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16F628A with MCP41010 problem

 
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FunkyLabRat



Joined: 13 Feb 2012
Posts: 10

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16F628A with MCP41010 problem
PostPosted: Thu Apr 12, 2012 2:33 pm     Reply with quote

Hi again everybody,

I'm trying to replace a foot switch (working with force-sensing resistors) with a MCP41010 digital potentiometer connected to a PIC 16F628A to control a device. It's connected with a TRS jack to the device.

I know this is not a hardware forum, but maybe I'm doing something wrong or missing a connection, so please check my schematics.



I'm not sure if I can hook up the Jack TRS connector straight to the PW0, PA0 and PB0 pins on the MCP41010? (see the question marks in the picture).

But either way, I've tried a lot of combinations but it won't work and I'm not measuring anything on the MCP pins.

And of course, here's the test program I'm using:

Code:
#include <16F628A.h>
#fuses INTRC_IO, NOWDT, NOPROTECT, PUT, NOBROWNOUT, NOMCLR, NOLVP, NOCPD
#use delay(clock=4M)
#use rs232(stream=MIDI,baud=31250,parity=N,xmit=PIN_B2,rcv=PIN_B1,stop=1,bits=8,BRGH1OK,ERRORS)

#define CS PIN_B3
#define SCLK PIN_A5
#define SI PIN_B0

void set_pot (int data) {
   BYTE i;
   BYTE cmd[2];

   cmd[0] = data;
   cmd[1] = 0x11;

   output_low(SCLK);
   output_low(CS);

   for(i=1;i<=16;++i) {
      output_bit(SI, shift_left(cmd,2,0));

      output_high(SCLK);
      output_low(SCLK);
   }
   output_high(CS);
}

void shutdown_pot () {
   BYTE i;
   BYTE cmd[2];

   cmd[0] = 0;
   cmd[1] = 0x21;

   output_low(SCLK);
   output_low(CS);

   for(i=1;i<=16;++i) {
      output_bit(SI, shift_left(cmd,2,0));

      output_high(SCLK);
      output_low(SCLK);
   }
   output_high(CS);
}

void FlashLED() {

  output_high(PIN_A1);
  delay_ms (100);
  output_low(PIN_A1);
}

void main() {
 
enable_interrupts(GLOBAL);

while(1) {

   shutdown_pot();
   delay_ms(10);
   
   set_pot(50);
   delay_ms(1000);
   FlashLED();
   
   shutdown_pot ();
   delay_ms(10);

   set_pot(100);
   delay_ms(1000);
   FlashLED();

   shutdown_pot ();
   delay_ms(10);

   set_pot(150);
   delay_ms(1000);
   FlashLED();

   shutdown_pot ();
   delay_ms(10);

   set_pot(200);
   delay_ms(1000);
   FlashLED();



   }

}




I hope someone can push me in the right direction.

Thanks in advance!
PCM programmer



Joined: 06 Sep 2003
Posts: 21658

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PostPosted: Thu Apr 12, 2012 3:32 pm     Reply with quote

Quote:

#define CS PIN_B3
#define SCLK PIN_A5
#define SI PIN_B0

Download the 16F628A data sheet:
http://ww1.microchip.com/downloads/en/DeviceDoc/40044G.pdf
Look at the pin descriptions in this section:
Quote:

TABLE 3-2: PIC16F627A/628A/648A PINOUT DESCRIPTION

Look closely at the capabilities of the PIC pin that you have chosen for
SCLK signal.


Quote:
void main() {

enable_interrupts(GLOBAL);

Also, why are you enabling interrupts when you have no interrupt
service routine ? What do you expect that line to do ?
FunkyLabRat



Joined: 13 Feb 2012
Posts: 10

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PostPosted: Thu Apr 12, 2012 7:53 pm     Reply with quote

Thanks for your reply PCM Programmer.

I've checked the datasheet and pins before posting, but I couldn't find anything wrong with the RA5 pin. I totally overlooked that it can only be used as a input pin. Thank you.

And the ''enable_interrupts(GLOBAL);'' line is a mistake. I'm sorry about that. I forgot to delete this line before pasting the code here. These interrupts ARE used in the ''full'' code.
Mike Walne



Joined: 19 Feb 2004
Posts: 1782
Location: Boston Spa UK

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Take a step or two backwards
PostPosted: Fri Apr 13, 2012 2:04 am     Reply with quote

Do you fully understand what the force sensing device is actually doing?

You're presenting two different sets of data when you compare the force sensor with the 25k potentiometer.

Then try replacing the force sensors with hardware potentiometer(s).

When you've got that working correctly, move on to MCP41010. It should, by then, be obvious how to connect.

Programming is a separate issue.

That's my two pennies worth.

Mike
temtronic



Joined: 01 Jul 2010
Posts: 8525
Location: Greensville,Ontario

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PostPosted: Fri Apr 13, 2012 5:11 am     Reply with quote

Also a 'B' style pot is a logarithmic type, while the digipot is linear so you'll have to create a table to get the response you need to mimic the force resistor.
Simple enough to do, just another 'detail' to work out.
FunkyLabRat



Joined: 13 Feb 2012
Posts: 10

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PostPosted: Sun Apr 15, 2012 8:09 pm     Reply with quote

The force sensing foot switch is just a fancy 20K linear potentiometer. The B25K pot was just to show how the connections work fine.

The digipot now works, but I can't change it's step value. It seems to be always in the middle (which seems to be the default state). I've tried some test programs with no success.

Here's my latest test program where there are two buttons added on pins B5 and A0 to change the step value of the MCP digipot which works in 256 steps. Every press should add or subtract 10 steps on the digipot.

I assume that set_pot() has to be used with values 0 to 255, but since I've had no success in changing the value, the ''pot_data'' value can go higher for testing purposes.

Code:

#include <16F628A.h>
#fuses INTRC_IO, NOWDT, NOPROTECT, PUT, NOBROWNOUT, NOMCLR, NOLVP, NOCPD
#use delay(clock=4M)
#use rs232(stream=MIDI,baud=31250,parity=N,xmit=PIN_B2,rcv=PIN_B1,stop=1,bits=8,BRGH1OK,ERRORS)

#define CS PIN_A3
#define SCLK PIN_A4
#define SI PIN_B0

int pot_data;

void set_pot (int data) {
   BYTE i;
   BYTE cmd[2];

   cmd[0] = data;
   cmd[1] = 0x11;

   output_low(SCLK);
   output_low(CS);

   for(i=1;i<=16;++i) {
      output_bit(SI, shift_left(cmd,2,0));

      output_high(SCLK);
      output_low(SCLK);
   }
   output_high(CS);
}

void shutdown_pot () {
   BYTE i;
   BYTE cmd[2];

   cmd[0] = 0;
   cmd[1] = 0x21;

   output_low(SCLK);
   output_low(CS);

   for(i=1;i<=16;++i) {
      output_bit(SI, shift_left(cmd,2,0));

      output_high(SCLK);
      output_low(SCLK);
   }
   output_high(CS);
}

void main() {
 
pot_data = 128;

while(1) {
     
   if (input(PIN_B5) == 0) {
   
      output_high(PIN_A1);
   
      pot_data = pot_data - 10;
   
      shutdown_pot();
      delay_ms(10);
   
      set_pot(pot_data);
     
      delay_ms(500);
      output_low(PIN_A1);
   }
   
 if (input(PIN_A0) == 0) {
   
      output_high(PIN_A1);
   
      pot_data = pot_data + 10;
   
      shutdown_pot();
      delay_ms(10);
   
      set_pot(pot_data);
     
      delay_ms(500);
      output_low(PIN_A1);
   }


   }

}
FunkyLabRat



Joined: 13 Feb 2012
Posts: 10

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PostPosted: Thu Apr 19, 2012 1:02 pm     Reply with quote

To answer my own question: PIN_A4 is a open-drain pin and also can't be used for the Digipot.

I used pin A2 instead of A4:
Code:

#define CS PIN_A3
#define SCLK PIN_A2
#define SI PIN_B0


And now it works!
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