Capacitive Touch With PIC12F675

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.

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.

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