power supply.

TIDM-1021 tidm-1021-testing-hardware-setup-photograph.gifFigure 18. Test Hardware Setup

Test Procedures

  1. A user touches each button from Button 1 to Button B.
  2. Turn on the water pump with continuous water flow.
  3. A user touches each button from Button 1 to Button B.
  4. Turn off the water pump.

Test Result

Figure 19 shows the sensor measurement count result for this test. The Y axis represents the sensor measurement count, and the X axis represents the number of samples. Each color represents the data for a specific button. The data shows that the sensor is calibrated at a baseline count of 350, and the touch increases the count to approximately 550 in dry conditions. 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 can still cause a distinguishable count increase for the touched button. Based on this test result and by using the software algorithm in Section 3.1.2, the system can reliably operate when exposed to continuous water flow, with no false touch detections and with accurate detection of a button touch.

TIDM-1021 D003_TIDUE90.gifFigure 19. Sensor Count Result With Continuous Water Flow (Well Grounded Power Supply)