-- Reading ROM Codes From 1-Wire Devices  --

If you're planning to use 1-wire components you need to identify individual components before placing them on the 1-wire network, this little project comes in handy for doing this.

Components required for this project:

You're not necessarily limited to using the components listed above. The code is easy to convert to any PIC, and the serial LCD can easily be replaced by using any RS-232 terminal program.

By first identifying each 1-wire device, and recording the unique 64-bit ROM number contained within each device, you can then communicate with individual 1-wire components on the 1-wire bus. To do this you use the "Match Rom" command followed by the unique 64-bit ROM number matching a specific 1-wire component on the network you wish to communicate with.

Here's an example of talking to a 1-wire DS2405 addressable switch located on the 1-wire network.

Note: You can toggle the output of the DS2405 by simply sending the Match Rom command followed by the 64-bit ROM code of the switch. This ROM code is unique from one DS2405 to the other.

Assuming you have first read the unique 64-bit ROM code from the DS2405 switch:

' Use the routine below to toggle the addressable switch
Toggle_Switch:
    OWOUT DQ, 1, [$55,$05,$C5,$C3,$08,$00,$00,$00,$CD]

The Match ROM command is $55 or 55h. Following the Match ROM command is the 64-bit ROM code read from the DS2405 as shown in the table below. Notice data is sent LSB first. When communicating with devices on the 1-wire network, data is always read/written LSB first.

Refer to the datasheet for the specific device you plan to use. The datasheet will show a table similar to the grey section of the table above. All 64-bits of data in the table above were first read from a DS2405 switch placed in circuit with the code example shown below. $55 is the Match Rom command. Following the Match Rom command is the unique 64-bit ROM code read from the DS2405 addressable switch. This allows the PicBasic Pro command OWOUT to address this particular DS2405 addressable switch on the 1-wire network.

The photo above shows the LCD display with the DS2405 inserted in the circuit. Here's the simple circuit to build for this project for connecting 1-wire components to the PIC.

After sending the unique ROM code matching the switch you wish to control, this device matching the unique ROM code will be the only one on the 1-wire net that responds to further commands. The remaining 1-wire components on the network will sit by waiting for a reset pulse.

Since the DS2405 doesn't require a specific command to toggle its switch output from 0-1 or 1-0, you're finished commanding the DS2405 to toggle its switch output. Issuing the exact same sequence [shown above] again, will command the DS2405 to toggle its output once again.

The DS18B20 Temperature Sensor

Below is a photo of the results shown on the serial LCD after we insert the DS18B20 temperature sensor into the 1-wire device identifier circuit.

Notice how each different 1-wire device contains a totally unique 64-bit ROM code that identifies the:

1. 8-Bit Family Code
2. Unique 48-Bit Serial Number
3. 8-Bit CRC Code

This project shows how to identify each 1-wire device prior to inserting them into the 1-wire network. By identifying each component you'll know the unique 64-bit ROM code for each 1-wire device, and be able to talk to individual components on your 1-wire network as necessary. You can use the 1-wire command Search ROM [$F0], but this doesn't tell you where each device is physically located on the 1-wire network. It only identifies all components on the network.

The next project will give more detailed explanations of how the PicBasic Pro OWIN & OWOUT mode numbers affect different devices on the 1-wire network.

LCD Connections

The 20 x 4 LCD used for this project is a standard backlit serial LCD similar to this Serial LCD , but any serial LCD should work. You may or may not need to change the LCD commands, depending on which type you use. Just connect PortB.0 to the serial 'data' input of the one you use.

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