-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]On Behalf
Of Shawn Wilton
Sent: Friday, November 03, 2006 12:23 PM
To: Microcontroller discussion list - Public.
Subject: [PIC] A/D on pic16f88


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  .
****
while disconnected what voltage do you have on porta.1 ?

****

  Is this typical for a PIC uC?  The A/D conversion works,
****
no. some things not right?

****

 but
it seems like I'm going to have to tie my sense pin to a very weak pull down

****
maybe you already have a pullup?
****

in order for it to register properly.  Does this sound right?

****
no
****
you need to add a delay to charge capacitors before conversion




Andre Abelian



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
-- 
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-- 
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View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist