Yes you need a pulldown resistor or the pin will
float.

John

--- Shawn Wilton <black9@gmail.com> wrote:

> I'm using a PIC16F88, trying to read a voltage off
> of what is essentially a
> trim pot.  It's actually a variably resistant touch
> panel.  Pins should not
> be tied together unless pressure is being applied to
> the panel and then you
> slide your finger up and down the panel to adjust
> the resistance.  This is a
> fabric panel.
> 
> The Vref+ is set for AVdd which is tied to Vdd and
> Vref- is set for AVss
> which is tied to Vss.
> 
> The board works, runs great.
> 
> Code is below.
> 
> Problem is that the sense pin (AN2, porta.2) is
> registering nearly the full
> AVdd when no panel is connected and then around 2-3V
> when the panel is
> connected  .  Is this typical for a PIC uC?  The A/D
> conversion works, but
> it seems like I'm going to have to tie my sense pin
> to a very weak pull down
> in order for it to register properly.  Does this
> sound right?
> 
> Relevant code below in BoostC format:
> 
> 
> 
> 
> 
> /********
>   Setup the I/O ports
>   ********/
>   /*
>   Set the io ports as follows:
>   Port A:
>   7 nu
>   6 nu
>   5 \mclr
>   4 nu
>   3 SW2 (input)
>   2 SW1 (input)
>   1 LED (output)
>   0 nu
>   */
>   //      76543210
>   trisa = 11111101b;
>   porta = 0;
>   ansel = 00000000b;
> 
>   /*
>   Port B:
>   7 nu
>   6 nu
>   5 txd (output)
>   4 clock (output)
>   3 nu
>   2 rxd (input)
>   1 sdi (input)
>   0 pwm (output)
>   */
>   //      76543210
>   trisb = 11011110b;
> 
> 
>   /********
>   Setup the ADC
>   ********/
>   //Set A/D ports 2 and 3 for analog inputs
>   set_bit(ansel, ANS2);
>   //set_bit(ansel, ANS3);
> 
>   /*
>   ADCon0
>   7 AD conversion clock
>   6
>   5 Channel select bits (2)
>   4
>   3
>   2 Go/\Done
>   1 nu
>   0 ADON - Yes
>   */
>   //       76543210
>   adcon0 = 11010001b;
> 
>   /*
>   ADCon1
>   7 Justification (left)
>   6 AD clock divide by 2 (yes)
>   5 Vref config bits
>   4
>   3 nu
>   2 nu
>   1 nu
>   0 nu
>   */
>   //       76543210
>   adcon1 = 11000000b;
> 
> 
> 
> 
> 
> 
> 
> /**
>   Read the specified ADC channel and return an
> unsigned int result
>  */
> unsigned int readADChannel(unsigned char channel)
> {
>   //unsigned int result;
>   unsigned int high = 0;
>   unsigned int low = 0;
> 
>   volatile unsigned char addr_h@ADRESH;
>   volatile unsigned char addr_l@ADRESL;
> 
>   //Set the ADC Channel
>   //adcon0 &= 00 << 2;
>   adcon0.CHS2 = 0;
>   adcon0.CHS1 = 0;
>   adcon0.CHS0 = 0;
>   adcon0 |= channel << 3;
> 
>   //Turn on the ADC
>   //adc_on = TRUE;
> 
>   //Delay to perform the acquisition
>   delay_ms(2);
> 
>   //Begin a conversion
>   adc_go = TRUE;
> 
>   //Begin the conversion and block until it's
> complete
>   while(adc_go == TRUE){}
> 
>   //Clear the interrupt flag
>   //clear_bit(pir1, ADIF);
> 
>   //Convert to 16 bit vars so we can shift properly
>   high = addr_h;
>   low = addr_l;
> 
>   //Turn off the ADC since we are no longer using it
>   //adc_on = FALSE;
> 
>   //Shift the upper bits up and the lower bits down
>   return (high << 8) | (low);
> }
> 
> 
> -- 
> 
> Shawn Wilton (b9 Systems)
> http://b9Systems.com  <- New web page
> -- 
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
> 



 
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