MQTT - Compass Bricklet

This is the description of the MQTT API bindings for the Compass Bricklet. General information and technical specifications for the Compass Bricklet are summarized in its hardware description.

An installation guide for the MQTT API bindings is part of their general description.

Examples

The example code below is Public Domain (CC0 1.0).

Simple

Download (example-simple.txt)

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# Change XYZ to the UID of your Compass Bricklet

setup:
    # Get current heading
    subscribe to tinkerforge/response/compass_bricklet/XYZ/get_heading
    publish '' to tinkerforge/request/compass_bricklet/XYZ/get_heading

Callback

Download (example-callback.txt)

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# Change XYZ to the UID of your Compass Bricklet

setup:
    # Handle incoming heading callbacks
    subscribe to tinkerforge/callback/compass_bricklet/XYZ/heading
    publish '{"register": true}' to tinkerforge/register/compass_bricklet/XYZ/heading # Register heading callback
    
    # Set period for heading callback to 0.1s (100ms) without a threshold
    publish '{"period": 100, "value_has_to_change": false, "option": "off", "min": 0, "max": 0}' to tinkerforge/request/compass_bricklet/XYZ/set_heading_callback_configuration

API

All published payloads to and from the MQTT bindings are in JSON format.

If an error occures, the bindings publish a JSON object containing the error message as attribute "_ERROR". It is published on the corresponding response topic: .../response/... for .../request/... and .../callback/... for .../register/....

Basic Functions

request/compass_bricklet/<UID>/get_heading
Response payload:
  • heading -- int

Returns the heading in 1/10 degree (north = 0 degree, east = 90 degree).

Alternatively you can use request/compass_bricklet/<UID>/get_magnetic_flux_density and calculate the heading with heading = atan2(y, x) * 180 / PI.

If you want to get the value periodically, it is recommended to use the register/compass_bricklet/<UID>/heading callback. You can set the callback configuration with request/compass_bricklet/<UID>/set_heading_callback_configuration.

request/compass_bricklet/<UID>/get_magnetic_flux_density
Response payload:
  • x -- int
  • y -- int
  • z -- int

Returns the magnetic flux density (magnetic induction) for all three axis in 1/10 mG (milli Gauss).

If you want to get the value periodically, it is recommended to use the register/compass_bricklet/<UID>/magnetic_flux_density callback. You can set the callback configuration with request/compass_bricklet/<UID>/set_magnetic_flux_density_callback_configuration.

request/compass_bricklet/<UID>/set_configuration
Request payload:
  • data_rate -- int (has symbols)
  • background_calibration -- bool

Configures the data rate and background calibration.

  • Data Rate: Sets the data rate that is used by the magnetometer. The lower the data rate, the lower is the noise on the data.
  • Background Calibration: Set to true to enable the background calibration and false to turn it off. If the background calibration is enabled the sensing polarity is flipped once per second to automatically calculate and remove offset that is caused by temperature changes. This polarity flipping takes about 20ms. This means that once a second you will not get new data for a period of 20ms. We highly recommend that you keep the background calibration enabled and only disable it if the 20ms off-time is a problem in your application.

Default values: Data rate of 100Hz and background calibration enabled.

The following symbols are available for this function:

for data_rate:

  • "100Hz" = 0
  • "200Hz" = 1
  • "400Hz" = 2
  • "600Hz" = 3
request/compass_bricklet/<UID>/get_configuration
Response payload:
  • data_rate -- int (has symbols)
  • background_calibration -- bool

Returns the configuration as set by request/compass_bricklet/<UID>/set_configuration.

The following symbols are available for this function:

for data_rate:

  • "100Hz" = 0
  • "200Hz" = 1
  • "400Hz" = 2
  • "600Hz" = 3

Advanced Functions

request/compass_bricklet/<UID>/set_calibration
Request payload:
  • offset -- [int,int,int]
  • gain -- [int,int,int]

Sets offset and gain for each of the three axes.

The Bricklet is factory calibrated. If you want to re-calibrate the Bricklet we recommend that you do the calibration through Brick Viewer.

The calibration is saved in non-volatile memory and only has to be done once.

request/compass_bricklet/<UID>/get_calibration
Response payload:
  • offset -- [int,int,int]
  • gain -- [int,int,int]

Returns the calibration parameters as set by request/compass_bricklet/<UID>/set_calibration.

request/compass_bricklet/<UID>/get_spitfp_error_count
Response payload:
  • error_count_ack_checksum -- int
  • error_count_message_checksum -- int
  • error_count_frame -- int
  • error_count_overflow -- int

Returns the error count for the communication between Brick and Bricklet.

The errors are divided into

  • ACK checksum errors,
  • message checksum errors,
  • framing errors and
  • overflow errors.

The errors counts are for errors that occur on the Bricklet side. All Bricks have a similar function that returns the errors on the Brick side.

request/compass_bricklet/<UID>/set_bootloader_mode
Request payload:
  • mode -- int (has symbols)
Response payload:
  • status -- int (has symbols)

Sets the bootloader mode and returns the status after the requested mode change was instigated.

You can change from bootloader mode to firmware mode and vice versa. A change from bootloader mode to firmware mode will only take place if the entry function, device identifier and CRC are present and correct.

This function is used by Brick Viewer during flashing. It should not be necessary to call it in a normal user program.

The following symbols are available for this function:

for mode:

  • "Bootloader" = 0
  • "Firmware" = 1
  • "BootloaderWaitForReboot" = 2
  • "FirmwareWaitForReboot" = 3
  • "FirmwareWaitForEraseAndReboot" = 4

for status:

  • "OK" = 0
  • "InvalidMode" = 1
  • "NoChange" = 2
  • "EntryFunctionNotPresent" = 3
  • "DeviceIdentifierIncorrect" = 4
  • "CRCMismatch" = 5
request/compass_bricklet/<UID>/get_bootloader_mode
Response payload:
  • mode -- int (has symbols)

Returns the current bootloader mode, see request/compass_bricklet/<UID>/set_bootloader_mode.

The following symbols are available for this function:

for mode:

  • "Bootloader" = 0
  • "Firmware" = 1
  • "BootloaderWaitForReboot" = 2
  • "FirmwareWaitForReboot" = 3
  • "FirmwareWaitForEraseAndReboot" = 4
request/compass_bricklet/<UID>/set_write_firmware_pointer
Request payload:
  • pointer -- int

Sets the firmware pointer for request/compass_bricklet/<UID>/write_firmware. The pointer has to be increased by chunks of size 64. The data is written to flash every 4 chunks (which equals to one page of size 256).

This function is used by Brick Viewer during flashing. It should not be necessary to call it in a normal user program.

request/compass_bricklet/<UID>/write_firmware
Request payload:
  • data -- [int,... (x64)]
Response payload:
  • status -- int

Writes 64 Bytes of firmware at the position as written by request/compass_bricklet/<UID>/set_write_firmware_pointer before. The firmware is written to flash every 4 chunks.

You can only write firmware in bootloader mode.

This function is used by Brick Viewer during flashing. It should not be necessary to call it in a normal user program.

request/compass_bricklet/<UID>/set_status_led_config
Request payload:
  • config -- int (has symbols)

Sets the status LED configuration. By default the LED shows communication traffic between Brick and Bricklet, it flickers once for every 10 received data packets.

You can also turn the LED permanently on/off or show a heartbeat.

If the Bricklet is in bootloader mode, the LED is will show heartbeat by default.

The following symbols are available for this function:

for config:

  • "Off" = 0
  • "On" = 1
  • "ShowHeartbeat" = 2
  • "ShowStatus" = 3
request/compass_bricklet/<UID>/get_status_led_config
Response payload:
  • config -- int (has symbols)

Returns the configuration as set by request/compass_bricklet/<UID>/set_status_led_config

The following symbols are available for this function:

for config:

  • "Off" = 0
  • "On" = 1
  • "ShowHeartbeat" = 2
  • "ShowStatus" = 3
request/compass_bricklet/<UID>/get_chip_temperature
Response payload:
  • temperature -- int

Returns the temperature in °C as measured inside the microcontroller. The value returned is not the ambient temperature!

The temperature is only proportional to the real temperature and it has bad accuracy. Practically it is only useful as an indicator for temperature changes.

request/compass_bricklet/<UID>/reset

Calling this function will reset the Bricklet. All configurations will be lost.

After a reset you have to create new device objects, calling functions on the existing ones will result in undefined behavior!

request/compass_bricklet/<UID>/write_uid
Request payload:
  • uid -- int

Writes a new UID into flash. If you want to set a new UID you have to decode the Base58 encoded UID string into an integer first.

We recommend that you use Brick Viewer to change the UID.

request/compass_bricklet/<UID>/read_uid
Response payload:
  • uid -- int

Returns the current UID as an integer. Encode as Base58 to get the usual string version.

request/compass_bricklet/<UID>/get_identity
Response payload:
  • uid -- string
  • connected_uid -- string
  • position -- char
  • hardware_version -- [int,int,int]
  • firmware_version -- [int,int,int]
  • device_identifier -- int (has symbols)
  • _display_name -- string

Returns the UID, the UID where the Bricklet is connected to, the position, the hardware and firmware version as well as the device identifier.

The position can be 'a', 'b', 'c' or 'd'.

The device identifier numbers can be found here. If symbolic output is not disabled, the device identifier is mapped to the corresponding name in the format used in topics.

The display name contains the Compass's name in a human readable form.

Callback Configuration Functions

request/compass_bricklet/<UID>/set_heading_callback_configuration
Request payload:
  • period -- int
  • value_has_to_change -- bool
  • option -- char (has symbols)
  • min -- int
  • max -- int

The period in ms is the period with which the register/compass_bricklet/<UID>/heading callback is triggered periodically. A value of 0 turns the callback off.

If the value has to change-parameter is set to true, the callback is only triggered after the value has changed. If the value didn't change within the period, the callback is triggered immediately on change.

If it is set to false, the callback is continuously triggered with the period, independent of the value.

It is furthermore possible to constrain the callback with thresholds.

The option-parameter together with min/max sets a threshold for the register/compass_bricklet/<UID>/heading callback.

The following options are possible:

Option Description
'x' Threshold is turned off
'o' Threshold is triggered when the value is outside the min and max values
'i' Threshold is triggered when the value is inside or equal to the min and max values
'<' Threshold is triggered when the value is smaller than the min value (max is ignored)
'>' Threshold is triggered when the value is greater than the min value (max is ignored)

If the option is set to 'x' (threshold turned off) the callback is triggered with the fixed period.

The default value is (0, false, 'x', 0, 0).

The following symbols are available for this function:

for option:

  • "Off" = 'x'
  • "Outside" = 'o'
  • "Inside" = 'i'
  • "Smaller" = '<'
  • "Greater" = '>'
request/compass_bricklet/<UID>/get_heading_callback_configuration
Response payload:
  • period -- int
  • value_has_to_change -- bool
  • option -- char (has symbols)
  • min -- int
  • max -- int

Returns the callback configuration as set by request/compass_bricklet/<UID>/set_heading_callback_configuration.

The following symbols are available for this function:

for option:

  • "Off" = 'x'
  • "Outside" = 'o'
  • "Inside" = 'i'
  • "Smaller" = '<'
  • "Greater" = '>'
request/compass_bricklet/<UID>/set_magnetic_flux_density_callback_configuration
Request payload:
  • period -- int
  • value_has_to_change -- bool

The period in ms is the period with which the register/compass_bricklet/<UID>/magnetic_flux_density callback is triggered periodically. A value of 0 turns the callback off.

If the value has to change-parameter is set to true, the callback is only triggered after the value has changed. If the value didn't change within the period, the callback is triggered immediately on change.

If it is set to false, the callback is continuously triggered with the period, independent of the value.

The default value is (0, false).

request/compass_bricklet/<UID>/get_magnetic_flux_density_callback_configuration
Response payload:
  • period -- int
  • value_has_to_change -- bool

Returns the callback configuration as set by request/compass_bricklet/<UID>/set_magnetic_flux_density_callback_configuration.

Callbacks

Callbacks can be registered to receive time critical or recurring data from the device. The registration is done with the corresponding .../register/... topic and an optional suffix. This suffix can be used to deregister the callback later.

Note

Using callbacks for recurring events is always preferred compared to using getters. It will use less USB bandwidth and the latency will be a lot better, since there is no round trip time.

register/compass_bricklet/<UID>/heading
Request payload:
  • register -- bool
Response payload:
  • heading -- int

A callback can be registered for this event by publishing to the .../register/compass_bricklet/<UID>/heading[/<SUFFIX>] topic with the payload "true". An added callback can be removed by publishing to the same topic with the payload "false". To support multiple (de)registrations, e.g. for message filtering, an optional suffix can be used.

If the callback is triggered, a message with it's payload is published under the corresponding .../callback/compass_bricklet/<UID>/heading[/<SUFFIX>] topic for each registered suffix.

This callback is triggered periodically according to the configuration set by request/compass_bricklet/<UID>/set_heading_callback_configuration.

The callback payload is the same as request/compass_bricklet/<UID>/get_heading.

register/compass_bricklet/<UID>/magnetic_flux_density
Request payload:
  • register -- bool
Response payload:
  • x -- int
  • y -- int
  • z -- int

A callback can be registered for this event by publishing to the .../register/compass_bricklet/<UID>/magnetic_flux_density[/<SUFFIX>] topic with the payload "true". An added callback can be removed by publishing to the same topic with the payload "false". To support multiple (de)registrations, e.g. for message filtering, an optional suffix can be used.

If the callback is triggered, a message with it's payload is published under the corresponding .../callback/compass_bricklet/<UID>/magnetic_flux_density[/<SUFFIX>] topic for each registered suffix.

This callback is triggered periodically according to the configuration set by request/compass_bricklet/<UID>/set_magnetic_flux_density_callback_configuration.

The callback payload members are the same as request/compass_bricklet/<UID>/get_magnetic_flux_density.