EBC Exercise 12 I2C

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thumb‎ Embedded Linux Class by Mark A. Yoder


3.8 Kernel

This page is for the 3.8 kernel. See EBC Exercise 12 I2C - xM for the 3.2 kernel.

I²C is a "two-wire interface" standard that is used to attach low-speed peripherals to an embedded system. In this exercise we will wire up a couple of I²C temperature sensors (TC74) and learn how to read their values.

The Hardware

bone

The AM3359 on the BeagleBone has three I²C controllers (Section 21 of the TRM). You can see which ones are configured at boot time by running the following on the Beagle:

beagle$ dmesg | grep i2c
[    0.153495] omap_i2c 44e0b000.i2c: bus 0 rev0.11 at 400 kHz
[    0.165269] omap_i2c 44e0b000.i2c: unable to select pin group
[    0.166036] omap_i2c 4819c000.i2c: bus 1 rev0.11 at 100 kHz
[    0.168388] omap_i2c 4819c000.i2c: unable to select pin group
[    0.418600] i2c /dev entries driver

Here we see two buses, one running at 400 kHz and the other at 100 kHz. Table 11 from the SRM shows buses 1 and 2 are brought out to the P9 Expansion Header, however what the table starts numbering with 1 and the software starts with 0, so these are really buses 0 and 1. We'll use 1 (called 2 in the table).

You can see what's on the i2c buses with

beagle$ i2cdetect -y -r 0
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- UU -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- UU -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: UU -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: UU -- -- -- -- -- -- -- 
beagle$ i2cdetect -y -r 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- 48 -- -- -- -- -- -- 4f 
50: -- -- -- -- UU UU UU UU -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: 70 -- -- -- -- -- -- -- 

I have something at addresses 0x48, 0x4f and 0x70 on bus 1.

.jpg

These signals are 3.3V and the TMP101 runs on 2.7 to 5.5V so we are in luck.

I²C is a two-wire bus. The two wires are

  1. Serial Clock (SCL), is an input to the TMP101 and is used to clock data into and out of the TMP101.
  2. Serial Data (SDA), is bidirectional and carries the data to and from the TMP101.

The only other two pins on the TMP101 that you need to use are the Power Supply (Vdd) and Ground, unless you want to use OS/ALERT, in which case you need to add a pull up resistor to Vdd, and then run another wire to a GPIO in order to properly trigger an interrupt.

Bone gpio.JPG BoneGPIO.png

Wire up the TMP101 to the Beagle by attaching the Vdd to the 3.3V + bus, the GND to the - bus and SDA to SDA (pin 20) and SCL to SCL (pin 19). You will also need to attach two pull-up resistors. Get two 4.7KΩ resistors. Attach one between SDA and Vdd. Attach the other between SCL and Vdd.

Your TMP101 should be labeled with T101. If you have another TMP101 you can wire it in parallel with the first be sure to wire the ADD0 pin differently. That is, attach SDA to SDA and SCL to SCL, etc. No need for additional pull up resistors.

The Software

Do this, to be sure you have all you needed installed:

beagle$ opkg update
beagle$ opkg install i2c-tools
beagle$ opkg install i2c-tools-dev

From the Shell

The Beagle brings out I²C bus 1 to the Expansion Header. You can see what devices are on the bus by using the i2cdetect command. On your Beagle try:

beagle$ i2cdetect -y -r 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- UU -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- 48 48 4a -- -- -- -- -- 
50: -- -- -- -- UU UU UU UU -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --  

What you see is a list of all the devices found on the bus. I've attached three TMP101's, and wired their ADD0 line differently. Their address are 1001 000, 1001 010 and 1001 010 respectively. Converting to hex you get 0x48, 0x49 and 0x4a. You can see the three appear in the ic2detect.

Each TMP101 has four registers. Check the TMP101 manual for details. We're interested in the TEMP register. You can read it with:

beagle$ i2cget -y 1 0x48 0
0x1b

The -y says don't ask me, just do it. 1 says use bus 1. 0x48 is the device address and 0 is the register number. The value returned is the temperature in degrees C.

  1. Convert the hex temperature to decimal. Is the value reasonable?
  2. Write a script to run ic2get in a loop and watch the temperature. Hold the device between your fingers. Does the temp go up?

From C

Another approach to using I²C on the Beagle is from a C program. You can open /dev/i2c-1 and do ioctl calls on it to read and write data.

Pull the exercises

beagle$ cd exercises
beagle$ git pull
beagle$ cd i2c

Compile and run myi2cget.c.

beagle$ gcc myi2cget.c -o myi2cget
beagle$ ./myi2cget
Usage:  ./myi2cget <i2c-bus> <i2c-address> <register>
beagle$ ./myi2cget 1 72 0

0x1b (27)

It takes many of the arguments as i2cget, but none of the flags. It's very stripped down version of i2cget. Note that 72 is the decimal representation of 0x48. myi2cget requires the use of decimal numbers, while the "traiditional" i2cget can use either decimal or hex representations.

The tools directory contains the original i2cget code. It came from here.

Challenge

  1. Look over myi2cget.
    1. Find the open which opens the device.
    2. Find the ioctl call that sets the address. What other values can be used instead of I2C_SLAVE? Hint: Look in the include files for the definition of I2C_SLAVE.
    3. Find the ioctl call reads the register. Hint: There are a couple of wrappers hiding it. Find where i2c_smbus_read_byte_data is defined and then keep going until you find ioctl. I2C_SLAVE is used by the previous ioctl to set the slave address. What's used at the 2nd argument to ioctl to read a byte?
  2. Challenge 1: Write a C program that will print the current temperature every time the USER button is pressed. Print the temp in F.
  3. Challenge 2: Modify your program to update the temperature every second if the USER button isn't pressed.

References

  1. Introduction to i2c and SPI
  2. TMP101 I2C Temperature Sensor information.
  3. I got a lot information from Interfacing with I2C Devices.
  4. This appears to have some nice I2C information for the Gumstix. It should also work for the Beagle.
  5. i2c via Python




thumb‎ Embedded Linux Class by Mark A. Yoder