Capacitive Touch With PIC12F675

About: Like geckos.

This tutorial will show you how to make a capacitive touch sensor using the PIC12F675. Due to its limited memory space the software is made in assembler.

Step 1: The Circuit

Make the connections according to the schematic above. If you're interested in how it works, check out my other tutorial on capacitive touch using a PIC16F886 since it uses a similar method to measure capacitance. A different frequency crystal should not be a problem as long as it's in the MHz range.

Step 2: The Code

Here's the code that you can copy-paste and upload to the microcontroller. Keep in mind that it's in assembler and not C language. Once you plug in power, the pic will perform a calibration to know when you are vs are not touching the pad, therefore the touch pad should be avoided during this moment. The calibration is complete after only a couple of milliseconds. In case you use the internal oscillator, change the '_HS_OSC' on the third row to '_FOSC_INTRCIO' instead. Using a different frequency crystal should not be a problem (needs no change in the code) as long as it is in the MHz range. Using a faster/slower crystal might create noticeable differances when the pad is being touched continuously (as the led will blink).

INCLUDE <p12f675.inc>

;configuration registers

__CONFIG _HS_OSC & _WDT_OFF & _PWRTE_OFF & _MCLRE_ON & _BOREN_ON & _CP_OFF & _CPD_OFF

;define register addreses ;holds calibration for charge time CA0 equ h'21' CA1 equ h'22' ;shadow register to gpio SHADGPIO equ h'23' ;gets set by CHARGE_TIME section (count) COUNT0 equ h'24' COUNT1 equ h'25' ;delay registers DELAY0 equ h'26' DELAY1 equ h'27' DELAY2 equ h'30' ;holds average calbration sum CA0_SUM equ h'28' CA1_SUM equ h'29' CA2_SUM equ h'2a' ;sets how many samples calibrate takes CA_SAM equ h'2b' CA_SAM_INI equ h'2c' ;holds differance between count and ca DIF_CA0 equ h'2d' DIF_CA1 equ h'2e' ;differance flag register DIF_FLAG equ h'2f' RES_VECT CODE 0x0000 GOTO START

MAIN_PROG CODE

START ;setup-------------------------------------------------------------------------- ;move to bank 0 bcf STATUS, 5 clrf GPIO ;04 sets up comparator with cvref non inverting input movlw h'04' movwf CMCON ;move to bank 1 bsf STATUS, 5 ;clear ansel and trisio (all output pins) except for gpio1 clrf ANSEL movlw h'2' movwf TRISIO ;initialize timer0 bsf OPTION_REG, 3 bcf OPTION_REG, 5 ;initialize comparator cvref movlw b'10001111' movwf VRCON ;move to bank 0 bcf STATUS, 5 ;clear registers in use clrf SHADGPIO clrf CA0 clrf CA1 clrf COUNT0 clrf COUNT1 clrf DELAY0 clrf DELAY1 clrf DELAY2 clrf CA0_SUM clrf CA1_SUM clrf CA2_SUM clrf CA_SAM clrf CA_SAM_INI clrf DIF_CA0 clrf DIF_CA1 clrf DIF_FLAG ;goto main code goto MAIN ;------------------------------------------------------------------------------- ;update charge time subroutine-------------------------------------------------- CHARGE_TIME ;clear COUNT0 and COUNT1 clrf COUNT0 clrf COUNT1 ;set GP0 to low (discharge capacitance) bcf SHADGPIO, 0 movf SHADGPIO, 0 movwf GPIO ;delay to let capacitance discharge movlw h'ff' movwf DELAY0 movlw h'14' movwf DELAY1 ;loop a couple of milliseconds call DELAY_MS ;clear timer overflow flag bcf INTCON, 2 OVERFLOW ;set timer0 to 0 clrf TMR0 ;clear timer overflow flag bcf INTCON, 2 ;set gp0 to high (start charging capacitance) bsf SHADGPIO, 0 movf SHADGPIO, 0 movwf GPIO ;update count0 with tmr0 all the time its charging UPDATE_CHARGE_TIME movf TMR0, 0 movwf COUNT0 ;add 1 to count1 when overflowed and clear t0if btfsc INTCON, 2 call INCREMENT_COUNT1 ;check if comparator has gone low btfsc CMCON, 6 goto UPDATE_CHARGE_TIME return ;------------------------------------------------------------------------------- ;calibration subroutine--------------------------------------------------------- CALIBRATE ;set ca_sam to 5 calibration samples, cant be more than 255 movlw h'5' movwf CA_SAM movwf CA_SAM_INI clrf CA0_SUM clrf CA1_SUM clrf CA2_SUM clrf CA0 clrf CA1 SAMPLE ;update count registers call CHARGE_TIME ;add ca0_sum and count0, store in ca0_sum movf COUNT0, 0 addwf CA0_SUM, 1 ;check if a carry has occured btfsc STATUS, 0 call ADD_CA1_CA2 ;add count1 to ca1_sum movf COUNT1, 0 addwf CA1_SUM, 1 btfsc STATUS, 0 incf CA2_SUM, 1 ;check if ca_sam has decreased to 0 decfsz CA_SAM goto SAMPLE DIVIDE ;check if ca2_sum, ca1_sum is zero and ca0 - ca_sam_ini has a borrow ;check if ca2 is 0 movlw h'1' subwf CA2_SUM, 0 ;move to subtract directly if its not zero btfsc STATUS, 0 goto SUBTRACT ;otherwise check if ca1 is 0 movlw h'1' subwf CA1_SUM, 0 ;move to subtract directly if its not zero btfsc STATUS, 0 goto SUBTRACT ;otherwise check if ca0 is smaller than ca_sam_ini movf CA_SAM_INI, 0 subwf CA0_SUM, 0 ;if ca0 is smaller than ca_sam_ini skip to continue btfss STATUS, 0 return SUBTRACT ;divide ca_sum by ca_sam_ini movf CA_SAM_INI, 0 subwf CA0_SUM, 1 ;check if borrow has occured btfss STATUS, 0 call SUB_CA1_CA2_SUM ;add 1 to ca0 ca1 when subtraction went through movlw h'1' addwf CA0, 1 ;check if ca0 has a carry btfsc STATUS, 0 incf CA1, 1 goto DIVIDE return ;------------------------------------------------------------------------------- ;delay subroutine--------------------------------------------------------------- DELAY_MS LOOP decfsz DELAY0, 1 goto LOOP decfsz DELAY1, 1 goto LOOP return ;------------------------------------------------------------------------------- ;other instructions------------------------------------------------------------- INCREMENT_COUNT1 incf COUNT1, 1 bcf INTCON, 2 return ADD_CA1_CA2 incf CA1_SUM, 1 ;check if ca1_sum has overflowed btfsc STATUS, 2 incf CA2_SUM, 1 return SUB_CA1_CA2_SUM ;set w register to 1 movlw h'1' ;subtract ca1 with 1 (w register) subwf CA1_SUM, 1 ;check if ca1 has a borrow btfsc STATUS, 0 return ;if continued here ca1 has a borrow ;subtract ca2 with 1 aswell then subwf CA2_SUM, 1 return ABB ;count is bigger than ca bsf DIF_FLAG, 1 goto AFTER_SUB BBA ;ca is bigger than count bcf DIF_FLAG, 1 goto AFTER_SUB DEC_DIF_CA1 ;decrement DIF_CA1 movlw h'1' subwf DIF_CA1 ;check if borrow occured btfss STATUS, 0 goto ABB return TOGGLE_IO ;check if gp2 is off/on btfss GPIO, 2 goto SET_IO ;otherwise clear gp2 bcf SHADGPIO, 2 movf SHADGPIO, 0 movwf GPIO ;skip set_io goto WAIT SET_IO ;turn gp2 on bsf SHADGPIO, 2 movf SHADGPIO, 0 movwf GPIO WAIT ;set delay values movlw h'ff' movwf DELAY0 movlw h'ff' movwf DELAY1 movlw h'c' movwf DELAY2 WAIT2 ;delay around 1 sec at 10 MHz oscillator frequency decfsz DELAY0, 1 goto WAIT2 decfsz DELAY1, 1 goto WAIT2 decfsz DELAY2, 1 goto WAIT2 return ;------------------------------------------------------------------------------- ;main code---------------------------------------------------------------------- MAIN ;set up calibration values call CALIBRATE ;add a constant to the calibrated value movlw h'a' addwf CA0, 1 ;check if carry occured btfsc STATUS, 0 incf CA1, 1 UPDATE ;set count values call CHARGE_TIME ;subtract count1 from ca1 movf COUNT1, 0 subwf CA1, 0 ;check if COUNT1 is bigger than CA1 btfss STATUS, 0 goto ABB ;move result from subtraction to DIF_CA1 movwf DIF_CA1 ;subtract count0 from ca0 movf COUNT0, 0 subwf CA0, 0 ;set result to DIF_CA0 movwf DIF_CA0 ;check if borrow occured btfss STATUS, 0 call DEC_DIF_CA1 ;check if DIF_CA is 0 ;check dif_ca0 movf DIF_CA0, 1 btfss STATUS, 2 goto CON_BBA ;check dif_ca1 movf DIF_CA1, 1 btfss STATUS, 2 goto CON_BBA ;dif_ca is zero (count equals ca) bsf DIF_FLAG, 0 goto AFTER_SUB CON_BBA ;otherwise ca is bigger than count goto BBA AFTER_SUB ;check if count is bigger than ca, if so toggle io btfsc DIF_FLAG, 1 call TOGGLE_IO ;otherwise update count values goto UPDATE ;------------------------------------------------------------------------------- END

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