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Onewire

Latest Version: 1.0.1

This library provides a means to interact with 1-Wire* devices connected to an imp via a UART bus. For more information on connecting 1-Wire devices this way, see ‘Implementing a 1-Wire Bus on the imp’.

The class provides a means to enumerate all the 1-Wire devices connected to the host imp, and to communicate with any one of them. It provides methods for the five key 1-Wire bus commands.

You can view the library’s source code on GitHub. Click here to see information on the available versions of this library.

To add this library to your project, add #require "Onewire.class.nut:1.0.1" to the top of your device code

*1-Wire is a registered trademark ® of Maxim Integrated.

Class Usage

Constructor: Onewire(impUart, [debug])

To instantiate a new Onewire object, just pass the imp UART bus object you are using to drive your 1-Wire device(s). This bus should not be pre-configured; the class will apply two separate configuations — one for testing the bus, another for interacting with devices — while it is running.

Optionally, you may also pass a debugging flag: pass true to trigger debug log messages. This is false by default.

The constructor returns null if no UART bus is passed into it.

#require "Onewire.class.nut:1.0.1"

ow <- Onewire(hardware.uart12);

Class Methods

init()

Call this method to prepare the 1-Wire/UART bus for use. It resets the bus and enumerates the bus’ devices.

It returns true for success, or false to indicate a bus-read failure:

local success = ow.init();
if (success) {
    // Report error. Note if the Onewire object instantiated in
    // debug mode, the object will report errors
    server.error("1-Wire error.");
    return;

    // Perform tasks on the 1-Wire bus...
}

reset()

This method reset the 1-Wire bus and checks for connected devices, but does not enumerate them. It returns true if it detects a 1-Wire bus with one or more 1-Wire peripherals, false otherwise. It is implicitly called by init().

reset() clears and, in it encounters an error, an internal error record. If a call to reset() returns false, this error record can be read using the method getErrorCode().

if (ow.reset()) {
    // Valid 1-Wire bus detected
    local temp = getTemperatureReading();
} else {
    // Display error code on LED
    local err = ow.getErrorCode();
    led.display("Error: " + err);
}

discoverDevices()

This method enumerates all the 1-Wire devices on the bus, storing connected devices’ unique IDs internally. See the further methods below for means to access these devices. It is implicitly called by init().

if (ow.reset()) {
    // Valid 1-Wire bus detected, so enumerate the bus
    ow.discoverDevices();
} else {
    // Display error code on LED
    local err = ow.getErrorCode();
    led.display("Error: " + err.msg);
}

getDeviceCount()

This method returns the number of 1-Wire devices on the bus. Note that this will be zero if you have not first called init() or reset() and discoverDevices().

local success = ow.init();
if (success) {
    local n = ow.getDeviceCount();
    server.log("You have " + n + " 1-Wire devices connected to your imp.");
}

getDevices()

This method returns an array in which the ID of each device on the 1-Wire bus is stored. If the returned array is empty, there are either no devices on the bus or your code has yet to enumerate them (using init() or discoverDevices()).

local success = ow.init();
if (success) {
    local devices = ow.getDevices();
    if (devices.len() > 0) {
        foreach (index, device in devices) {
            server.log("Device " + index + ": " + device);
        }
    }
}

getDevice(deviceIndex)

This method returns the ID of the device at index deviceIndex in the list, or null if the value of deviceIndex is out of range.

local success = ow.init();
if (success) {
    local numDevs = getDeviceCount();
    for (local i = 0 ; i < numDevs ; i++) {
      local device = ow.getDevice();
      server.log("Device " + index + ": " + device);
    }
}

getErrorCode()

This method returns error information arising from the last bus reset; if init() or reset() returns false, you can use this method to learn more about the cause of the failure. getErrorCode() returns a table with two keys, code and msg:

Code Message
0 No error.
1 No 1-Wire devices connected.
2 No 1-Wire circuit detected.

 
See reset(), above, for example code.

writeByte(byte)

This method writes the supplied byte value to the 1-Wire bus. The value of byte is tyically an integer; the method only writes the first eight bits of the 32-bit Squirrel integer.

// Signal the sensor to take a reading
ow.reset();
ow.skipRom();
ow.writeByte(0x44);

// Give the sensor enough time for sampling
imp.sleep(0.8);

// Tell the sensor to transmit the reading
ow.reset();
ow.skipRom();
ow.writeByte(0xBE);

readByte()

This method reads a eight bits of data from the 1-Wire bus and returns it as an integer.

// Read the temperature
local tempLSB = onew.readByte();
local tempMSB = onew.readByte();

skipRom()

This method writes the 1-Wire command ‘Skip ROM’ to the bus, which tells it to ignore a supplied device ID in the next transaction. It is intended to be used if there is only one device or to send subsequent commands to all devices.

// Signal the sensor to take a reading
ow.reset();
ow.skipRom();
ow.writeByte(0x44);

readRom()

This method writes the 1-Wire command ‘Read ROM’ to the bus: read a device’s ID. It is used when there is only one slave device on the bus.

searchRom()

This method writes the 1-Wire command ‘Search ROM’ to the bus, to prepare it for device enumeration.

matchRom()

This method writes the 1-Wire command ‘Match ROM’ to the bus. This indicates a specific device is to be selected and that the device’s ID will be the next 64 bits to be written.

local devices = ow.getDevices();
foreach (index, device in devices) {
    // Run through the list of discovered slave devices, getting the temperature
    // if a given device is of the correct family number: 0x28 for BS18B20
    if (device[7] == 0x28) {
        ow.reset();

        // Issue 1-Wire MATCH ROM command to select device by ID
        ow.matchRom();

        // Write out the 64-bit ID from the array's eight bytes
        for (local i = 7 ; i >= 0 ; i--) {
            ow.writeByte(device[i]);
        }

        // Issue the DS18B20's READ SCRATCHPAD command (0xBE) to get temperature
        ow.writeByte(0xBE);

        // Read the temperature value from the sensor's RAM
        local tempLSB = ow.readByte();
        local tempMSB = ow.readByte();

        // Signal that we don't need any more data by resetting the bus
        onewireReset();

        // Calculate the temperature from LSB and MSB
        local tempCelsius = ((tempMSB * 256) + tempLSB) / 16.0;
        server.log("The temperature is " + tempCelsius + "C");
    }
}

Release History

The Electric Imp Dev Center documents the latest version of the library. For past versions, please see the Electric Imp public GitHub repos listed below.

Version Source Code Notes
1.0.0 GitHub Initial release
1.0.1 GitHub Minor code changes

License

The Onewire library is licensed under the MIT License.