16F726 Capacitive sensing module

[Normnet]

I have been using capacitive touch switches for several years with little problem except the
Q prox chips became obsoleted after a couple of years.

Any Microchip can be used as the requirements are unbelievably simple:
An I/O pin to charge with supply voltage and an AD pin to read the cap sense value.

I sometimes take 20 to 100 readings, bubble sort them highest to lowest, then average
only array elements 20% to 80% to to remove any errant readings.
The outcome is stable, varies only one or two at 8 bit.
The AD needs to be coded for power conservation however.

The following code is derived almost directly from AN1286:

'PIC18F2520  
subLOAD_AD_ARRAY:
    ADCON0 = %00000001  'ENABLES AD
    For i = 0 To 99
        LOW PORTA.0
        LOW PORTA.2 
        ASM 
        NOP 
        ENDASM 
        TRISA.0 = 1 'INPUT
        HIGH PORTA.2
        ASM 
        NOP
        ENDASM 

        'AD BY REGISTER'S  
        ADCON0.1 = 1   ' START AD  GO/DONE BIT
        While ADCON0.1 = 1 : Wend  ' AD ENDED GO/DONE Bit
        wAD.LowByte = ADRESL       ' 2520 10 BIT AD ONLY
        wAD.HighByte = ADRESH  
        yAD_ARRAY100[i] = wAD >> 2
    Next
    ADCON0 = %00000000  'DISABLE AD
    Return

The touch button is simply a 3/16" stainless steel machine screw through the faceplate with
a double nut tightened around a soldered wire loop connected to the AD.
I add a little grommet for looks.
The charge lead is simply wound several times around the screw behind the faceplate.

Another version is individual metal tabs of sheet metal adhered to the underside of the
enclosure for buttons.
The AD wire is soldered to each tab and the charge wire only needs to wrap around the
AD wire or all AD wires.

The cap sensor readings can vary over time and the base line to compare the readings against
should be re zeroed and a new max value redetermined at regular intervals.

I laminate an inkjet photo quality paper (not photo paper) printout for the faceplate.

The last consideration is ESD.
A simple 100k resistor in line on both leads is a good solution.

Norm

[Ioannis]

Hi Norm.

I think you can shorten your code by Left justify the AD results and take only the ADRESH. Same as with the lines:

        wAD.LowByte = ADRESL       ' 2520 10 BIT AD ONLY
        wAD.HighByte = ADRESH  
        yAD_ARRAY100[i] = wAD >> 2

Ioannis

[rsocor01]

Norm,

There are many ways to do it. You can also use the interrupt TMR1 to detect changes in the output frequency due to a pad being touched by a finger. This is explained in AN1101 "Introduction to Capacitive Sensing".

http://ww1.microchip.com/downloads/e...tes/01101a.pdf

The following code is derived almost directly from AN1286:

I see that you are using AN1286 "Water-Resistant Capacitive Sensing". If you are using your touch sensors in a wet environment, then you might want to consider using inductive sensors. Take a look at the table I attached to this post.

http://www.microchip.com/stellent/id...param=en538303



Robert

[George]

I'm trying to use capacitive touch too - was kinda hoping that the inbuilt capacitive sensing was a bit more straight forward than the convoluted mess and complexity it looks like. Would it be too hard for microchip to make it a bit user friendly so you just enable named pins as capacitive, then read the register to see which were pressed and had inbuilt filtering etc??

I've used Mister E's capacitive touch idea before and it seems so much simpler than the "integrated' solution.

Anyrate - enough of my ramblings, I'd like to learn to use it - hopefully it's not as complex as it appears from outset. I've copied byte butchers code as above and pasted it directly into MCS and I have errors all over the place. I have a fresh install of PBP2.6 and have also just downloaded DTs' ints-14.

How come it works for him and not for me? I had the chip type 16F726 like his also

While I'm on a roll, why wont MCS allow you to copy and paste the error list???

I think many of the issues come from the Interrupt routine but asm and interrupts are well over my head. Is there any reason Byte Butcher used interrupts rather than just checking pins periodically?

Thanks

Error[118]c:\pbp...asm 213 : Overwriting previous address contents (2007)
Error[101]c:\pbp...asm 342 : ERROR: (wsave variable not found)
Error[101]c:\pbp...asm 308 : ERROR: (" Add:" wsave VAR BYTE $20 SYSTEM)
Error[101]c:\pbp...asm363 : ERROR: (Chip has RAM in BANK1, but WSAVE was not found)
Error[101]c:\pbp...asm315 : :ERROR: (" Add:" wsave VAR BYTE $A0 SYSTEM)
Error[128]c:\pbp...asm 989 : Missing argument(s)
Error[101]c:\pbp...asm672 : ERROR: ("INT_Handler" - Interrupt Flag (FlagReg,FlagBit) not found)
Error[128]c:\pbp...asm 1000 : Missing argument(s)
Error[101]c:\pbp...asm 672 : ERROR: (INT_ENABLE" - Interrupt Flag ( FlagReg,FlagBit) not found)

[rsocor01]

I think many of the issues come from the Interrupt routine but asm and interrupts are well over my head. Is there any reason Byte Butcher used interrupts rather than just checking pins periodically?

He used interrupts because that is the way to do it. You need to count the number of pulses in the signal for a fixed amount of time and determine if the switch has been touched or not. This app note might help you to understand the basics of CSM

http://ww1.microchip.com/downloads/e...tes/01101a.pdf

Robert

[Ioannis]

Don't you have to do better things?

Seriously now, I mean it, while the timers do their jobs, that is to count, you can do many other tasks in your program.

That is why the so hated interrupts are needed.

As for your errors, do post your code. Do you need help or guesses?

Ioannis

[HankMcSpank]

I experimented with capacitive touch on a 16f690, my related thread about it here (you might glean some stuff from it)...

http://www.picbasic.co.uk/forum/showthread.php?t=12196

My (short!) youtube video of the end result here (using a guitar's scratchplate screw's 'head', as a 'toggle switch')...



Re the process itself...it might initially seem a lot to swallow (especially if it's your first sortie using interrupts - which are pretty much essential for capacitive touch), but like everything else, if you break it down into smaller 'building blocks' and approach each block one at a time, then it becomes much more digestible.

You obviously need to get the relaxation oscillator working first - an oscilloscope is *extremely* useful to make sure this is working (& getting it working proved to be the most troublesome part for me....there's a whole heap of registers that need to be set 'oh so right', once I got it running, I was off to the races).

re those wsave errors you're seeing when compiling - you need to comment out some lines in the DTS_INTS-14.BAS (search on the error wsave VAR BYTE $20 SYSTEM on this forum to find out what needs doing!)

[Ioannis]

Finally I got my first touch demo working with 16F1827 chip. The CSM module is easy to use and makes program small enough.

But, I have one problem with the stability of the output that shows whether one is touching, close to touch or not touching at all.

Although I have included Darrel's Average routine there is a grey region that makes deciding if one is touching not very clear.

The following test code has a LED indicator to show when touch pad is really touched or not. If finger is very close, value is playing up or down the threshold and makes the LED indicator blink.

OPTION_REG = %10000110    'Tmr0 from 256 presc and Int Clock
  
 T1CON = %11000001 '%00110000    ' TMR1 1:8 prescale, timer1 off

led         var portb.4       
value       var word
AvgCount CON 32  ' = Number of samples to average
FAspread CON 10000 ' = Fast Average threshold +/-
ADavg  VAR WORD
flag        var bit
'Interrupt Engine Setup
':::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
ASM
INT_LIST  macro    ; IntSource,     Label,      Type,       ResetFlag?
  INT_Handler   TMR0_INT,       _Timer_0,     PBP,    yes

 endm
 INT_CREATE               ; Creates the interrupt processor
    INT_ENABLE   TMR0_INT
ENDASM
':::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::       

goto main

Timer_0:
    t1con.0=0
    vALUE.lowbyte=tmr1l:VALUE.highbyte=tmr1h
    flag=1
    tmr1l=0:tmr1h=0
    t1con.0=1
@ INT_RETURN    

main:
clear
pause 1000

start:
if flag=1 then
    flag=0
    gosub Average
endif

hserout [27,"[H",27,"[10;0H","Value is: ",#vALUE," "]

if VALUE<319 then
    high led
else
    low led
endif
goto start

Average:
    IF Value = ADavg Then NoChange
    IF ABS (Value - ADavg) > FAspread OR Value < AvgCount Then FastAvg
    IF ABS (Value - ADavg) < AvgCount Then RealClose
    ADavg = ADavg - (ADavg/AvgCount)
    ADavg = ADavg + (Value/AvgCount)
    GoTo AVGok
  FastAvg:
    ADavg = Value
    GoTo AVGok
  RealClose:
    ADavg = ADavg - (ADavg/(AvgCount/4))
    ADavg = ADavg + (Value/(AvgCount/4))
  AVGok:
    Value = ADavg  ' Put Average back into Value
  NoChange:
Return

Any ideas welcome.
Ioannis