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Diffstat (limited to 'kernel/Documentation/w1/w1.generic')
| -rw-r--r-- | kernel/Documentation/w1/w1.generic | 113 |
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diff --git a/kernel/Documentation/w1/w1.generic b/kernel/Documentation/w1/w1.generic new file mode 100644 index 000000000..b2033c64c --- /dev/null +++ b/kernel/Documentation/w1/w1.generic @@ -0,0 +1,113 @@ +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 tries to load the driver for its family +and check if it 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 +w1_master_max_slave_count + - maximum number of slaves to search for at a time +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) +w1_master_slave_count + - 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. |