Arduino - Using digital potentiometers (AD8403)

I have seen several blog posts covering the use of digital potentiometers with an Arduino but I haven't seen any that demonstrate digital pots that contain a shutdown circuit. I'm working on a project that needs this particular feature.

The shutdown circuit is of interest to me because I plan on interfacing with something that controls movement. In that application any value on the digital pot will cause a motor to move. Lower resistive values move forward and higher resistive values move back. I need a way to completely disable the potentiometer. My first prototype used two relays, two transistors and two resistors with different values. The Arduino would turn off and on the relays to cause movement. It worked because I could simply turn off both relays but it was quite bulky and the mechanical relays were noisy. A friend recommend a digital potentiometer as a replacement. A single digital pot chip would replace six components. I began researching them and found that not all of them include a shutdown circuit. I settled on the Analog Devices AD8403 chip. It is a quad channel, 256 position digitally controlled variable resistor. Communication is done through an SPI interface.

So lets start with a demonstration of what happens when there is no shutdown circuit on a digital potentiometer and the Arduino is reset. The circuit and code cycles through each digital pot varying the brightness of an LED by running up and down all 255 positions of the pot. While this is running I press the reset button on the Arduino.



In this situation the digital pot chip still has power and the value of one the pots is stuck while the Arduino resets (demonstrated by the LED staying lit). In my real application this would cause movement to continue until the board finished restarting which is highly undesirable. This effect could be minimized somewhat by including code to reset values of the four pots on startup but there would still be a second or two of uncontrolled movement while the board restarts.

Next is a demonstration of the same circuit but with the shutdown circuit enabled. The shutdown pin is connected to a pull down resistor. This causes the chip to go into shutdown mode if the Arduino resets or loses power. Again I press the reset button on the Arduino as it is running the code.



This is getting close to what I want but there is still the problem of the pot being set to some unknown value when the AD8B403 is taken out of shutdown mode. To handle this I added code to the setup function that sets the pots to a known value (zero in this case) before taking the chip out of shutdown mode.

Circuit

Here is a photo and Fritzing diagram of the circuit.

The connections are:

  * All A pins of AD8403 connected to +5V

  * All B pins of AD8403 connected to ground

  * An LED and a 220-ohm resisor in series connected from each W pin to ground

  * RS - to +5v

  * SHDN - to digital pin 7 and a 10k ohm pull down resistor

  * CS - to digital pin 10  (SS pin)

  * SDI - to digital pin 11 (MOSI pin)

  * CLK - to digital pin 13 (SCK pin)

The AD8403 is the 10k ohm version.

The Fritzing diagram is available here: https://github.com/matt448/arduino/raw/master/SPI_Digital_Pot_AD8403/SPI_Digital_Pot_AD8403.fzz

Code

The most update to date version of the code is available here: https://github.com/matt448/arduino/blob/master/SPI_Digital_Pot_AD8403/SPI_Digital_Pot_AD8403.ino

The code to control this isn't very complex. This is my first time using an SPI device and I found it to be very straight forward. The Arduino IDE has an example sketch for controlling SPI digital pots under File > Examples > SPI > DigitalPotControl. I started with the example and modified it to include shutdown control. That example code also ran a bit slow because of the serial console messages so I removed all the serial communication code.

Comment on shutdown pin

So after getting through all this testing I noticed a limitation of the shutdown pin, it shuts down all four pots at the same time. In my real application I plan on controlling movement on four separate motors and moving them to a certain positions. Once the motor reaches it's position I want it to stop. But the only way to stop the motor is activate the shutdown which deactivates all four pots. This is a problem because the other three motors may not have reached their position when I need to shutdown. I did some more looking and found that Microchip makes a digital pot that has individually controllable software shutdown for each pot. I'm going to order a couple MCP4251 chips and I'll write up another post when I test them out.

UPDATE 3/17/2014: Post about the MCP4251 is available here
UPDATE 3/27/2014: Added list of connections and resistor values. Also added links to github repo.