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OneWire

The idea is to configure Arduino Pins as OneWire Busmaster. The may be more than one pin configured for OneWire and there may be more than one device connected to such a pin.

Each one-wire-device has a unique identifier which is 8 bytes long and comes factory-programmed into the the device. To scan all devices connected to a pin configured for onewire a SEARCH-request message is sent. The response contains all addresses of devices found. Having the address of a device OneWire-command-messages may be sent to this device.

The actual commands executed on the OneWire-bus are 'reset', 'skip', 'select', 'read', 'delay' and 'write' All these commands may be executed with a single OneWire-command-message. The subcommand-byte contains these commands bit-encoded. The data required to execute each bus-command must only be included in the message when the corresponding bit is set.

The order of execution of bus commands is: 'reset'->'skip'->'select'->'write'->'read'->'delay' (remember: each of these steps is optional. Also some combinations don't make sense and in fact are mutual exclusive in terms of OneWire bus protocol, so you cannot run a 'skip' followed by a 'select') The delay is useful for OneWire-commands included into taskdata (see Firmata-scheduler proposal).

Some OneWire-devices require some time to carry out e.g. a a/d-conversion after receiving the appropriate command. Including a delay into a OneWire-message saves some bytes in the taskdata (in comparism to the inclusion of a 'delay_task' scheduler message). OneWire Read- and ReadReply messages are correlated using a correlationid (16bits). The reply contains the correlationid-value that was sent with the original request.

Added in Firmata protocol version 2.4.0.

Example files:

Compatible host implementations

Compatible client librairies

Protocol details

OneWire SEARCH request

0  START_SYSEX      (0xF0)
1  OneWire Command  (0x73)
2  search command   (0x40|0x44) 0x40 normal search for all devices on the bus
                                0x44 SEARCH_ALARMS request to find only those
                                devices that are in alarmed state.
3  pin              (0-127)
4  END_SYSEX        (0xF7)

OneWire SEARCH reply

0  START_SYSEX      (0xF0)
1  OneWire Command  (0x73)
2  search reply command (0x42|0x45) 0x42 normal search reply
                                    0x45 reply to a SEARCH_ALARMS request
3  pin              (0-127)
4  bit 0-6   [optional] address bytes encoded using 8 times 7 bit for 7 bytes of 8 bit
5  bit 7-13  [optional] 1.address[0] = byte[0]    + byte[1]<<7 & 0x7F
6  bit 14-20 [optional] 1.address[1] = byte[1]>>1 + byte[2]<<6 & 0x7F
7  ....                 ...
11 bit 49-55            1.address[6] = byte[6]>>6 + byte[7]<<1 & 0x7F
12 bit 56-63            1.address[7] = byte[8]    + byte[9]<<7 & 0x7F
13 bit 64-69            2.address[0] = byte[9]>>1 + byte[10]<<6 &0x7F
n  ... as many bytes as needed (don't exceed MAX_DATA_BYTES though)
n+1  END_SYSEX      (0xF7)

OneWire CONFIG request

0  START_SYSEX      (0xF0)
1  OneWire Command  (0x73)
2  config command   (0x41)
3  pin              (0-127)
4  power            (0x00|0x01) 0x00 = leave pin on state high after write to support
                                parasitic power
                                0x01 = don't leave pin on high after write
5  END_SYSEX (0xF7)

OneWire COMMAND request

0  START_SYSEX      (0xF0)
1  OneWire Command  (0x73)
2  command bits     (0x00-0x2F) bit 0 = reset, bit 1 = skip, bit 2 = select,
                                bit 3 = read, bit 4 = delay, bit 5 = write
3  pin              (0-127)
4  bit 0-6   [optional] data bytes encoded using 8 times 7 bit for 7 bytes of 8 bit
5  bit 7-13  [optional] data[0] = byte[0]   + byte[1]<<7 & 0x7F
6  bit 14-20 [optional] data[1] = byte[1]>1 + byte[2]<<6 & 0x7F
7  ....                 data[2] = byte = byte[2]>2 + byte[3]<<5 & 0x7F ...
n  ... as many bytes as needed (don't exceed MAX_DATA_BYTES though)
n+1  END_SYSEX      (0xF7)

// data bytes within OneWire Request Command message
0  address[0]                    [optional, if bit 2 set]
1  address[1]                              "
2  address[2]                              "
3  address[3]                              "
4  address[4]                              "
5  address[5]                              "
6  address[6]                              "
7  address[7]                              "
8  number of bytes to read (LSB) [optional, if bit 3 set]
9  number of bytes to read (MSB)           "
10 request correlationid byte 0            "
11 request correlationid byte 1            "
10 delay in ms      (bits 0-7)   [optional, if bit 4 set]
11 delay in ms      (bits 8-15)            "
12 delay in ms      (bits 16-23)           "
13 delay in ms      (bits 24-31)           "
14 data to write    (bits 0-7)   [optional, if bit 5 set]
15 data to write    (bits 8-15)            "
16 data to write    (bits 16-23)           "
n  ... as many bytes as needed (don't exceed MAX_DATA_BYTES though)

OneWire READ reply

0  START_SYSEX          (0xF0)
1  OneWire Command      (0x73)
2  read reply command   (0x43)
3  pin                  (0-127)
4  bit 0-6   [optional] data bytes encoded using 8 times 7 bit for 7 bytes of 8 bit
5  bit 7-13  [optional] correlationid[0] = byte[0]   + byte[1]<<7 & 0x7F
6  bit 14-20 [optional] correlationid[1] = byte[1]>1 + byte[2]<<6 & 0x7F
7  bit 21-27 [optional] data[0] = byte[2]>2 + byte[3]<<5 & 0x7F
8  ....                 data[1] = byte[3]>3 + byte[4]<<4 & 0x7F
n  ... as many bytes as needed (don't exceed MAX_DATA_BYTES though)
n+1  END_SYSEX          (0xF7)
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