Arduino Uno and MCP4260

I am working on a project and part of it involves using an Arduino Uno to send data to a MCP4260 dual potentiometer. I have hooked up a separate volt meter to each wiper. My problem is that when I run the sketch below I get the expected voltage readings on one of the volt meters - .2v to 4.68v, while on the second volt meter I only get .19 - .2 v.

I’ve tried different MCP4260 chips and switch volt meters around but I keep getting the same results.

I would really appreciate some suggestions.

Thanks

/*
Digital Pot Control

Based on the original sketch for AD5206…

This example controls a Microchip digital potentiometer.
The MCP42 has 2 potentiometer channels. Each channel’s pins are labeled
PAx - connect this to voltage
PWx - this is the pot’s wiper, which changes when you set it
PBx - connect this to ground.

The MCP42 is SPI-compatible,and to command it, you send two bytes:

The first byte is the Command Byte which has this format when
the next byte is to be data: XX01XXpp
note these bits …^^… the 01 means the next byte is data
(where pp = potentiometer selection, X= don’t care)
pp= 00 = dummy code, no pot selected
pp= 01 = pot0
pp= 10 = pot1
pp= 11 = both pots

Simplest case is to have X= 0 so the Command Byte will be:
pp= 00: 00010000 = 16
pp= 01: 00010001 = 17
pp= 10: 00010010 = 18
pp= 11: 00010011 = 19

The second byte is the resistance value for the channel (0 - 255).

The circuit:

  • All PA pins of MCP42 connected to +5V
  • All PB pins of MCP42 connected to ground
  • An LED and a 220-ohm resisor in series connected from each PW pin to ground
  • CS - to digital pin 10 (SS pin)
  • SI - to digital pin 11 (MOSI pin)
  • SCK - to digital pin 13 (SCK pin)

created 10 Aug 2010
by Tom Igoe

Thanks to Heather Dewey-Hagborg for the original tutorial, 2005

Version for MCP42xx April 2013, Jim Brown

*/

// include the SPI library:
#include <SPI.h>

// set pin 10 as the slave select for the digital pot:
const int slaveSelectPin = 10;

void setup() {
// set the slaveSelectPin as an output:
pinMode (slaveSelectPin, OUTPUT);
// initialize SPI:
SPI.begin();
}

void loop() {
// go through pp=01, 10, 11 for pot0, pot1 and both together of the digital pot:
for (int CommandByte = 17; CommandByte < 20; CommandByte++) { // 17, 18 and 19
// change the resistance on this pot from min to max:
for (int level = 0; level < 255; level++) {
digitalPotWrite(CommandByte, level);
delay(10);
}
// wait at the top:
delay(100);
// change the resistance on this channel from max to min:
for (int level = 0; level < 255; level++) {
digitalPotWrite(CommandByte, 255 - level);
delay(10);
}
}

}

int digitalPotWrite(int CommandByte, int value) {
// take the SS pin low to select the chip:
digitalWrite(slaveSelectPin,LOW);
// send in the address and value via SPI:
SPI.transfer(CommandByte);
SPI.transfer(value);
// take the SS pin high to de-select the chip:
digitalWrite(slaveSelectPin,HIGH);
}