The Arduino Uno makes serial communication with digital potentiometers (digipots) such as the Microchip MCP4241-103E/P, relatively easy.
The MCP4241-103E/p is available at www.mouser.com in through hole technology which makes experimenting a little more easy.
Here are what the the digipots look like inside the IC.
There are the typical three terminals to a regular pot available on the IC. ย I am using the center wiper to control the voltage to the anode of the LED. ย Resistors R3 and R4 can be 470 ohm to current limit the LED. ย The MCP4241 can source 25mA of current on any output pin, such as the wiper.
The first order of business is the code. ย Here is some code I modified that runs two LEDs through a range of voltages by connecting the anode of each to the wiper of the digipot.
/*
Digital Pot Control
Based on the original sketch for AD5206….
and based on the Version for MCP42xx April 2013, Jim Brown,
This example controls a Microchip digital potentiometer.
The MCP4241 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 MCP4241 is SPI-compatible,and to command it, you send two bytes:
*** Here is where my analysis changes from Mr. Brown’s ***
8 bit Command: *see page 45 of the MCP4241 data sheet*
A=address, C=Command bit, D=data
AAAACCDD
I chose for AAAA 00h for Volatile Wiper 0 and then 01h for wiper 1
Command bits for both were Write Data so CC is 00
Data bit D9 is not used. D8 is zero, I have a 7bit reistor resolution.
So,
AAAACCDD is 00000000 to Write to Wiper 0 and 00010000 to Write to Wiper 1
The decimal values are 0 and 16 for CommandByte in the code below.
I swing the resistance value with the Data byte in my example from 0 to 80h or 0-127 because I use a 7 bit pot.
The circuit:
* All PA pins of MCP4241 connected to +5V
* All PB pins of MCP4241 connected to ground
* An LED and a 390-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)
Written November 29, 2014 with big thanks to the above mentioned people.
Robert Keeley
*/
// 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 Pot 1 first then Pot 2, CommandByte = 16 and then CommandByte = 0:
digitalPotWrite(0, 0); ย //turn off Pot 0 so it doesn’t float, partially lit:
delay(10);
int CommandByte = 16; // to Write to Pot 1
// change the resistance on this pot from min to max:
for (int level = 0; level < 127; 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 < 127; level++) {
digitalPotWrite(CommandByte, 127 – level);
delay(10);
}
digitalPotWrite(16, 0); //make sure Pot 1 is off:
delay(10);
CommandByte = 0; // to Write to Pot 0
// change the resistance on this pot from min to max:
for (int level = 0; level < 127; 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 < 127; level++) {
digitalPotWrite(CommandByte, 127 – 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);
}
Here is a simple version of the schematic:
Working Circuit:
MCP4241 and Arduino Uno
To Convert Analog Pot to Digipot Value:
Here’s the schematic using the ADC of the Arduino to read the analog pot and the SPI connection to the MCP4241 to control LED voltage using the internal digital potentiometer:
Analog pot read by ADC in Arduino. SPI to MCP4241 and set digipot value.
Here is a simple method to take the analog pot and convert it to something the MCP4241 will accept. ย Note with an 8 bit pot I would have divided the variable potValue by 4 instead of 8.
CODE FOR ARDUINO
/*
Analog Pot Read for Digital Pot Control
Written November 30, 2014
Robert Keeley
*/
// include the SPI library:
#include <SPI.h>
const int slaveSelectPin = 10; // set pin 10 as the slave select for the digital pot:
float valueAnalogPot; // potValue divided by 8. 1024 values/128 7 bit resistor resolution:
int digiPotData; // value of analog pot scaled and cast into an integer:
void setup() {
pinMode (slaveSelectPin, OUTPUT); // set the slaveSelectPin as an output:
SPI.begin(); // initialize SPI:
Serial.begin(9600); //serial communication to screen:
// turn pots off, when the 4241 powers up it may float to a wiper value and LED will be partially lit:
digitalPotWrite(0, 0);
delay(1);
digitalPotWrite(16, 0);
delay(1);
}
void loop() {
int potValue = analogRead(A0); //store ADC of pot value 0-1023 in potValue:
Serial.println(potValue); //print value between 0-1023 to screen int datatype:
delay(1); //stability
valueAnalogPot = potValue/8; // scaled to fit 7 bit pot:
digiPotData = (int) valueAnalogPot; //cast to integer to be used as data byte for digipot
int CommandByte = 0; // Pot 0 – Write Mode:
// change the resistance on this digital pot 0 to scaled version of the analog pot:
digitalPotWrite(CommandByte, digiPotData);
delay(1);
}
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);
}
The code now uses the analog input A(0) of the Arduino to read the analog pot and send a scaled version of it to the MCP4241 digital potentiometer.
Here’s a little video: