-- Communicating With Multiple 1-Wire Devices  --

The Schematic

Shown above is a simple schematic for a 1-wire network. This shows how you might connect multiple 1-wire components on a small network such as the bread board, or short wire network.

Notice from the schematic how the DS2405 addressable switch outputs are connected to 2N3904 NPN transistors. This is because the DS2405 outputs can only provide about 4mA sink [ground] output current. The 2N3904 NPN transistors allow the DS2405 limited capacity outputs to easily control higher current devices.

Since the open-drain outputs of the DS2405 addressable switches cannot produce a logic 1 output, the 10K pull-up resistors are included to pull the NPN transistor base to logic 1, and turn on the LEDs. When the DS2405 output is on, it will apply ground to the base of the NPN transistors, turning the LEDs off.

PicBasic Pro 1-Wire Command Mode Switches

The 1-wire commands in PicBasic Pro include mode switches for controlling the reset pulses, and data formats for the 1-wire bus. The printed PBP manual shows the mode configurations like this:

Some people seem confused by this, so I'll show it in a different format like this:

This effectively gives us 8 possible modes of operation with the PicBasic Pro 1-wire commands. Here's why we would want separate modes when communicating with 1-wire components.

Once we have established communications with a specific 1-wire device on the network by issuing the Match ROM command, followed by the unique 64-bit ROM code for a specific 1-wire component, the rest of the 1-wire devices on the network will ignore all data placed on the 1-wire communications bus until a 1-wire bus RESET has been issued.

Reset, Bit, and Byte Modes & Why We Need Them

The reset pulse is used to end communications with a specific device on the network, and also to get the attention all 1-wire devices on the network. Think of a reset pulse as being OK, we're finished, now everyone else listen up. Byte-sized data informs the compiler that all data being sent/received on the 1-wire network will be in byte-sized data packets.

The reset pulse simply resets all 1-wire devices on the network, and gets them all to ready receive commands.

Example:

OWOUT DQ, 1, [$33]  ' Read ROM command, byte-sized data, reset before data

This will reset the 1-wire bus, [Send reset pulse before byte-sized data], and issue the byte-sized Read ROM command. This will work [only] if there is a single 1-wire device on the network. If there are more than a single 1-wire device on the network, we need to do the following:

Switch_1:
    OWOUT DQ, 1, [$55,$05,$C5,$C3,$08,$00,$00,$00,$CD]' Match ROM, byte-sized, reset before
    OWIN DQ, 4, [Stat]' Get bit-sized response, no reset pulse
    IF Stat = 0 THEN
       DEBUG Ins, Line2, "Switch #1 = OFF"
    ELSE
       DEBUG Ins, Line2, "Switch #1 = ON "
    ENDIF

The example above shows how to individually address 1 DS2405 addressable 1-wire switch on the network. Notice we have substituted the Read ROM command with the Match ROM command. The red hexadecimal number $55 is the command for Match ROM. The blue hexadecimal numbers $05,$C5,$C3,$08,$00,$00,$00,$CD are the unique 64-bit ROM code numbers inside the specific DS2405 addressable 1-wire switch we're trying to communicate with.

What we're doing with --> OWOUT DQ, 1, [$55,$05,$C5,$C3,$08,$00,$00,$00,$CD]is: Attention, Match the following ROM code. Were leaving the 1-wire bus open for communications since we didn't RESET the 1-wire bus after we have sent several byte-sized data packets containing the ROM code. The device matching the ROM code [as shown in blue above], will respond if it's present on the 1-wire bus.

What we're doing with --> OWIN DQ, 4, [Stat]is: Reading the bit-sized data from the 1-wire bus into the bit-sized variable Stat, without issuing a reset pulse. The DS2405 will return a logic 0 on the 1-wire bus if its switch output PIO is ON or at logic 0. If the switch output is OFF, the DS2405 will return bit-sized data at logic 1, indicating to the Master controller that its switch output is OFF. Refer to the schematic at top again, and you'll notice that logic 0 will turn the NPN transistors OFF, and logic 1 will turn them ON again.

If you jumped into this page without reading the previous article showing how to read individual ROM codes from 1-wire components, visit this link HERE to see how this is done. You'll need to individually identify each of the 1-wire component unique 64-bit ROM codes before you can make sense of this article. Knowing the unique 64-bit ROM codes for all devices you're using [if you're building this project], is essential.

Let's move on to more of the mode switches, and see how they work with the DS18B20 temperature sensor on the 1-wire network.

Temp_1:
    OWOUT DQ, 1, [$55,$28,$B1,$FE,$22,$00,$00,$00,$5D,$44]
W1:
    OWIN DQ, 4, [Stat]' Check for still busy converting
    IF Stat = 0 THEN W1' Still busy?, then loop
    OWOUT DQ, 1,[$55,$28,$B1,$FE,$22,$00,$00,$00,$5D,$BE]
    OWIN DQ, 2, [Temp.LOWBYTE,Temp.HIGHBYTE]' Read two bytes, then end communications
    GOSUB Convert_Temp
    DEBUG Ins,Line1, "Temp #1 = ", Sign, DEC (Temp / 100),".", DEC2 Temp, Deg, "F "

Notice from the code sample above that we have again issued the Match ROM command $55, and left the 1-wire bus open for communications. We next use mode 4 [bit-sized data, no reset pulse], and OWIN command to read the bit-sized response from the device addressed with the unique 64-bit ROM code $28,$B1,$FE,$22,$00,$00,$00,$5D.

Did you notice that the $44 is missing from end of the original first line of code? $44 is the last byte of data sent to the DS18B20 after the ROM code, and is telling the DS18B20 to begin a temperature conversion. I have made commands RED, and data BLUE for easy recognition.

What we're doing here --> OWIN DQ, 4, [Stat]: is reading the bit-sized response from the DS18B20. A logic 0 means the DS18B20 is still performing the temperature conversion, so we wait until it's finished before proceeding.

Notice how mode 4 allows us to read single-bit responses from the 1-wire bus without resetting the bus. This is important to remember if you're going to build a reliable 1-wire network. Sometimes the Master 1-wire controller will be required to read single-bit data, and be able to control [reset or no reset] the 1-wire bus as necessary. That's why it's important to carefully read the datasheet for any 1-wire device you're planning to use, and learn how to use the PicBasic Pro 1-wire command mode switches.

Now We'll Use Mode 2

Remember how mode 2 will send a reset pulse after sending data, and mode 1 sends the reset pulse before sending data? This is important, and here's why:

The reset pulse gets the attention of each device on the 1-wire network, and also terminates any communications with a previous device. The next couple lines of code perform the following tasks.

OWOUT DQ, 1,[$55,$28,$B1,$FE,$22,$00,$00,$00,$5D,$BE]

OWIN DQ, 2, [Temp.LOWBYTE,Temp.HIGHBYTE]' Read two bytes, then end communications

The sample does pretty much the same thing we have been doing previously with the first line of code, but does something a little different in line #2. If you look at the datasheet for the DS18B20, you'll notice that once the Read Scratchpad Memory command is issued, the DS18B20 will output all 9-bytes of data in the scratchpad memory, [unless] the master issues a reset pulse during the transaction. Once the master issues the reset pulse, the device will end communications.

We're only concerned with the first two data-bytes [as shown below] containing the temperature results, so we end communications after reading only these two bytes by issuing the reset pulse [after reading two bytes].

Mode #2 allows reading only as much data from a device as necessary for a specific operation, then terminating communications with the 1-wire component. Mode 2 can cut down the time required for 1-wire bus communications, and is handy to remember. No sense reading 7 additional bytes of data if we really don't need it for a specific task.

That pretty much covers it. Examine the complete code sample below, and you'll see how first knowing the unique 64-bit numbers [ROM Code] contained within each Dallas 1-wire component comes in handy for accessing each device on the 1-wire network individually.

The code sample below has several features.

Possible uses for this project include, but are not limited to:

Get creative......!

Connections to the PIC

Notes:

Have fun with the project. You may want to visit the Dallas/Maxim web site, and download datasheets for each 1-wire device you're using. You can find this information here http://www.maxim-ic.com I most highly recommend downloading every datasheet for the 1-wire devices you're using, and thoroughly reading each one to become familiar with specific register settings, and the specifics for each 1-wire component. Explaining this all in detail here would take me months....;o]

The Code

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