Introduction to Capacitive Sensing Part2

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Introduction of the CSA and CSD Capacitive Sensing

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Introduction to Capacitive Sensing Part2

  1. 1. Introduction to Capacitive Sensing <ul><li>Source: Cypress </li></ul><ul><ul><li>Part2: CSA And CSD CapSense </li></ul></ul>
  2. 2. Module Introduction <ul><li>Purpose </li></ul><ul><ul><li>This module introduce the CSA and CSD Capacitive Sensing. </li></ul></ul><ul><li>Outline </li></ul><ul><ul><li>Introduction to CSA Capacitive Sensing. </li></ul></ul><ul><ul><li>Introduction to CSD Capacitive Sensing. </li></ul></ul><ul><li>Contents </li></ul><ul><ul><li>32 pages </li></ul></ul><ul><li>Duration </li></ul><ul><ul><li>15 Minutes </li></ul></ul>
  3. 3. Sidebar: Switched Capacitors <ul><li>Method for developing analog components with basic CMOS technologies </li></ul>
  4. 4. Switched Capacitors (cont.)
  5. 5. Switched Capacitors (cont.)
  6. 6. Switched Capacitors (cont.)
  7. 7. Switched Capacitors (cont.)
  8. 8. Switched Capacitors (cont.)
  9. 9. Switched Capacitors Example
  10. 10. Making Sense of It <ul><li>Detect the difference between the presence or absence of conductive element: </li></ul><ul><ul><li>Continuously read the sensor capacitance </li></ul></ul><ul><ul><li>Digitize the readings </li></ul></ul><ul><ul><li>Cancel out the noise </li></ul></ul><ul><ul><li>Compare to a threshold </li></ul></ul><ul><ul><li>Make a decision based on the value </li></ul></ul>
  11. 11. CapSense Sensing Methods <ul><li>CapSense Successive Approximation (CSA) </li></ul><ul><li>CapSense Sigma-Delta (CSD) </li></ul>
  12. 12. CSA Sensing Method
  13. 13. CSA Sensing Method
  14. 14. CSA Sensing Method
  15. 15. CSA Waveform
  16. 16. Equivalent Resistance Replace the sensor capacitor (C X ) with its equivalent resistance
  17. 17. Region 1 <ul><li>Find an iDAC current value where voltage across R CX (V Start ) is just below V REF </li></ul><ul><li>Successive approximation used to find golden iDAC current </li></ul><ul><li>Done once for each sensor in the CSA_Start() API </li></ul>
  18. 18. Count long it takes for the voltage return to V REF Region 2
  19. 19. <ul><li>Update the baseline </li></ul><ul><li>Compare measurement to the baseline </li></ul><ul><li>Determine button status / centroid location </li></ul>Region 3 Process measured values in firmware
  20. 20. With Finger Present
  21. 21. Change in Capacitance
  22. 22. CSA Summary <ul><li>Increase in C x reduces R Cx , lowering the starting voltage, V start </li></ul><ul><li>Lower starting voltage causes longer charge time </li></ul><ul><li>Large charge time enables the counter for a longer time </li></ul><ul><li>Due to the counter’s longer enable time, higher raw counts are measured </li></ul><ul><li>Comparator triggers when V start is equal to V REF , stopping counter </li></ul><ul><li>The raw counts are processed by firmware </li></ul><ul><ul><li>Update the baseline </li></ul></ul><ul><ul><li>Compare measurement to the base line </li></ul></ul><ul><ul><li>Determine button status / centroid location </li></ul></ul>
  23. 23. CSD Sensing Method
  24. 24. CSD Sensing Method
  25. 25. CSD Sensing Method
  26. 26. CSD Sensing Method
  27. 27. Equivalent Resistance
  28. 28. Sigma-Delta Modulator
  29. 29. Sigma-Delta Modulator
  30. 30. Bit Stream Filter
  31. 31. CSD Method
  32. 32. CSD Summary <ul><li>Increase in C x reduces R Cx , allowing more current to charge C Mod </li></ul><ul><li>More current causes C Mod to charge faster </li></ul><ul><li>Faster charge time increases the duty cycle </li></ul><ul><li>Larger duty cycle enables the counter for a longer time </li></ul><ul><li>Due to the counter’s longer enable time, higher raw counts are measured </li></ul><ul><li>The raw counts are processed by firmware </li></ul><ul><ul><li>Update the baseline </li></ul></ul><ul><ul><li>Compare measurement to the baseline </li></ul></ul><ul><ul><li>Determine button status / centroid location </li></ul></ul>
  33. 33. Additional Resource <ul><li>For ordering PSoC, please call our sales hotline </li></ul><ul><li>For additional inquires contact our technical service hotline </li></ul><ul><li>For more product information go to </li></ul><ul><li>http:// www.cypress.com/capsense / </li></ul>

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