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-The 1-wire (w1) subsystem
-The 1-wire bus is a simple master-slave bus that communicates via a single
-signal wire (plus ground, so two wires).
-Devices communicate on the bus by pulling the signal to ground via an open
-drain output and by sampling the logic level of the signal line.
-The w1 subsystem provides the framework for managing w1 masters and
-communication with slaves.
-All w1 slave devices must be connected to a w1 bus master device.
-Example w1 master devices:
- DS9490 usb device
- W1-over-GPIO
- DS2482 (i2c to w1 bridge)
- Emulated devices, such as a RS232 converter, parallel port adapter, etc
-What does the w1 subsystem do?
-When a w1 master driver registers with the w1 subsystem, the following occurs:
- - sysfs entries for that w1 master are created
- - the w1 bus is periodically searched for new slave devices
-When a device is found on the bus, w1 core checks if driver for its family is
-loaded. If so, the family driver is attached to the slave.
-If there is no driver for the family, default one is assigned, which allows to perform
-almost any kind of operations. Each logical operation is a transaction
-in nature, which can contain several (two or one) low-level operations.
-Let's see how one can read EEPROM context:
-1. one must write control buffer, i.e. buffer containing command byte
-and two byte address. At this step bus is reset and appropriate device
-is selected using either W1_SKIP_ROM or W1_MATCH_ROM command.
-Then provided control buffer is being written to the wire.
-2. reading. This will issue reading eeprom response.
-It is possible that between 1. and 2. w1 master thread will reset bus for searching
-and slave device will be even removed, but in this case 0xff will
-be read, since no device was selected.
-W1 device families
-Slave devices are handled by a driver written for a family of w1 devices.
-A family driver populates a struct w1_family_ops (see w1_family.h) and
-registers with the w1 subsystem.
-Current family drivers:
-w1_therm - (ds18?20 thermal sensor family driver)
- provides temperature reading function which is bound to ->rbin() method
- of the above w1_family_ops structure.
-w1_smem - driver for simple 64bit memory cell provides ID reading method.
-You can call above methods by reading appropriate sysfs files.
-What does a w1 master driver need to implement?
-The driver for w1 bus master must provide at minimum two functions.
-Emulated devices must provide the ability to set the output signal level
-(write_bit) and sample the signal level (read_bit).
-Devices that support the 1-wire natively must provide the ability to write and
-sample a bit (touch_bit) and reset the bus (reset_bus).
-Most hardware provides higher-level functions that offload w1 handling.
-See struct w1_bus_master definition in w1.h for details.
-w1 master sysfs interface
-<xx-xxxxxxxxxxxxx> - a directory for a found device. The format is family-serial
-bus - (standard) symlink to the w1 bus
-driver - (standard) symlink to the w1 driver
-w1_master_add - Manually register a slave device
-w1_master_attempts - the number of times a search was attempted
- - the maximum slaves that may be attached to a master
-w1_master_name - the name of the device (w1_bus_masterX)
-w1_master_pullup - 5V strong pullup 0 enabled, 1 disabled
-w1_master_remove - Manually remove a slave device
-w1_master_search - the number of searches left to do, -1=continual (default)
- - the number of slaves found
-w1_master_slaves - the names of the slaves, one per line
-w1_master_timeout - the delay in seconds between searches
-If you have a w1 bus that never changes (you don't add or remove devices),
-you can set the module parameter search_count to a small positive number
-for an initially small number of bus searches. Alternatively it could be
-set to zero, then manually add the slave device serial numbers by
-w1_master_add device file. The w1_master_add and w1_master_remove files
-generally only make sense when searching is disabled, as a search will
-redetect manually removed devices that are present and timeout manually
-added devices that aren't on the bus.
-w1 slave sysfs interface
-bus - (standard) symlink to the w1 bus
-driver - (standard) symlink to the w1 driver
-name - the device name, usually the same as the directory name
-w1_slave - (optional) a binary file whose meaning depends on the
- family driver
-rw - (optional) created for slave devices which do not have
- appropriate family driver. Allows to read/write binary data.