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Evgeniy Polyakov47546392005-06-04 01:31:47 +04001The 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 it's family is
29loaded. If so, the family driver is attached to the slave.
Evgeniy Polyakovf522d232006-03-23 19:11:58 +030030If 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.
Evgeniy Polyakov47546392005-06-04 01:31:47 +040043
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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070058
59You can call above methods by reading appropriate sysfs files.
Evgeniy Polyakov47546392005-06-04 01:31:47 +040060
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_attempts - the number of times a search was attempted
83w1_master_max_slave_count
84 - the maximum slaves that may be attached to a master
85w1_master_name - the name of the device (w1_bus_masterX)
86w1_master_search - the number of searches left to do, -1=continual (default)
87w1_master_slave_count
88 - the number of slaves found
89w1_master_slaves - the names of the slaves, one per line
90w1_master_timeout - the delay in seconds between searches
91
92If you have a w1 bus that never changes (you don't add or remove devices),
93you can set w1_master_search to a positive value to disable searches.
94
95
96w1 slave sysfs interface
97------------------------------------------------------------------
98bus - (standard) symlink to the w1 bus
99driver - (standard) symlink to the w1 driver
100name - the device name, usually the same as the directory name
101w1_slave - (optional) a binary file whose meaning depends on the
102 family driver
Evgeniy Polyakovf522d232006-03-23 19:11:58 +0300103rw - (optional) created for slave devices which do not have
104 appropriate family driver. Allows to read/write binary data.