Capacitive Sense - DIY Realforce 106s controller

This scans capacitive switch matrix using Atmel QMatrix method.

TODO

• Calibration
• Threshold
• Use sense value for analog input
• Test: voltage change, temperature change
• Low voltage power supply
• look into Topre capsense chip

Realforce 106s Matrix

My controller scans matrix with 16 drive lines and 8 sense lines. Sense lines are multiplexed by ADC MUX of AT90USB1286.

Row: Burst(X)*16 Col: Sense(Y)*8 0 1 2 3 4 5 6 7 R14 R15 R16 R17 R18 R19 R20 R21 0 R22 58 44 30 16 1 2 90A 110 1 R23 60 46 31 17 3 4 112 113 2 R24 47 33 32 18 19 5 - 114 3 R25 49 48 34 20 21 6 131 115 4 R26 50 35 36 22 7 0B 0A - 5 R27 61 51 37 23 8 9 117 116 6 R28 52 38 24 25 10 - 132 118 7 R29 54 53 39 26 11 - 133 119

8 R33 62 55 40 27 12 - 120 121 9 R34 57 56 41 28 13 - 122 123 A R35 64 42 43 - 15 14 90B - B R36 79 83 81 76 75 - - - C R37 84 89 - 86 85 80 - - D R38 99 93 92 91 90 - - - E R39 104 98 97 96 100 95 125 124 F R40 108 103 102 106 101 105 - 126

Meanwhile original Topre controller scans with 8 drive lines and 16 sense lines. Sense lines are multiplexed by two 74HC4051 analog multiplexers.

Row: Burst(X)*8 Col: Sense(Y)*16 R22 R23 R24 R25 R26 R27 R28 R29 R33 R34 R35 R36 R37 R38 R39 R40 R14 58 60 47 49 50 61 52 54 62 57 64 79 84 99 104 108 R15 44 46 33 48 35 51 38 53 55 56 42 83 89 93 98 103 R16 30 31 32 34 36 37 24 39 40 41 43 81 - 92 97 102 R17 16 17 18 20 22 23 25 26 27 28 - 76 86 91 96 106 R18 1 3 19 21 7 8 10 11 12 13 15 75 85 90 100 101 R19 2 4 5 6 0B 9 - - - - 14 - 80 - 95 105 R20 90A 112 - 131 0A 117 132 133 120 122 90B - - - 125 - R21 110 113 114 115 - 116 118 119 121 123 - - - - 124 126

Where Rxx means marking of resiter on PCB.

Memo

• Other drive lines than bursted one should be grounded to remove influence of capacitances not under test.
• Drive lines with resistor in series avoid divergence of sensing values. R=1KOhm
• Measured value of sense lines(Y) are electromagnetically infuenced depending on physical distance from drive lines(X)
  I had to place sensing compoenets(Csample and Rs) apart from drive lines(PC0-7) to remove EMS influence.
• Vaule changes
  • at startup: calculate threshold value of each node
    Boot magic - keys can be holded while keyboard is starting up
    - how holded keys are determined?
    - use average of all nodes as threashold
    - use last threashold valued stored in eeprom
    - error when many keys are stacked
  • long time change: >10s? tune threshold values (runtime recalibration)
  • short time change: recognized as human operation
  • key press - change in short time
    • threashold
      average(from long time data except for values while pressed)
    • hysteresis - threshold is changed when key is pressed
  • cross-talk on both adjacent X and Y line
    • cross-talk between adjacent X lines
      • removed by pulling other X lines down to LOW while the burst
      • HIZ causes cross-talk severely between adjacent X lines
    • cross-talk between adjacent Y lines
      • reduced much by sensing one of even/odd Y line group(0,2,4,6/1,3,5,7) at a time
      • both Ya and Yb are LOW when other line is measured
• Slow Scan
  • burst: 64 cycles, 6us per cycle = 380us/line (3us/200us with inline optimisation)
  • 16ms for 16X*8Y, 2 burst/sense cycles per line 09/21
  • if keys are pressed sense time becomes longer hence scan takes longer
  • how improve: fast burst pulse, short burst length, small Rsmp

Resources

Burst length number of charge pulses in a burst measurement Increasing BL increases signal resolution and sensitivity. Sampling Resistor discharges Sampling Capacitor(Cs) and controls discharge slope. Increasing Rsmp increases sensitivity. X-Y X transmitter, burst, drive line Y receiver, sense line YA port: charge transfer control(top) YB port: ADC mux input(bottom) self/mutual capacitance sense self mutual ATMEL QTouch Library supports GCC tool chain works with AVR with 2K flash(QTouch only) QTouch self a button per two pins QMatrix mutual http://www.atmel.com/products/TouchSolutions/bsw/qmatrix.aspx QTouchADC QTouchADC only requires 1-series resistor and 1-pin per channel. No other external components are required. QTouchADC offers faster acquisition times with shorter burst lengths, resulting in lower power consumption. Also can be used promixity sensing. http://www.atmel.com/products/TouchSolutions/bsw/qtouchadc.aspx Charge transfer QTouch Composer extension for Atmel Studio 6

3.3 Tracking Base Capacitance Value http://www.ti.com/lit/an/slaa379/slaa379.pdf TI explains a patent expired method here. (not mutual sensing)

Calibrating compensate capacitance drift http://www.embedded.com/design/prototyping-and-development/4008781/Getting-in-touch-with-capacitance-sensor-algorithms

Voltage of Cs http://forum.atmel.com/showthread.php?t=2023 The terminal voltage that appears across the Cs capacitor has to be in the range of -50mV to -150mV. The terminal voltage should not exceed -250mV due to diode i/o conduction.

QTouch and QMatrix Sesitivity Tuning: http://www.atmel.com/images/qtan0062.pdf X-Y electrode design(capacitance) Bust length Sampling Capacitor Cs = 4.7-10nF Sampling Resistor Rsmp = 470K(220K-1M)Ohm Sensor Design Guide http://www.atmel.com/images/doc10752.pdf Promixity sensing http://www.atmel.com/Images/doc10760.pdf QTouch 180mm QTouchADC 250mm QMatrix 50mm

mTouch Theory http://ww1.microchip.com/downloads/en/DeviceDoc/93064A.pdf Capacitive sensing library for Arduino http://playground.arduino.cc/Main/CapacitiveSensor?from=Main.CapSense

How Oscilloscope Probes Affect Your Measurement Input capacitance of Probes really affect circuit of capacitive sensor. http://www.tek.com/dl/51W_30013_0_MR_Letter_0.pdf

Analog Devices CapTouch http://www.analog.com/media/en/technical-documentation/data-sheets/AD7147.pdf

IBM patent: Keyboard multiple switch assembly http://www.google.co.ug/patents/US4274752 http://deskthority.net/f-o-r-s-a-l-e-f58/xwhatsit-s-grand-unified-ibm-capsense-usb-controller-thread-t7993-120.html#p165846

QMatrix API Flow - QTouch Library

qm_burst_length[]:  retains burst length for all channels
measurement_period_ms

touch_qm_sensor_init()
touch_qm_sensor_config()
touch_qm_sensor_calibrate()
loop:
    touch_qm_sensor_dispatcher():   dispatches measure_complete_callback()
    touch_qm_sensor_start_aquisition()
    ... host app ...