human-readable-spec.md

ev3dev Language Wrapper Specification v1.0.0

This is an unofficial specification that defines a unified interface for language wrappers to expose the ev3dev device APIs.

General Notes

Because this specification is meant to be implemented in multiple languages, the specific naming conventions of properties, methods and classes are not defined here. Depending on the language, names will be slightly different (ex. "touchSensor" or "TouchSensor" or "touch-sensor") so that they fit the language's naming conventions.

Some concepts that apply to multiple classes are described as "abstracts". These abstract sections explain how the class should handle specific situations, and do not necessarily translate in to their own class in the wrapper.

Implementation Notes (important)

• File access. There should be one class that is used or inherited from in all other classes that need to access object properties via file I/O. This class should check paths for validity, do basic error checking, and generally implement as much of the core I/O functionality as possible.
• Errors. All file access and other error-prone calls should be wrapped with error handling. If an error thrown by an external call is fatal, the wrapper should throw an error for the caller that states the error and gives some insight in to what actually happened.
• Naming conventions. All names should follow the language's naming conventions. Keep the names consistent, so that users can easily find what they want.
• Attribute types. int and string attributes are read-write files containing a single value that is representable either as an integer or as a single word. A string array attribute is a readonly file that contains space-separated list of words, where each word is a possible value of some other string atribute. And a string selector attribute is a read-write file that contains space-separated list of possible values, where the currently selected value is enclosed in square brackets. Another value may be selected by writing a single word to the file.

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Classes

Motor

The motor class provides a uniform interface for using motors with positional and directional feedback such as the EV3 and NXT motors. This feedback allows for precise control of the motors. This is the most common type of motor, so we just call it motor.

ev3dev docs link: http://www.ev3dev.org/docs/drivers/tacho-motor-class/

System properties

Property Name	Type	Accessibility	Description
Command	string	Write	Sends a command to the motor controller. See commands for a list of possible values.
Commands	string array	Read	Returns a list of commands that are supported by the motor controller. Possible values are run-forever, run-to-abs-pos, run-to-rel-pos, run-timed, run-direct, stop and reset. Not all commands may be supported. - run-forever will cause the motor to run until another command is sent. - run-to-abs-pos will run to an absolute position specified by position_sp and then stop using the command specified in stop_command. - run-to-rel-pos will run to a position relative to the current position value. The new position will be current position + position_sp. When the new position is reached, the motor will stop using the command specified by stop_command. - run-timed will run the motor for the amount of time specified in time_sp and then stop the motor using the command specified by stop_command. - run-direct will run the motor at the duty cycle specified by duty_cycle_sp. Unlike other run commands, changing duty_cycle_sp while running will take effect immediately. - stop will stop any of the run commands before they are complete using the command specified by stop_command. - reset will reset all of the motor parameter attributes to their default value. This will also have the effect of stopping the motor.
Count Per Rot	int	Read	Returns the number of tacho counts in one rotation of the motor. Tacho counts are used by the position and speed attributes, so you can use this value to convert rotations or degrees to tacho counts. In the case of linear actuators, the units here will be counts per centimeter.
Driver Name	string	Read	Returns the name of the driver that provides this tacho motor device.
Duty Cycle	int	Read	Returns the current duty cycle of the motor. Units are percent. Values are -100 to 100.
Duty Cycle SP	int	Read/Write	Writing sets the duty cycle setpoint. Reading returns the current value. Units are in percent. Valid values are -100 to 100. A negative value causes the motor to rotate in reverse. This value is only used when speed_regulation is off.
Encoder Polarity	string	Read/Write	Sets the polarity of the rotary encoder. This is an advanced feature to all use of motors that send inversed encoder signals to the EV3. This should be set correctly by the driver of a device. It You only need to change this value if you are using a unsupported device. Valid values are normal and inversed.
Polarity	string	Read/Write	Sets the polarity of the motor. With normal polarity, a positive duty cycle will cause the motor to rotate clockwise. With inversed polarity, a positive duty cycle will cause the motor to rotate counter-clockwise. Valid values are normal and inversed.
Address	string	Read	Returns the name of the port that this motor is connected to.
Position	int	Read/Write	Returns the current position of the motor in pulses of the rotary encoder. When the motor rotates clockwise, the position will increase. Likewise, rotating counter-clockwise causes the position to decrease. Writing will set the position to that value.
Position P	int	Read/Write	The proportional constant for the position PID.
Position I	int	Read/Write	The integral constant for the position PID.
Position D	int	Read/Write	The derivative constant for the position PID.
Position SP	int	Read/Write	Writing specifies the target position for the run-to-abs-pos and run-to-rel-pos commands. Reading returns the current value. Units are in tacho counts. You can use the value returned by counts_per_rot to convert tacho counts to/from rotations or degrees.
Speed	int	Read	Returns the current motor speed in tacho counts per second. Note, this is not necessarily degrees (although it is for LEGO motors). Use the count_per_rot attribute to convert this value to RPM or deg/sec.
Speed SP	int	Read/Write	Writing sets the target speed in tacho counts per second used when speed_regulation is on. Reading returns the current value. Use the count_per_rot attribute to convert RPM or deg/sec to tacho counts per second.
Ramp Up SP	int	Read/Write	Writing sets the ramp up setpoint. Reading returns the current value. Units are in milliseconds. When set to a value > 0, the motor will ramp the power sent to the motor from 0 to 100% duty cycle over the span of this setpoint when starting the motor. If the maximum duty cycle is limited by duty_cycle_sp or speed regulation, the actual ramp time duration will be less than the setpoint.
Ramp Down SP	int	Read/Write	Writing sets the ramp down setpoint. Reading returns the current value. Units are in milliseconds. When set to a value > 0, the motor will ramp the power sent to the motor from 100% duty cycle down to 0 over the span of this setpoint when stopping the motor. If the starting duty cycle is less than 100%, the ramp time duration will be less than the full span of the setpoint.
Speed Regulation Enabled	string	Read/Write	Turns speed regulation on or off. If speed regulation is on, the motor controller will vary the power supplied to the motor to try to maintain the speed specified in speed_sp. If speed regulation is off, the controller will use the power specified in duty_cycle_sp. Valid values are on and off.
Speed Regulation P	int	Read/Write	The proportional constant for the speed regulation PID.
Speed Regulation I	int	Read/Write	The integral constant for the speed regulation PID.
Speed Regulation D	int	Read/Write	The derivative constant for the speed regulation PID.
State	string array	Read	Reading returns a list of state flags. Possible flags are running, ramping holding and stalled.
Stop Command	string	Read/Write	Reading returns the current stop command. Writing sets the stop command. The value determines the motors behavior when command is set to stop. Also, it determines the motors behavior when a run command completes. See stop_commands for a list of possible values.
Stop Commands	string array	Read	Returns a list of stop modes supported by the motor controller. Possible values are coast, brake and hold. coast means that power will be removed from the motor and it will freely coast to a stop. brake means that power will be removed from the motor and a passive electrical load will be placed on the motor. This is usually done by shorting the motor terminals together. This load will absorb the energy from the rotation of the motors and cause the motor to stop more quickly than coasting. hold does not remove power from the motor. Instead it actively try to hold the motor at the current position. If an external force tries to turn the motor, the motor will 'push back' to maintain its position.
Time SP	int	Read/Write	Writing specifies the amount of time the motor will run when using the run-timed command. Reading returns the current value. Units are in milliseconds.

Large Motor (inherits from motor)

EV3 large servo motor

Target driver(s): lego-ev3-l-motor

Medium Motor (inherits from motor)

EV3 medium servo motor

Target driver(s): lego-ev3-m-motor

DC Motor

The DC motor class provides a uniform interface for using regular DC motors with no fancy controls or feedback. This includes LEGO MINDSTORMS RCX motors and LEGO Power Functions motors.

ev3dev docs link: http://www.ev3dev.org/docs/drivers/dc-motor-class/

System properties

Property Name	Type	Accessibility	Description
Command	string	Write	Sets the command for the motor. Possible values are run-forever, run-timed and stop. Not all commands may be supported, so be sure to check the contents of the commands attribute.
Commands	string array	Read	Returns a list of commands supported by the motor controller.
Driver Name	string	Read	Returns the name of the motor driver that loaded this device. See the list of [supported devices] for a list of drivers.
Duty Cycle	int	Read	Shows the current duty cycle of the PWM signal sent to the motor. Values are -100 to 100 (-100% to 100%).
Duty Cycle SP	int	Read/Write	Writing sets the duty cycle setpoint of the PWM signal sent to the motor. Valid values are -100 to 100 (-100% to 100%). Reading returns the current setpoint.
Polarity	string	Read/Write	Sets the polarity of the motor. Valid values are normal and inversed.
Address	string	Read	Returns the name of the port that this motor is connected to.
Ramp Down SP	int	Read/Write	Sets the time in milliseconds that it take the motor to ramp down from 100% to 0%. Valid values are 0 to 10000 (10 seconds). Default is 0.
Ramp Up SP	int	Read/Write	Sets the time in milliseconds that it take the motor to up ramp from 0% to 100%. Valid values are 0 to 10000 (10 seconds). Default is 0.
State	string array	Read	Gets a list of flags indicating the motor status. Possible flags are running and ramping. running indicates that the motor is powered. ramping indicates that the motor has not yet reached the duty_cycle_sp.
Stop Command	string	Write	Sets the stop command that will be used when the motor stops. Read stop_commands to get the list of valid values.
Stop Commands	string array	Read	Gets a list of stop commands. Valid values are coast and brake.
Time SP	int	Read/Write	Writing specifies the amount of time the motor will run when using the run-timed command. Reading returns the current value. Units are in milliseconds.

Servo Motor

The servo motor class provides a uniform interface for using hobby type servo motors.

ev3dev docs link: http://www.ev3dev.org/docs/drivers/servo-motor-class/

System properties

Property Name	Type	Accessibility	Description
Command	string	Write	Sets the command for the servo. Valid values are run and float. Setting to run will cause the servo to be driven to the position_sp set in the position_sp attribute. Setting to float will remove power from the motor.
Driver Name	string	Read	Returns the name of the motor driver that loaded this device. See the list of [supported devices] for a list of drivers.
Max Pulse SP	int	Read/Write	Used to set the pulse size in milliseconds for the signal that tells the servo to drive to the maximum (clockwise) position_sp. Default value is 2400. Valid values are 2300 to 2700. You must write to the position_sp attribute for changes to this attribute to take effect.
Mid Pulse SP	int	Read/Write	Used to set the pulse size in milliseconds for the signal that tells the servo to drive to the mid position_sp. Default value is 1500. Valid values are 1300 to 1700. For example, on a 180 degree servo, this would be 90 degrees. On continuous rotation servo, this is the 'neutral' position_sp where the motor does not turn. You must write to the position_sp attribute for changes to this attribute to take effect.
Min Pulse SP	int	Read/Write	Used to set the pulse size in milliseconds for the signal that tells the servo to drive to the miniumum (counter-clockwise) position_sp. Default value is 600. Valid values are 300 to 700. You must write to the position_sp attribute for changes to this attribute to take effect.
Polarity	string	Read/Write	Sets the polarity of the servo. Valid values are normal and inversed. Setting the value to inversed will cause the position_sp value to be inversed. i.e -100 will correspond to max_pulse_sp, and 100 will correspond to min_pulse_sp.
Address	string	Read	Returns the name of the port that this motor is connected to.
Position SP	int	Read/Write	Reading returns the current position_sp of the servo. Writing instructs the servo to move to the specified position_sp. Units are percent. Valid values are -100 to 100 (-100% to 100%) where -100 corresponds to min_pulse_sp, 0 corresponds to mid_pulse_sp and 100 corresponds to max_pulse_sp.
Rate SP	int	Read/Write	Sets the rate_sp at which the servo travels from 0 to 100.0% (half of the full range of the servo). Units are in milliseconds. Example: Setting the rate_sp to 1000 means that it will take a 180 degree servo 2 second to move from 0 to 180 degrees. Note: Some servo controllers may not support this in which case reading and writing will fail with -EOPNOTSUPP. In continuous rotation servos, this value will affect the rate_sp at which the speed ramps up or down.
State	string array	Read	Returns a list of flags indicating the state of the servo. Possible values are: * running: Indicates that the motor is powered.

LED

Any device controlled by the generic LED driver. See https://www.kernel.org/doc/Documentation/leds/leds-class.txt for more details.

System properties

Property Name	Type	Accessibility	Description
Max Brightness	int	Read	Returns the maximum allowable brightness value.
Brightness	int	Read/Write	Sets the brightness level. Possible values are from 0 to max_brightness.
Triggers	string array	Read	Returns a list of available triggers.
Trigger	string selector	Read/Write	Sets the led trigger. A trigger is a kernel based source of led events. Triggers can either be simple or complex. A simple trigger isn't configurable and is designed to slot into existing subsystems with minimal additional code. Examples are the ide-disk and nand-disk triggers. Complex triggers whilst available to all LEDs have LED specific parameters and work on a per LED basis. The timer trigger is an example. The timer trigger will periodically change the LED brightness between 0 and the current brightness setting. The on and off time can be specified via delay_{on,off} attributes in milliseconds. You can change the brightness value of a LED independently of the timer trigger. However, if you set the brightness value to 0 it will also disable the timer trigger.
Delay On	int	Read/Write	The timer trigger will periodically change the LED brightness between 0 and the current brightness setting. The on time can be specified via delay_on attribute in milliseconds.
Delay Off	int	Read/Write	The timer trigger will periodically change the LED brightness between 0 and the current brightness setting. The off time can be specified via delay_off attribute in milliseconds.

Button

Provides a generic button reading mechanism that can be adapted to platform specific implementations. Each platform's specific button capabilites are enumerated in the 'platforms' section of this specification.

Sensor

The sensor class provides a uniform interface for using most of the sensors available for the EV3. The various underlying device drivers will create a lego-sensor device for interacting with the sensors.

Sensors are primarily controlled by setting the mode and monitored by reading the value<N> attributes. Values can be converted to floating point if needed by value<N> / 10.0 ^ decimals.

Since the name of the sensor<N> device node does not correspond to the port that a sensor is plugged in to, you must look at the address attribute if you need to know which port a sensor is plugged in to. However, if you don't have more than one sensor of each type, you can just look for a matching driver_name. Then it will not matter which port a sensor is plugged in to - your program will still work.

ev3dev docs link: http://www.ev3dev.org/docs/drivers/lego-sensor-class/

System properties

Property Name	Type	Accessibility	Description
Command	string	Write	Sends a command to the sensor.
Commands	string array	Read	Returns a list of the valid commands for the sensor. Returns -EOPNOTSUPP if no commands are supported.
Decimals	int	Read	Returns the number of decimal places for the values in the value<N> attributes of the current mode.
Driver Name	string	Read	Returns the name of the sensor device/driver. See the list of [supported sensors] for a complete list of drivers.
Mode	string	Read/Write	Returns the current mode. Writing one of the values returned by modes sets the sensor to that mode.
Modes	string array	Read	Returns a list of the valid modes for the sensor.
Num Values	int	Read	Returns the number of value<N> attributes that will return a valid value for the current mode.
Address	string	Read	Returns the name of the port that the sensor is connected to, e.g. ev3:in1. I2C sensors also include the I2C address (decimal), e.g. ev3:in1:i2c8.
Units	string	Read	Returns the units of the measured value for the current mode. May return empty string

I2C Sensor (inherits from sensor)

A generic interface to control I2C-type EV3 sensors.

Target driver(s): nxt-i2c-sensor

System properties

Property Name	Type	Accessibility	Description
FW Version	string	Read	Returns the firmware version of the sensor if available. Currently only I2C/NXT sensors support this.
Poll MS	int	Read/Write	Returns the polling period of the sensor in milliseconds. Writing sets the polling period. Setting to 0 disables polling. Minimum value is hard coded as 50 msec. Returns -EOPNOTSUPP if changing polling is not supported. Currently only I2C/NXT sensors support changing the polling period.

Power Supply

A generic interface to read data from the system's power_supply class. Uses the built-in legoev3-battery if none is specified.

System properties

Property Name	Type	Accessibility	Description
Measured Current	int	Read	The measured current that the battery is supplying (in microamps)
Measured Voltage	int	Read	The measured voltage that the battery is supplying (in microvolts)
Max Voltage	int	Read
Min Voltage	int	Read
Technology	string	Read
Type	string	Read

Lego Port

The lego-port class provides an interface for working with input and output ports that are compatible with LEGO MINDSTORMS RCX/NXT/EV3, LEGO WeDo and LEGO Power Functions sensors and motors. Supported devices include the LEGO MINDSTORMS EV3 Intelligent Brick, the LEGO WeDo USB hub and various sensor multiplexers from 3rd party manufacturers.

Some types of ports may have multiple modes of operation. For example, the input ports on the EV3 brick can communicate with sensors using UART, I2C or analog validate signals - but not all at the same time. Therefore there are multiple modes available to connect to the different types of sensors.

In most cases, ports are able to automatically detect what type of sensor or motor is connected. In some cases though, this must be manually specified using the mode and set_device attributes. The mode attribute affects how the port communicates with the connected device. For example the input ports on the EV3 brick can communicate using UART, I2C or analog voltages, but not all at the same time, so the mode must be set to the one that is appropriate for the connected sensor. The set_device attribute is used to specify the exact type of sensor that is connected. Note: the mode must be correctly set before setting the sensor type.

Ports can be found at /sys/class/lego-port/port<N> where <N> is incremented each time a new port is registered. Note: The number is not related to the actual port at all - use the address attribute to find a specific port.

System properties

Property Name	Type	Accessibility	Description
Driver Name	string	Read	Returns the name of the driver that loaded this device. You can find the complete list of drivers in the [list of port drivers].
Modes	string array	Read	Returns a list of the available modes of the port.
Mode	string	Read/Write	Reading returns the currently selected mode. Writing sets the mode. Generally speaking when the mode changes any sensor or motor devices associated with the port will be removed new ones loaded, however this this will depend on the individual driver implementing this class.
Address	string	Read	Returns the name of the port. See individual driver documentation for the name that will be returned.
Set Device	string	Write	For modes that support it, writing the name of a driver will cause a new device to be registered for that driver and attached to this port. For example, since NXT/Analog sensors cannot be auto-detected, you must use this attribute to load the correct driver. Returns -EOPNOTSUPP if setting a device is not supported.
Status	string	Read	In most cases, reading status will return the same value as mode. In cases where there is an auto mode additional values may be returned, such as no-device or error. See individual port driver documentation for the full list of possible values.

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Constants / Enums

Due to the inherent differences between the various languages that we support here, the enums listed below can also be declared as global constants.

###Ports

Name	Value	Description
INPUT_AUTO	instance-specific (see below for details)	Automatic input selection
OUTPUT_AUTO	instance-specific (see below for details)	Automatic output selection
INPUT_1	"in1"	Sensor port 1
INPUT_2	"in2"	Sensor port 2
INPUT_3	"in3"	Sensor port 3
INPUT_4	"in4"	Sensor port 4
OUTPUT_A	"outA"	Motor port A
OUTPUT_B	"outB"	Motor port B
OUTPUT_C	"outC"	Motor port C
OUTPUT_D	"outD"	Motor port D

The values for the *_AUTO constants can be chosen by the implementation. They can have any value that signifies an auto-search.

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IO Device (abstract)

An IO Device handles control tasks for a single port or index. These classes must chose one device out of the available ports to control. Given an IO port (in the constructor), an implementation should:

• If the specified port is blank or unspecified/undefined/null, the available devices should be enumerated until a suitable device is found. Any device is suitable when it's type is known to be compatible with the controlling class, and it meets any other requirements specified by the caller.
• If the specified port name is not blank, the available devices should be enumerated until a device is found that is plugged in to the specified port. The supplied port name should be compared directly to the value from the file, so that advanced port strings will match, such as in1:mux3.

All IO devices should have a connected variable. If a valid device is found while enumerating the ports, the connected variable should be set to true (by default, it should be false). If connected is false when an attempt is made to read from or write to a property file, an error should be thrown (except while in the consructor).

If an error occurs after the initial connection, an exception should be thrown by the binding informing the caller of what went wrong. Unless the error is fatal to the application, no other actions should be taken.

Compatibility

Compatibility table:

Spec Version|Fully Supported Kernel Version ---|---|--- v0.9.1|v3.16.1-7-ev3dev v0.9.2|v3.16.7-ckt10-4-ev3dev v1.0.0|v3.16.7-ckt21-9-ev3dev