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

1.1 Key System Specifications

Table 1. Key System Specifications

PARAMETER SPECIFICATIONS
Buttons 12 square buttons, 10 mm x 10 mm each
Measurement mode Mutual capacitance mode
Scan rate 25 Hz
Input voltage 3.3 VDC
Average current consumption(1) 3.63 mW (1.1 mA)
Serial interface 250-kbps UART
MSP430FR2633 FRAM code footprint 8314 bytes (54%)
Printed circuit board (PCB) size (W x L) 63 mm x 109 mm
PCB thickness 1.6-mm 2-layer PCB
Noise immunity test IEC 61000-4-6 conducted noise immunity (CNI) with 3-Vrms noise voltage
(1) This reference design is optimized for noise immunity and liquid tolerance but is not optimized for low power consumption. For more information on low-power optimization, refer to the CapTIvate Technology Guide.

Although the hardware and firmware design can be different depending on specific design requirements, this reference design is optimized to use mutual capacitance topology for 12 buttons with a well grounded power supply. The design works reliably when exposed to a continuous stream of water and operates under these conditions without false detections and with accurate detection of a touched button.