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Using I2C Sensors
The I2C standards only specify how data is sent from device to device. It does not specify the layout of the registers of a device. LEGO, however, has guidelines for 3rd party manufactures so that they can provide sensors with a (fairly) uniform register layout.
We call sensors that were designed following LEGO's guidelines I2C/M sensors (M is for Mindstorms). This common register layout lets us autodetect the type of sensor and proves access to the sensor via the msensor class.
We refer to sensors that do not conform to LEGO's specifications as I2C/S sensors (S is for Standard). There are so many types of I2C chips in the wild that are already supported on Linux, that we do not attempt to autodetect them. To use them, we just need to find a compatible driver and manually load it.
The I2C address that is used in ev3dev is different from the other EV3/NXT programming languages/environments. See I2C Sensor Addressing to find out how and why. Since most sensors are autodetected, you don't usually have to worry about the address. However, for manually loading devices, you need to know what the address is. Also, if you had multiple sensors connected to a single port using a port splitter, you would need to know the address to make sure your are using the correct sensorN device node. NOTE: A port splitter is not the same as a sensor MUX.
See Using the Mindstorms Sensor Device Class for general usage. This is the I2C specific usage.
By default, I2C/M sensors are polled every 100 milliseconds. This value can be changed via a module parameter. More on that later.
When we say "polled", we just mean that the EV3 brick initiates a read command to read data from the sensor. The data that is read depends on the current mode that is selected. You can change the polling rate using the poll_ms. You can also disable polling by setting poll_ms to 0. When polling is disabled, you can initiate a data read by setting the mode again.
WARNING! Be very careful when writing to your sensors. It is theoretically possible to break them if you write to the wrong register.
We can write data to I2C sensors using the bin_data attribute. The first byte is the address of the register you want to write to and the following bytes are the data that is written to that register.
Example: Sending a "calibrate white" command to the mindsensor.com Light Sensor Array.
$ echo -e -n "\x41W" > bin_dataThis writes the ascii character 'W' to register 0x41
All of the I2C/M drivers are part of the nxt-i2c-sensor module. This module allows control over some of its behaviors via module parameters.
| Parameter | Default | Description |
|---|---|---|
| allow_autodetect | Y | Setting to N disables autodetection of sensors. |
| default_poll_ms | 100 | This provides the default value for the poll_ms attribute. A value of 0 will disable polling by default. Values of less that the minimum 50 msec will be rounded up to 50 msec. Changes only affect sensors plugged in after the change was made. |
You can change the values at any time using /sys/module/nxt_i2c_sensor/parameters/* or you can make the changes permanent by adding a .conf file to /etc/modprobe.d.
If you have autodetection disabled or if you have managed to change the I2C address of your sensor to something other than the default, you will have to manually load a device in order to be able to use your sensor. We just have to tell the I2C adapter which driver to use and the address of the device. (You read the addressing section didn't you?)
The I2C adapter device nodes are at /sys/bus/i2c/devices/i2c-N where N is the number of the input port plus 2. To load a device, we write to the new_device attribute. NOTE: These nodes only exist when you have an I2C sensor plugged into an input port.
$ echo nxt-i2c-sensor 0x0B > /sys/bus/i2c/devices/i2c-5/new_deviceAs we already discussed, I2C/S sensors generally have an existing Linux driver that you can use. This means that each sensor will work a bit differently. You can load a device just like for manually loading an I2C/M device, except we use a different driver name. You can find the names of drivers here.
Example: Using the mindsensors.com Realtime Clock Sensor on input port 2.
$ echo ds1307 0x68 > /sys/bus/i2c/devices/i2c-4/new_device
$ dmesg | tail
...
i2c-legoev3 i2c-legoev3.4: registered on input port 2
i2c i2c-4: new_device: Instantiated device ds1307 at 0x68
rtc-ds1307 4-0068: SET TIME!
rtc-ds1307 4-0068: rtc core: registered ds1307 as rtc1
rtc-ds1307 4-0068: 56 bytes nvram
$ cd /sys/class/rtc
$ ls
rtc0 rtc1
$ cd rtc1
$ ls
date device max_user_freq since_epoch time
dev hctosys name subsystem ueventNow, I just need to figure out what to do with TWO realtime clocks!
You actually don't need a driver to use your I2C sensors. Drivers do make it much safer and easier, but if you really want full control, it is yours for the taking. There are symlinks for each I2C adapter to make finding them easy. NOTE: The symlinks and the underlying I2C device are only present when an I2C sensor is plugged into a port.
$ ls /dev/i2c-in*
/dev/i2c-in2 /dev/i2c-in3You can use the i2c-tools package or an I2C library in your programming language of choice to communicate with I2C devices this way. You don't want to do this if a device is already loaded so you will want to disable autodetection first if the sensor is the autodetected type. Beware that many sensors, including the NXT Ultrasonic Sensor use an address of 0x01, which is illegal according to the I2C standards. i2c-tools and any library that does some error checking may prevent you from accessing the sensor. In ev3dev-jessie, the i2c-tools package has been patched to work around this.
Using the NXT Ultrasonic Sensor:
$ cat poll_ms
100
$ while true; do cat value0; done
22
23
26
27
30
25
...
22
24
26
26
22
22
^C
$ echo 1000 > poll_ms
$ while true; do cat value0; done
22
22
22
22
22
22
22
25
25
25
25
25
25
25
25
...
^C
$ echo 0 > poll_ms
$ cat value0 # value0 will be last value measured before polling stopped
23
$ cat value 0 # move the sensor and try again
23
$ cat mode
[NXT-US-CM] NXT-US-IN NXT-US-SI-CM NXT-US-SI-IN NXT-US-LIST
$ echo NXT-US-CM > mode # reads data
$ cat value0
29
$ cat value0 # move the sensor and try again
29^C means you have to press CTRL+C to make the loop stop.
# Module configuration for nxt-i2c-sensor
# Uncomment this line to disable polling
#options nxt-i2c-sensor default_poll_ms=0
# Uncomment this line to disable autodetection
#options nxt-i2c-sensor allow_autodetect=N$ IN2_I2C_ADAP=$(udevadm info -q path -n /dev/i2c-in2)"/../.."
$ echo $IN2_I2C_ADAP
/devices/platform/legoev3-ports/in2/in2:nxt-i2c-host/i2c-legoev3.4/i2c-4/i2c-dev/i2c-4/../..With the mindsensors.com Realtime Clock Sensor on input port 2.
& i2cdump 4 0x68
No size specified (using byte-data access)
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-4, address 0x68, mode byte
Continue? [Y/n] y
0 1 2 3 4 5 6 7 8 9 a b c d e f 0123456789abcdef
00: 11 35 00 01 01 01 00 03 50 71 48 60 f5 01 6b 0c ?5.???.?PqH`??k?
10: 78 e3 2d 4e 92 6e c7 69 25 61 6b 5b 04 34 15 05 x?-N?n?i%ak[?4??
20: cc 3e 4e 4b 41 8a 59 09 1b f3 1a 2a 7c 47 a7 90 ?>NKA?Y????*|G??
30: 20 6a 95 7a 3b da 5b de 73 31 a2 3a 6e 59 ed f8 j?z;?[?s1?:nY??
40: 11 35 00 01 01 01 00 03 50 71 48 60 f5 01 6b 0c ?5.???.?PqH`??k?
50: 78 e3 2d 4e 92 6e c7 69 25 61 6b 5b 04 34 15 05 x?-N?n?i%ak[?4??
60: cc 3e 4e 4b 41 8a 59 09 1b f3 1a 2a 7c 47 a7 90 ?>NKA?Y????*|G??
70: 20 6a 95 7a 3b da 5b de 73 31 a2 3a 6e 59 ed f8 j?z;?[?s1?:nY??
80: 11 35 00 01 01 01 00 03 50 71 48 60 f5 01 6b 0c ?5.???.?PqH`??k?
90: 78 e3 2d 4e 92 6e c7 69 25 61 6b 5b 04 34 15 05 x?-N?n?i%ak[?4??
a0: cc 3e 4e 4b 41 8a 59 09 1b f3 1a 2a 7c 47 a7 90 ?>NKA?Y????*|G??
b0: 20 6a 95 7a 3b da 5b de 73 31 a2 3a 6e 59 ed f8 j?z;?[?s1?:nY??
c0: 12 35 00 01 01 01 00 03 50 71 48 60 f5 01 6b 0c ?5.???.?PqH`??k?
d0: 78 e3 2d 4e 92 6e c7 69 25 61 6b 5b 04 34 15 05 x?-N?n?i%ak[?4??
e0: cc 3e 4e 4b 41 8a 59 09 1b f3 1a 2a 7c 47 a7 90 ?>NKA?Y????*|G??
f0: 20 6a 95 7a 3b da 5b de 73 31 a2 3a 6e 59 ed f8 j?z;?[?s1?:nY??
$ i2cget -y 4 0x68 0x01 | sed s/0x// # read minutes
35
$ i2cset -y 4 0x68 0x08 0x46 0x72 0x65 0x65 0x20 0x72 0x61 0x6d 0x20 0x73 0x70 0x61 0x63 0x65 0x21 i
$ i2cdump -y -r 0x08-0x16 4 0x68
No size specified (using byte-data access)
0 1 2 3 4 5 6 7 8 9 a b c d e f 0123456789abcdef
00: 46 72 65 65 20 72 61 6d Free ram
10: 20 73 70 61 63 65 21 space!