Capacitive Touch Button by using ADC channel (the CVD system)

[mister_e]

USB device are often simple... could be anything... at very least Lots of tranceiver possibilities... IR/SPI/I2C/DMX/MIDI/Serial/RF...DAC/ADC? ICSP? ICD? name it

Sorry guys to be OT

[Ioannis]

I was thinking a Capacitive entry something with USB. On a 8-pin would be really small...

Ioannis

[Max Power]

Hank I'd love to see how you set it up. Thanks

[HankMcSpank]

Hank I'd love to see how you set it up. Thanks

Alrighty, a couple of salient points about capacitive touch.

1. You need an oscillator.

2, You need something to count the 'pulses' from that oscillator

3. You need something set as an accurate time 'window' to count the pulses within

4. You need something to notice when the oscillator 'count' has diverged from the 'normal' count.

For step 1, the series of chips I mention (12f1822, 16f1824 etc), have a built in capacitive touch module, which makes it very easy (vs say the 16f690)

To set it up....

CPSCON0 = %10001100 'set the CPS module highest frequency for vcc mode + timer0clock sourced from CPS module. (BIT 1)
CPSCON1 = %00000000 'select CPS Ch0 - which is pin 7 on a 12lf1822  (or whichever pin you care to use)

...simple as eh? (the cps oscillator is now running) whatever CPS pin you choose to use needs to be an input & analogue, so for Ch0 on a 12lf1822 it's RA0....

TRISA.0 = 1
ANSEL.0 = 1

ok, next step 2 ....you need something to count the pulses (a timer), in the above snippets I set the CPS module oscillator output pulses to feed into Timer0.

(note: the prescaler setting will depend on your sensor &* enviromentals ...best to dabble & see what oscillator 'count's you're getting between' successive interrupts

OPTION_REG = %10000111 'use timer0 as interrupt (timebase) TMR0 Prescaler on (bit3) 256:1 prescale value (bits 2-0) 
T1CON = %11000001 'enable timer 1 (bit 0) & source the clock from the CPS module (bit 6 & 7 =11

next step 3, we need an accurate set time window ...this is where DT's fab interrupt's come in....

ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler   TMR0_INT,  _Timer0_Int,   pbp,  yes
    endm
    INT_CREATE               ; Creates the interrupt processor
ENDASM
 
TMR0 = 0                 'clear down TIMER0
@ INT_ENABLE  TMR0_INT  'enable timer0 interrupts

We're almost there- last step! In the interrupt routine, you basically keep a check of the count between 'interrupt's, when the count drops by say 20% (the count goes down when you touch your sensor), then that's a switch press.

(note: all the stuff below are my variables...call 'em what you like!)

Timer0_Int:
@ INT_DISABLE  TMR0_INT   ' stop timer0 interrupts while we're in here
CPS0_PRESENTCOUNT = TMR1     ' take a snapshot of Timer0's present count.
CPS0_THRESHOLD = CPS0_LASTCOUNT - ((CPS0_LASTCOUNT/100)*2)   ' this sets the 'trigger' up for a 20% diversion (finger press)
CPS0_LASTCOUNT = CPS0_PRESENTCOUNT ' store away the present timer0count for the next time we come into the interrupt routine
if CPS0_PRESENTCOUNT <  CPS0_THRESHOLD then 'if the present incoming timer0 count is 20% below the last count, then a finger has been placed on the sensor - go do 'stuff'
blah blah   
 
TMR0 = 0 ' clear timer0 down
@ INT_ENABLE  TMR0_INT  ' re-enable interrupt
@ INT_RETURN

(sorry about the formating - looks ok before I post it....horrible after I do!

[HankMcSpank]

I wish the 'edit' period was open for a little longer here (hint!), I've just noticed an error in that last part (cut/pastes are a hack from a much larger program)

Timer0_Int:
@ INT_DISABLE  TMR0_INT   ' stop timer0 interrupts while we're in here
CPS0_PRESENTCOUNT = TMR0     ' take a snapshot of Timer0's present count.
CPS0_THRESHOLD = CPS0_LASTCOUNT - ((CPS0_LASTCOUNT/100)*2)   ' this sets the 'trigger' up for a 20% diversion (finger press)
CPS0_LASTCOUNT = CPS0_PRESENTCOUNT ' store away the present timer0count for the next time we come into the interrupt routine
if CPS0_PRESENTCOUNT <  CPS0_THRESHOLD then 'if the present incoming timer0 count is 20% below the last count, then a finger is on the sensor,go do 'stuff'
'your stuff goes here'
endif
 
 
TMR0 = 0 ' clear timer0 down
@ INT_ENABLE  TMR0_INT  ' re-enable interrupt
@ INT_RETURN

One thing to point out, depending on how reactive you want the 'sensor touch' to be trapped, you may need quite a high interrupt rate, which can bring the PIC to it's knees using the interrupt method above - not a problem if your PIC isn't doing much else, but really hampers things if it's a busy little thing!

Edit: Damn my speed typing, re this entry...

T1CON = %11000001 'enable timer 1 (bit 0) & source the clock from the CPS module (bit 6 & 7 =11

remove the above entry in my explanation post above - it relates to timer1! (which I was using too, but not needed in my example above, since I was using timer0)

[ivanrosales]

Returning to the mister_e's idea of the USB 8-pin chip, I wish to see that too!
You can do a lot even with NO extra I/O pins: http://www.thinkgeek.com/interests/retro/c208/

[Max Power]

Hank, awesome. you -seriously-made my day. I'm picking up the 12F1822, and 16F1823s this afternooon, and I should be up and running in no time. will I have to modify my DT file like you did a while back? I can't find that post right now.

[rmteo]

You can read multiple touch buttons (and perform other functions at the same time) with a PIC16F690/88/887 type device. This example does the keyboard scanning (30+ keys), polyphonic tone generation, PWM, etc. with a single PIC all concurrently.