Thursday, October 24, 2013

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.



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.

Thursday, October 10, 2013

Arduino - Sending data over a CAN bus

I have been tinkering with CAN buses due to my interest in cars. It's fascinating to me that packets are flying around a modern vehicle controlling nearly everything. Gauges, lights, locks, engine sensors, etc. To have a better understanding of the basics of a CAN bus I wanted to build the simplest possible setup to send and receive CAN messages. I chose two Arduino Uno's with a Seeed Studio CAN-BUS shield attached to each Uno. The Seeed shield is very straight forward and inexpensive. The Sparkfun CAN-BUS shield has an SD card slot, LCD connector and GPS connector. All of which are cool but drive up the price and complexity. The Seeed shield only does CAN bus and includes screw terminals which are handy for testing.

Arduino Uno R3Seeed CAN-BUS Shield

What I wanted to do with this experiment was transmit the value of an analog pin hooked up to a linear potentiometer. The data would be sent from one Arduino to another over a CAN bus and then display that value on an LCD connected to the second Arduino. Here is a picture of my setup. (Ignore the Mega2560 above the LCD. It's not used here.)


And here is a Fritzing diagram minus the CAN-BUS shields.



CAN bus termination

A CAN bus requires 120 Ohm termination resistors at each end of the bus. The Seeed Studio shields have built in termination resistors. When you connect two Seeed CAN bus shields togther like I did in this example you will have a properly terminated CAN bus. If you plan on connecting into an existing CAN bus that already has termination you can disable the built in termination resistors. To disable termination you can cut trace P1 or you can desolder resistor R1.


Close up view of the Seeed CAN bus shield
 termination resistors.
**Note: I have recently discovered the Seeed Studio CAN-BUS shield v1.0 uses a 60 ohm termination resistor for R3. While that worked for this small demo I later ran into issues when trying to use this shield with other nodes on a CAN bus. This 60 ohm resistor caused me many hours of frustration. If you are going to use this shield with on a bus with multiple nodes I would recommend desoldering R3 and using the correct 120 ohm resistance at the ends of your bus. 

Connecting into an existing CAN bus

If you are planning on connecting into an existing CAN bus (like in a car) you need to remove/disable the termination resistor on the shield as explained above. The CAN bus in a vehicle already has termination resistors. Adding a new node with a termination resistor will cause errors and disrupt communication on the bus.

Another important step is to connect a common ground between your Arduino board and the vehicle. If you are connecting at the OBD2 port pin 5 provides a signal ground. If you can't find a signal ground wire a chassis ground will suffice.

CAN bus messages

So I should probably explain a bit about CAN bus messages. Each message is made up of an id and some data. The id's in hex start at 0x000 and go to 0x7FF or 0 to 2047 in decimal. In most systems lower id values are considered more important. The bus handles collisions by letting the lower id win the collision. The data can be between 1 and 8 bytes for each message. Each byte can have a value from 0 to 255 or in hex 0x00 to 0xFF. When you send a CAN bus message you transmit the id, how many bytes you are sending (this is called DLC) and the actual data. The receiver will only read the number of bytes you said should be in the message. So if you send a DLC of 4 but the message contains 8 bytes the receiver will only read the first 4 bytes. Eight bytes per message is a bit limiting but the tradeoff is the high reliability of the bus. So sometimes you have to be creative with stuffing data into those bytes. If the value you are sending is less than 255 you can just use a single byte. Larger numbers will require using multiple bytes. Ascii codes can be sent but only eight characters per message. Whatever method you use to stuff the data in will also have to be used to un-stuff the data on the receiver. In my simple example here I did some math to limit the range of values to 0-255. An analog pin produces values between 0-1024. I simply divided the result by four to give me data I could send in a single byte.
CAN buses can operate at several different speeds up to 1 Mbit/s. Typical rates are 100 kbit/s, 125 kbit/s and 500 kbit/s. Slower rates allow for longer length buses. All devices on a bus must transmit at the same speed. The CAN bus wikipedia page is a good place to start if you want to learn more about the CAN protocol.


Code

I started with the example code provided by Seeed and modified it to add in the LCD output on the 'receiver' device and added reading of the potentiometer on A0 for the value that is transmitted. They have basic examples for send and receive. You can find some good info on their wiki page. Their libraries are available here. On my Mac I created the directory ~/Documents/Arduino/libraries/CAN_BUS_Shield for the library files. I unzipped the file and copied over the .h and .cpp files into that new directory. The zip file also contains the send and receive examples.

Note that normally devices on a CAN bus are both receivers and transmitters of data. This is a simplified example where each device is only doing one task.



Sender code


Receiver code


Video




[Updated 2014-05-25: Noted value of A0 potentiometer in the Fritzing diagram]
[Updated 2014-07-21: Added section about termination resistors]
[Updated 2014-09-25: Added note about incorrect value of resistor R3 on Seeed's shield]
[Updated 2015-03-10: Added additional notes about termination resistors]
[Updated 2017-03-27: Added new section 'Connecting into an existing CAN bus']



Monday, October 7, 2013

Nagios monitoring for Amazon SQS queue depth

I have found that a bunch of messages stacking up in my SQS queue's can be the first sign of something breaking. Several things can cause messages to stack up in the queue. I have seen malformed messages, slow servers and dead processes all cause this at different times. So to monitor the queue depth I wrote this Nagios check / plug-in. The check simply queries the SQS api and finds out the count of messages in each queue. Then it compares the count to the warning and critical levels.

This check is written in python and uses the boto library. It includes perfdata output so you can graph the number of messages in the queue. The the AWS API for SQS does wildcard matching of queue names so you can monitor a bunch of queues with one check if they have some sort of common prefix to the name. The way I use this is I have several individual checks using the complete explicit name of the queue and then a catchall using a wildcard set to a higher number that will catch any queues that have been added. Make sure you have a .boto file for the user that will be running this nagios check. It only requires read permissions.

Some queues may be more time sensitive than others. That is the case for my setup. For queues that are time sensitive I set the warning and critical counts to low values. Less time sensitive queues are set to higher count values. This screenshot is an example of that:


Config


Here is the command definition I use for Naigos:

# 'check_sqs_depth' command definition
define command{
        command_name    check_sqs_depth
        command_line    /usr/lib/nagios/plugins/check_sqs_depth.py --name '$ARG1$' --region '$ARG2$' --warn '$ARG3$' --crit '$ARG4$'
        }

and here is the service definition I'm using

define service{
        use                                 generic-service   
        host_name                      sqs.us-east-1
        service_description          example_name SQS Queue
        contact_groups                admins,admins-page,sqs-alerts
        check_command             check_sqs_depth!example_name!us-east-1!150!300!
        }


Code


The code is available on my github nagios-checks repository here: https://github.com/matt448/nagios-checks and I have posted it as a gist below. My git repository will have the most up-to-date version