16F726 Capacitive sensing module

[Byte_Butcher]

OK, I was gonna sit on this for a few more days until I had a chance to perfect it a little more, but here it is now in it's crude form. I'm sure it needs lots of fixing to be "good" code, but it's functional as is and hopefully will give a starting place for folks who want to play with the capacitive sensing module on the 16F72X PIC's. Please feel free to critique it and fix it up. I need the education...

This program sets up Capacitive Sensing pin0 (CPS0/RB0) on the 16F726 / 16F727 to be a "Touch Sensor" pin
When the sensor pad is touched, the raw count (from TMR1) will drop below the running average and a LED on PORTA.1 will flash. If the finger is held on the pad long enough, eventually the running average will catch up with the current value and the LED will turn off.

Relevant Data Sheets for the CSM module include:

http://ww1.microchip.com/downloads/e...Doc/41341B.pdf 16F72X / 16LF72X Data Sheet.
http://ww1.microchip.com/downloads/e...tes/01101a.pdf AN1101 "Introduction to Capacitive Sensing "
http://ww1.microchip.com/downloads/e...tes/01103a.pdf AN1103 'Software Handling for Capacitive Sensing"
http://ww1.microchip.com/downloads/e...tes/01171B.pdf AN1171 "Using the Capacitive Sensing Module on the PIC16F72X"

Crude Code follows.... Enjoy!

Steve

------------

'-------------------------------------------------------------------------------
'Trying to learn to use the Capacitive Sensing Module on a 16F727 --------------
'-------------------------------------------------------------------------------

@  __config _CONFIG1, _DEBUG_OFF & _PLL_EN & _BORV_2_5 & _BOR_ON & _CP_OFF & _MCLRE_OFF & _PWRT_EN & _WDT_OFF & _INTOSCIO
@  __config _CONFIG2, _VCAP_RA0

Include "MODEDEFS.BAS"    ' Include Shiftin/out modes
INCLUDE "DT_INTS-14.bas"  ' Base Interrupt System
INCLUDE "ReEnterPBP.bas"  ' Include if using PBP interrupts

DEFINE LCD_DREG PORTA    ' Set LCD Data port
DEFINE LCD_DBIT 4        ' Set starting Data bit (0 or 4) if 4-bit bus
DEFINE LCD_RSREG PORTA   ' Set LCD Register Select port
DEFINE LCD_RSBIT 2       ' Set LCD Register Select bit
DEFINE LCD_EREG PORTC    ' Set LCD Enable port                                                   
DEFINE LCD_EBIT 0        ' Set LCD Enable bit
DEFINE LCD_BITS 4        ' Set LCD bus size (4 or 8 bits)
DEFINE ADC_BITS 8        ' Set number of bits in ADC result
DEFINE ADC_CLOCK 3       ' Set clock source for ADC (rc = 3)
DEFINE ADC_SAMPLEUS 100  ' Set ADC sampling time in microseconds 
DEFINE OSC 8

OSCCON = $10         'set oscillator to 8 Mhz

TRISA= %00000000     'Set 'em all to outputs
ANSELA= %00000000	 'Set 'em all to digital
TRISB= %11111111   	 'all input
ANSELB= %11111111    'all analog
TRISC= %00000000     'Set portC all outputs   

CPSCON0 = %10001101  'Cap sense on, high range oscillator
CPSCON1 = %00000000  'Cap sense channel input 0 is selected

'-----Alias Pins
RW           var    PORTC.3 : LOW rw   'Read/Write for LCD
LED          var    PORTA.1      'LED to display when button is pressed

'-----Allocate Variables
timercount   var    word  ' raw count from TMR1
timerave     var    word  ' long term average of timercount
AvgCount     var    word : AvgCount = 32 'number of samples to average 

'-----Set up Interrrupts
ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler   TMR1GATE_INT,  _CheckCount,   PBP,  yes
     endm    
    INT_CREATE               ; Creates the interrupt processor
ENDASM

@ INT_ENABLE  TMR1GATE_INT     ; enable Timer 2 interrupts

High LED
Pause 200       'Let the LCd wake up
lcdout $fe, 1   'Clear LCD
lcdout $fe, 2, " Awake! "
pause 500

'----- Timer Setup
T2CON = %01110110 'bit7=unimplemented, bit6-3=postscaler, bit2=TMRON, bit1-0=prescaler 
PR2 = %11111111       'give PR2 a number for TMR2 to match
PIR1.1 = 0      'Clear the TMR2 interupt flag
PIE1.1 = 1      'Turn TMR2 interrupt enable on
T1CON = %11000101 'Timer clock source=CAPOSC, prescale=1:1, dedicated OSC disabled, no external clock synchronize, timer on
T1GCON = %11100010 'Timer1 gate init/ Toggle Mode
PIR1.7 = 0   'Clear Gate Interrupt Flag
PIR1.1 = 0   'clear the TMR2 interupt flag

Goto main   'Just get it going...

'---[TMR2 - interrupt handler]--------------------------------------------------
    disable   
CheckCount:
    T2CON.2 = 0   'stop timer2
    T1CON.0 = 0   'stop timer1
    timercount = TMR1L + TMR1H << 8   'stuff the contents of the timer register into a word
    Gosub Fingercheck
    
    TMR1L = 0     'reset counter to 0...
    TMR1H = 0     'upper 1/2 too
    TMR2 = 0       'rest timer 2
    PIR1.1 = 0   'Clear TMR2 Interrupt Flag
    T1CON.0 = 1  'restart timer1
    T2CON.2 = 1  'restart timer2
@ INT_RETURN
    enable 
    
'----Average the timercount result----------------------------------------------

Fingercheck:
    timerave = timerave - (timerave/AvgCount)    'average the count so you have a reference to compare....
    timerave = timerave + (timercount/AvgCount)  '...with yimercount
        
    If timerave - (timerave /40) > timercount then  'If timercount drops more than 2.5% below the average, signal that a button is pressed.
        low LED      'make the LED do a little dance.
        pause 50
        High LED
        pause 25
    endif
    
    If timercount > timerave +50 then    'do a "fast recovery" on the average when finger is removed.
       timerave = timercount
    endif
    return     
    
'----Main loop-----------------------------------------------------------
Main:
    LCDOUT $fe,2, " Count= ", dec timercount,"    ",$fe,$C0,"Average = ", dec timerave,"    "
    pause 50  
GOTO Main

    end

[gmglickman]

Anybody else working with capacitive sensing? Any updates?

[rmteo]

I have used capacitive sensing in a handheld terminal with a graphic LCD. It scans 24 keys (although it will handle up to 32 keys) and does require a special MCU - I'm using a PIC18F44K20. Does auto-repeat, audible feedback, etc.

[Bruce]

Nice looking project rmteo. Do you have a PBP example for it?

[rmteo]

The project was done in SF BASIC for a client so I cannot go into too much detail. As for the capacitive touch part, the keyboard is implemented as PCB copper pads on the top side (see picture). Each of the 24 pads is routed through a pair of 16-to-1 multiplexers (74HCT4067) to act as the timing capacitor to a TS555 (CMOS version of 555) timer running at about 2MHz in astable mode.

A timer in the PIC is set up as a counter. It counts the number of pulses in a fixed time frame (I use 2 ms). When a pad is touched, its capacitance increases thereby lowering the frequency (lesser pulse count) of the TS555. You will typically see 10-15% change when a key is touched. Because the capacitance can (and will) drift due to temperature (and the fact that each key will have a slightly different capacitance due to trace length, etc.), you will need to employ some algorithms to ensure accurate readings. There is an app note on the MC website that describes this.

The nice thing about this method is that you can use any PIC - it does not need to have a capacitive sense module to work and uses fewer I/O pins. I implemented a similar scheme using the ARM Cortex M0/M3 but the same principle can be applied to work with just about any microcontroller.

[Bruce]

Good looking unit you made there. Nice work. I guess i'm going to have to start playing a bit with cap sensing on the 16F1827 here shortly...

[rsocor01]

rmteo,

Nice project. I have a project using CSM (capacitive sensing module) on a 16F727. In that project I'm using only two buttons. Yes, you can do it with any PIC but you will need some extra external components.

How did you do the external plastic cover for the buttons? Was it custom made by some company?

Robert