linux/Documentation/w1/w1.generic
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   1The 1-wire (w1) subsystem
   2------------------------------------------------------------------
   3The 1-wire bus is a simple master-slave bus that communicates via a single
   4signal wire (plus ground, so two wires).
   5
   6Devices communicate on the bus by pulling the signal to ground via an open
   7drain output and by sampling the logic level of the signal line.
   8
   9The w1 subsystem provides the framework for managing w1 masters and
  10communication with slaves.
  11
  12All w1 slave devices must be connected to a w1 bus master device.
  13
  14Example w1 master devices:
  15    DS9490 usb device
  16    W1-over-GPIO
  17    DS2482 (i2c to w1 bridge)
  18    Emulated devices, such as a RS232 converter, parallel port adapter, etc
  19
  20
  21What does the w1 subsystem do?
  22------------------------------------------------------------------
  23When a w1 master driver registers with the w1 subsystem, the following occurs:
  24
  25 - sysfs entries for that w1 master are created
  26 - the w1 bus is periodically searched for new slave devices
  27
  28When a device is found on the bus, w1 core checks if driver for its family is
  29loaded. If so, the family driver is attached to the slave.
  30If there is no driver for the family, default one is assigned, which allows to perform
  31almost any kind of operations. Each logical operation is a transaction
  32in nature, which can contain several (two or one) low-level operations.
  33Let's see how one can read EEPROM context:
  341. one must write control buffer, i.e. buffer containing command byte
  35and two byte address. At this step bus is reset and appropriate device
  36is selected using either W1_SKIP_ROM or W1_MATCH_ROM command.
  37Then provided control buffer is being written to the wire.
  382. reading. This will issue reading eeprom response.
  39
  40It is possible that between 1. and 2. w1 master thread will reset bus for searching
  41and slave device will be even removed, but in this case 0xff will
  42be read, since no device was selected.
  43
  44
  45W1 device families
  46------------------------------------------------------------------
  47Slave devices are handled by a driver written for a family of w1 devices.
  48
  49A family driver populates a struct w1_family_ops (see w1_family.h) and
  50registers with the w1 subsystem.
  51
  52Current family drivers:
  53w1_therm - (ds18?20 thermal sensor family driver)
  54    provides temperature reading function which is bound to ->rbin() method
  55    of the above w1_family_ops structure.
  56
  57w1_smem - driver for simple 64bit memory cell provides ID reading method.
  58
  59You can call above methods by reading appropriate sysfs files.
  60
  61
  62What does a w1 master driver need to implement?
  63------------------------------------------------------------------
  64
  65The driver for w1 bus master must provide at minimum two functions.
  66
  67Emulated devices must provide the ability to set the output signal level
  68(write_bit) and sample the signal level (read_bit).
  69
  70Devices that support the 1-wire natively must provide the ability to write and
  71sample a bit (touch_bit) and reset the bus (reset_bus).
  72
  73Most hardware provides higher-level functions that offload w1 handling.
  74See struct w1_bus_master definition in w1.h for details.
  75
  76
  77w1 master sysfs interface
  78------------------------------------------------------------------
  79<xx-xxxxxxxxxxxxx> - a directory for a found device. The format is family-serial
  80bus                - (standard) symlink to the w1 bus
  81driver             - (standard) symlink to the w1 driver
  82w1_master_add      - Manually register a slave device
  83w1_master_attempts - the number of times a search was attempted
  84w1_master_max_slave_count
  85                   - the maximum slaves that may be attached to a master
  86w1_master_name     - the name of the device (w1_bus_masterX)
  87w1_master_pullup   - 5V strong pullup 0 enabled, 1 disabled
  88w1_master_remove   - Manually remove a slave device
  89w1_master_search   - the number of searches left to do, -1=continual (default)
  90w1_master_slave_count
  91                   - the number of slaves found
  92w1_master_slaves   - the names of the slaves, one per line
  93w1_master_timeout  - the delay in seconds between searches
  94
  95If you have a w1 bus that never changes (you don't add or remove devices),
  96you can set the module parameter search_count to a small positive number
  97for an initially small number of bus searches.  Alternatively it could be
  98set to zero, then manually add the slave device serial numbers by
  99w1_master_add device file.  The w1_master_add and w1_master_remove files
 100generally only make sense when searching is disabled, as a search will
 101redetect manually removed devices that are present and timeout manually
 102added devices that aren't on the bus.
 103
 104
 105w1 slave sysfs interface
 106------------------------------------------------------------------
 107bus                - (standard) symlink to the w1 bus
 108driver             - (standard) symlink to the w1 driver
 109name               - the device name, usually the same as the directory name
 110w1_slave           - (optional) a binary file whose meaning depends on the
 111                     family driver
 112rw                 - (optional) created for slave devices which do not have
 113                     appropriate family driver. Allows to read/write binary data.
 114
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