In a previous post I discussed how to read the temperature from a MCP9808 temperature sensor. In this post I want to back up a bit to demonstrate some techniques for figuring out how to interface with an i2c chip and some basic verification techniques. I will be using the Intel Edison on the mini-breakout board, but these techniques will apply to most Linux based embedded systems.
The MCP9808 is an i2c bus device. I2C is short for Inter-Integrated Circuit, a two wire bus used for connecting components to each other on electronic devices. I2C is just one option used for interconnecting devices. Other options include. SPI, 1-wire, and Serial. When using devices for an embedded project we generally find SPI or I2C on devices. I tend to go for I2C devices when I can because they allow me to get away with using less wires to hook up.
When we do integrate a device it is handy to ensure the device is recognized by the system before we try to write code for it. There are tools that are already installed on the Edison called I2C tools. A part of the LM-Sensors project, I2C tools allow low level access to I2C busses and the devices connected to them.
Before we get started lets make sure our Edison is up to date. We can do this by logging in via ssh (or use the terminal) and executing the command:
You need to have your Edison attached to a network.
Let’s get started. There are four commands we are interested in that are summarized in the table here:
|Device register dumping||i2cdump|
|Device register reading||i2cget|
|Device register setting||i2cset|
We use the i2cdetect command to both find the busses available as well as the devices on the bus. If we execute
we can get a list of the currently installed I2C busses. The output is as as follows:
We can see we have 8 I2C buses on the Edison. But according to the docs we are only able to use bus 1 and 6 on the mini-break out board. I have only used bus 1 in my projects so far. If we wanted to see the devices on bus 1, we would enter:
i2cdetect -r 1
and get something like the following output:
When executing this command we get the standard warning that executing this command may jack up our device. It’s true, we can cause errors if we run this command on some of the other busses, but it has always cleared up for me with a power cycle of the Edison. Usually the device just hangs.
Looking at the output we can see we have four devices on bus 1. At 0x18 we have a MCP9808 temperature sensor, 0x3C is a SSD1306 display, 0x48 is an ADS1819 ADC , and 0x77 is a BMP085 barometric pressure and temperature sensor.
Device register dumping:
Now that we know what devices are on the board (or more accurately all the devices we wired to the Edison are recognized by our system) we can start working with the devices. Each device has internal registers that control the device and provide output from the device. We use the i2cdump command to take a look at these registers. Let’s take a look at the MCP9808
i2cdump 1 0x18 w
The form of the command is 12cdump (bus | device address | option). The ‘w’ option give us word output which makes it easier (for me at least) to read the registers. Executing the command gives us output that looks like this:
To understand what we are seeing in the dump we need to study the data sheet for our MCP9808.
Keeping in mind that the MCP9808 is a little endian device, we see the 16 bit data registers storing their values with the least significant byte (LSB) first and most significant byte (MSB) second. We just need to do a quick byte swap to look at register 0 to see the value is 0x001d. According to the spec sheet the MSB should always be 0x00 as well as the LSB bits 7-4. Bits 3-0 in the LSB are the pointer used to select which register we read and write to when communicating to the device. The MCP9808 is a surprisingly complex device, so we won’t look at all the registers. But a thorough explanation of the registers and their function begins on page 16 of the data sheet.
Read a specific register:
We could certainly read the registers from the output of i2cdump but there is a specific command for reading a single register. If we wanted to read the the current temperature the data sheet tells us the register containing this value is 0x05. To read this register we would issue this command:
i2cget 1 0x18 0x05 w
The command is in the form i2cget (bus | device address | register | option) . This gives is the following output:
We get the requisite warning and then the little endian contents of the register. Swapping the bytes to 0xC152 and referencing page 25 of the data sheet we can get the current temperature in Celsius. Briefly — Bits 15-13 are flags that are used when we set the device to monitor a high and low set point. They can tell us if a threshold was passed, and which one. We don’t have those set so the values are not significant to us in this case. Bit 12 of the word is the sign of the two’s compliment value, since it is 0 our value is positive. Stripping those four bits out we are left with 0x0152 which is the temperature value. The rest is a base conversion and a little math — the MSB is 1 decimal, the LSB is 82. So using the data sheet formula –> 1 * 16 + 82 / 16 = 21 degrees Celsius. Thats a cool 69.8 F in the office today.
Set a register:
If we want to manually control the MCP9808 we can set its registers with the i2cset command. Before we use the command a little back ground.
The MCP9808 has two modes of operation — continuous conversion and shutdown mode. Shutdown mode puts the device into a low power state and , as you might have guessed, stops updating the temperature measurement. If we read the temperature register while the device is in shutdown we will retrieve the last measurement made. In a battery powered device it might be a good idea to shutdown the device and only power it up when we need to update the temperature. This is controlled by bit 8 of the config register located at 0x01.
We can use the following command to enter into shutdown mode:
i2cset 1 0x18 0x01 0x0001 w
Remember that we need to swap the bytes in the word due to endianness. This can be tested by reading the temperature while heating up the sensor (I just put my finger on it). You can use i2cget or i2cdump to read register 0x05 of the device. To set the sensor to continuous conversion mode use this command:
i2cset 1 0x18 0x01 0x0000 w
This post covered the basics of verifying a device is detected on an I2C bus and determining if the device is functioning properly. We used the four I2C tools commands to detect the busses and devices installed, dump a device registers, read a specific register, and to write a register. With these four commands we can certainly see if our devices are working correctly as we implement an embedded system.
The i2c tools do have other modes of operation that I did not touch on here. These modes can be read about on the I2C tools docs page. Armed with these tools and the data sheet for a device we are ready to get coding.