16F726 Capacitive sensing module

[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

[mackrackit]

Just a wild guess...

What if bit 6 of the T1GCON register is set to 0?

[Darrel Taylor]

Ioannis,

Try this modification to the averaging routine for the CSM.
http://www.picbasic.co.uk/forum/show...598#post107598

[Ioannis]

I wanted to correct minor mistakes in the comments of the code I posted but it is not possible. Can any of the moderators or Admins see the whole code? I cannot for sure (in the Edit Post mode).

Dave, I have not changed the T1GCON register which by default on Power up is reset to zero. I do not use the gate anyway.

Darrel, I missed that. Will try it an get back.

Thanks both,
Ioannis

[HankMcSpank]

I use the CSMs with great success without having to average.

I basically store away the last cap sense output reading (ie TMR1 count), take the present reading (the TMR1 count again) and just have the program calculate an ongoing 'trigger' threshold count (the threshold being the last TMR1 reading reading minus a certain percentage)

And then just code it along the lines of if the present reading just takenis less than the threshold (ie the last reading minus a preset percentage), then the sensor has been touched....set a flag variable.

There should be a huge amount of difference between the non touched count & the touched count ...I set my threshold at something like 30% and it trigger everytime.

One thing i've found helps greatly with CSM ...is to adjust the CSM oscillator settings & also the sample period so that you get a 'normal' reading in the 10,000->15,000 count ballpark.

[Ioannis]

Thanks Hank. Well, 30% seems too much to me. I have managed up to 16%...

Thanks Darrel too. Yes your suggestions did made alot of difference.

Ioannis

[Ioannis]

After some testing, it seems that with the modification Darrel suggested, it needs to touch very quickly to get a 'pressed=1' flag.

If you approach relatively slow the touch pad it won't get triggered. I may have to play with the constants in the CSM and average too.

But it made a very reliable system now.

Thanks for the tips.

PIC16f1827 with the internal CSM module makes it very easy to make a touch keyboard. Only a couple of registers to setup and you are ready.

Ioannis