C/C++ - Load Cell Bricklet

Note

This Bricklet is currently in the prototype stage and the software/hardware as well as the documentation is in an incomplete state.

This is the description of the C/C++ API bindings for the Load Cell Bricklet. General information and technical specifications for the Load Cell Bricklet are summarized in its hardware description.

An installation guide for the C/C++ API bindings is part of their general description.

Examples

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

Simple

Download (example_simple.c)

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#include <stdio.h>

#include "ip_connection.h"
#include "bricklet_load_cell.h"

#define HOST "localhost"
#define PORT 4223
#define UID "XYZ" // Change to your UID

int main() {
    // Create IP connection
    IPConnection ipcon;
    ipcon_create(&ipcon);

    // Create device object
    LoadCell lc;
    load_cell_create(&lc, UID, &ipcon); 

    // Connect to brickd
    if(ipcon_connect(&ipcon, HOST, PORT) < 0) {
        fprintf(stderr, "Could not connect\n");
        exit(1);
    }
    // Don't use device before ipcon is connected

    // Get current weight (unit is g)
    int32_t weight;
    if(load_cell_get_weight(&lc, &weight) < 0) {
        fprintf(stderr, "Could not get value, probably timeout\n");
        exit(1);
    }

    printf("Weight: %f kg\n", weight/1000.0);

    printf("Press key to exit\n");
    getchar();
    ipcon_destroy(&ipcon); // Calls ipcon_disconnect internally
}

Callback

Download (example_callback.c)

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#include <stdio.h>

#include "ip_connection.h"
#include "bricklet_load_cell.h"

#define HOST "localhost"
#define PORT 4223
#define UID "XYZ" // Change to your UID

// Callback function for weight callback (parameter has unit g/10)
void cb_weight(uint16_t weight, void *user_data) {
    (void)user_data; // avoid unused parameter warning

    printf("Weight: %f kg\n", weight/1000.0);
}

int main() {
    // Create IP connection
    IPConnection ipcon;
    ipcon_create(&ipcon);

    // Create device object
    LoadCell lc;
    load_cell_create(&lc, UID, &ipcon); 

    // Connect to brickd
    if(ipcon_connect(&ipcon, HOST, PORT) < 0) {
        fprintf(stderr, "Could not connect\n");
        exit(1);
    }
    // Don't use device before ipcon is connected

    // Set Period for weight callback to 1s (1000ms)
    // Note: The weight callback is only called every second if the 
    //       weight has changed since the last call!
    load_cell_set_weight_callback_period(&lc, 1000);

    // Register weight callback to function cb_weight
    load_cell_register_callback(&lc,
                                LOAD_CELL_CALLBACK_WEIGHT,
                                (void *)cb_weight,
                                NULL);

    printf("Press key to exit\n");
    getchar();
    ipcon_destroy(&ipcon); // Calls ipcon_disconnect internally
}

Threshold

Download (example_threshold.c)

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#include <stdio.h>

#include "ip_connection.h"
#include "bricklet_load_cell.h"

#define HOST "localhost"
#define PORT 4223
#define UID "XYZ" // Change to your UID

// Callback for weight greater than 200 g
void cb_reached(uint16_t weight, void *user_data) {
    (void)user_data; // avoid unused parameter warning

    printf("Weight %f kg\n", weight/1000.0);
}

int main() {
    // Create IP connection
    IPConnection ipcon;
    ipcon_create(&ipcon);

    // Create device object
    LoadCell lc;
    load_cell_create(&lc, UID, &ipcon); 

    // Connect to brickd
    if(ipcon_connect(&ipcon, HOST, PORT) < 0) {
        fprintf(stderr, "Could not connect\n");
        exit(1);
    }
    // Don't use device before ipcon is connected

    // Get threshold callbacks with a debounce time of 10 seconds (10000ms)
    load_cell_set_debounce_period(&lc, 10000);

    // Register threshold reached callback to function cb_reached
    load_cell_register_callback(&lc,
                                LOAD_CELL_CALLBACK_WEIGHT_REACHED,
                                (void *)cb_reached,
                                NULL);

    // Configure threshold for "greater than 200 g"
    load_cell_set_weight_callback_threshold(&lc, '>', 200, 0);

    printf("Press key to exit\n");
    getchar();
    ipcon_destroy(&ipcon); // Calls ipcon_disconnect internally
}

API

Every function of the C/C++ bindings returns an integer which describes an error code. Data returned from the device, when a getter is called, is handled via call by reference. These parameters are labeled with the ret_ prefix.

Possible error codes are:

  • E_OK = 0
  • E_TIMEOUT = -1
  • E_NO_STREAM_SOCKET = -2
  • E_HOSTNAME_INVALID = -3
  • E_NO_CONNECT = -4
  • E_NO_THREAD = -5
  • E_NOT_ADDED = -6 (unused since bindings version 2.0.0)
  • E_ALREADY_CONNECTED = -7
  • E_NOT_CONNECTED = -8
  • E_INVALID_PARAMETER = -9
  • E_NOT_SUPPORTED = -10
  • E_UNKNOWN_ERROR_CODE = -11

as defined in ip_connection.h.

All functions listed below are thread-safe.

Basic Functions

void load_cell_create(LoadCell *load_cell, const char *uid, IPConnection *ipcon)

Creates the device object load_cell with the unique device ID uid and adds it to the IPConnection ipcon:

LoadCell load_cell;
load_cell_create(&load_cell, "YOUR_DEVICE_UID", &ipcon);

This device object can be used after the IP connection has been connected (see examples above).

void load_cell_destroy(LoadCell *load_cell)

Removes the device object load_cell from its IPConnection and destroys it. The device object cannot be used anymore afterwards.

int load_cell_get_weight(LoadCell *load_cell, int32_t *ret_weight)

Returns the currently measured weight in grams.

If you want to get the weight periodically, it is recommended to use the callback LOAD_CELL_CALLBACK_WEIGHT and set the period with load_cell_set_weight_callback_period().

int load_cell_led_on(LoadCell *load_cell)

Turns the LED on.

int load_cell_led_off(LoadCell *load_cell)

Turns the LED off.

int load_cell_is_led_on(LoadCell *load_cell, uint8_t *ret_value)

Returns true if the led is on, false otherwise.

int load_cell_tare(LoadCell *load_cell)

Sets the currently measured weight as tare weight.

Advanced Functions

int load_cell_set_moving_average(LoadCell *load_cell, uint8_t average)

Sets the length of a moving averaging for the weight value.

Setting the length to 1 will turn the averaging off. With less averaging, there is more noise on the data.

The range for the averaging is 1-40.

The default value is 4.

int load_cell_get_moving_average(LoadCell *load_cell, uint8_t *ret_average)

Returns the length moving average as set by load_cell_set_moving_average().

int load_cell_calibrate(LoadCell *load_cell, uint32_t weight)

To calibrate your Load Cell Bricklet you have to

  • empty the scale and call this function with 0 and
  • add a known weight to the scale and call this function with the weight in grams.

The calibration is saved in the EEPROM of the Bricklet and only needs to be done once.

We recommend to use the Brick Viewer for calibration, you don't need to call this function in your source code.

int load_cell_set_configuration(LoadCell *load_cell, uint8_t rate, uint8_t gain)

The measurement rate and gain are configurable.

The rate can be either 10Hz or 80Hz. A faster rate will produce more noise. It is additionally possible to add a moving average (see load_cell_set_moving_average()) to the measurements.

The gain can be 128x, 64x or 32x. It represents a measurement range of ±20mV, ±40mV and ±80mV respectively. The Load Cell Bricklet uses an excitation voltage of 5V and most load cells use an output of 2mV/V. That means the voltage range is ±15mV for most load cells (i.e. gain of 128x is best). If you don't know what all of this means you should keep it at 128x, it will most likely be correct.

The configuration is saved in the EEPROM of the Bricklet and only needs to be done once.

We recommend to use the Brick Viewer for configuration, you don't need to call this function in your source code.

The default rate is 10Hz and the default gain is 128x.

The following defines are available for this function:

  • LOAD_CELL_RATE_10HZ = 0
  • LOAD_CELL_RATE_80HZ = 1
  • LOAD_CELL_GAIN_128X = 0
  • LOAD_CELL_GAIN_64X = 1
  • LOAD_CELL_GAIN_32X = 2
int load_cell_get_configuration(LoadCell *load_cell, uint8_t *ret_rate, uint8_t *ret_gain)

Returns the configuration as set by load_cell_set_configuration().

The following defines are available for this function:

  • LOAD_CELL_RATE_10HZ = 0
  • LOAD_CELL_RATE_80HZ = 1
  • LOAD_CELL_GAIN_128X = 0
  • LOAD_CELL_GAIN_64X = 1
  • LOAD_CELL_GAIN_32X = 2
int load_cell_get_api_version(LoadCell *load_cell, uint8_t ret_api_version[3])

Returns the version of the API definition (major, minor, revision) implemented by this API bindings. This is neither the release version of this API bindings nor does it tell you anything about the represented Brick or Bricklet.

int load_cell_get_response_expected(LoadCell *load_cell, uint8_t function_id, bool *ret_response_expected)

Returns the response expected flag for the function specified by the function ID parameter. It is true if the function is expected to send a response, false otherwise.

For getter functions this is enabled by default and cannot be disabled, because those functions will always send a response. For callback configuration functions it is enabled by default too, but can be disabled by load_cell_set_response_expected(). For setter functions it is disabled by default and can be enabled.

Enabling the response expected flag for a setter function allows to detect timeouts and other error conditions calls of this setter as well. The device will then send a response for this purpose. If this flag is disabled for a setter function then no response is send and errors are silently ignored, because they cannot be detected.

See load_cell_set_response_expected() for the list of function ID defines available for this function.

int load_cell_set_response_expected(LoadCell *load_cell, uint8_t function_id, bool response_expected)

Changes the response expected flag of the function specified by the function ID parameter. This flag can only be changed for setter (default value: false) and callback configuration functions (default value: true). For getter functions it is always enabled and callbacks it is always disabled.

Enabling the response expected flag for a setter function allows to detect timeouts and other error conditions calls of this setter as well. The device will then send a response for this purpose. If this flag is disabled for a setter function then no response is send and errors are silently ignored, because they cannot be detected.

The following function ID defines are available for this function:

  • LOAD_CELL_FUNCTION_SET_WEIGHT_CALLBACK_PERIOD = 2
  • LOAD_CELL_FUNCTION_SET_WEIGHT_CALLBACK_THRESHOLD = 4
  • LOAD_CELL_FUNCTION_SET_DEBOUNCE_PERIOD = 6
  • LOAD_CELL_FUNCTION_SET_MOVING_AVERAGE = 8
  • LOAD_CELL_FUNCTION_LED_ON = 10
  • LOAD_CELL_FUNCTION_LED_OFF = 11
  • LOAD_CELL_FUNCTION_CALIBRATE = 13
  • LOAD_CELL_FUNCTION_TARE = 14
  • LOAD_CELL_FUNCTION_SET_CONFIGURATION = 15
int load_cell_set_response_expected_all(LoadCell *load_cell, bool response_expected)

Changes the response expected flag for all setter and callback configuration functions of this device at once.

int load_cell_get_identity(LoadCell *load_cell, char ret_uid[8], char ret_connected_uid[8], char *ret_position, uint8_t ret_hardware_version[3], uint8_t ret_firmware_version[3], uint16_t *ret_device_identifier)

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. There is also a constant for the device identifier of this Bricklet.

Callback Configuration Functions

void load_cell_register_callback(LoadCell *load_cell, uint8_t id, void *callback, void *user_data)

Registers a callback with ID id to the function callback. The user_data will be given as a parameter of the callback.

The available IDs with corresponding function signatures are listed below.

int load_cell_set_weight_callback_period(LoadCell *load_cell, uint32_t period)

Sets the period in ms with which the LOAD_CELL_CALLBACK_WEIGHT callback is triggered periodically. A value of 0 turns the callback off.

LOAD_CELL_CALLBACK_WEIGHT is only triggered if the weight has changed since the last triggering.

The default value is 0.

int load_cell_get_weight_callback_period(LoadCell *load_cell, uint32_t *ret_period)

Returns the period as set by load_cell_set_weight_callback_period().

int load_cell_set_weight_callback_threshold(LoadCell *load_cell, char option, int16_t min, int16_t max)

Sets the thresholds for the LOAD_CELL_CALLBACK_WEIGHT_REACHED callback.

The following options are possible:

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

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

The following defines are available for this function:

  • LOAD_CELL_THRESHOLD_OPTION_OFF = 'x'
  • LOAD_CELL_THRESHOLD_OPTION_OUTSIDE = 'o'
  • LOAD_CELL_THRESHOLD_OPTION_INSIDE = 'i'
  • LOAD_CELL_THRESHOLD_OPTION_SMALLER = '<'
  • LOAD_CELL_THRESHOLD_OPTION_GREATER = '>'
int load_cell_get_weight_callback_threshold(LoadCell *load_cell, char *ret_option, int16_t *ret_min, int16_t *ret_max)

Returns the threshold as set by load_cell_set_weight_callback_threshold().

The following defines are available for this function:

  • LOAD_CELL_THRESHOLD_OPTION_OFF = 'x'
  • LOAD_CELL_THRESHOLD_OPTION_OUTSIDE = 'o'
  • LOAD_CELL_THRESHOLD_OPTION_INSIDE = 'i'
  • LOAD_CELL_THRESHOLD_OPTION_SMALLER = '<'
  • LOAD_CELL_THRESHOLD_OPTION_GREATER = '>'
int load_cell_set_debounce_period(LoadCell *load_cell, uint32_t debounce)

Sets the period in ms with which the threshold callback

is triggered, if the threshold

keeps being reached.

The default value is 100.

int load_cell_get_debounce_period(LoadCell *load_cell, uint32_t *ret_debounce)

Returns the debounce period as set by load_cell_set_debounce_period().

Callbacks

Callbacks can be registered to receive time critical or recurring data from the device. The registration is done with the load_cell_register_callback() function. The parameters consist of the device object, the callback ID, the callback function and optional user data:

void my_callback(int p, void *user_data) {
    printf("parameter: %d\n", p);
}

load_cell_register_callback(&load_cell, LOAD_CELL_CALLBACK_EXAMPLE, (void *)my_callback, NULL);

The available constants with corresponding callback function signatures are described below.

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.

LOAD_CELL_CALLBACK_WEIGHT
void callback(uint32_t weight, void *user_data)

This callback is triggered periodically with the period that is set by load_cell_set_weight_callback_period(). The parameter is the weight as measured by the load cell.

LOAD_CELL_CALLBACK_WEIGHT is only triggered if the weight has changed since the last triggering.

LOAD_CELL_CALLBACK_WEIGHT_REACHED
void callback(int16_t weight, void *user_data)

This callback is triggered when the threshold as set by load_cell_set_weight_callback_threshold() is reached. The parameter is the weight as measured by the load cell.

If the threshold keeps being reached, the callback is triggered periodically with the period as set by load_cell_set_debounce_period().

Constants

LOAD_CELL_DEVICE_IDENTIFIER

This constant is used to identify a Load Cell Bricklet.

The load_cell_get_identity() function and the IPCON_CALLBACK_ENUMERATE callback of the IP Connection have a device_identifier parameter to specify the Brick's or Bricklet's type.

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