134 lines
5.6 KiB
ReStructuredText
134 lines
5.6 KiB
ReStructuredText
=========================================
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Introduction to the 1-wire (w1) subsystem
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=========================================
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The 1-wire bus is a simple master-slave bus that communicates via a single
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signal wire (plus ground, so two wires).
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Devices communicate on the bus by pulling the signal to ground via an open
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drain output and by sampling the logic level of the signal line.
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The w1 subsystem provides the framework for managing w1 masters and
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communication with slaves.
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All w1 slave devices must be connected to a w1 bus master device.
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Example w1 master devices:
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- DS9490 usb device
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- W1-over-GPIO
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- DS2482 (i2c to w1 bridge)
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- Emulated devices, such as a RS232 converter, parallel port adapter, etc
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What does the w1 subsystem do?
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------------------------------
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When a w1 master driver registers with the w1 subsystem, the following occurs:
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- sysfs entries for that w1 master are created
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- the w1 bus is periodically searched for new slave devices
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When a device is found on the bus, w1 core tries to load the driver for its family
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and check if it is loaded. If so, the family driver is attached to the slave.
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If there is no driver for the family, default one is assigned, which allows to perform
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almost any kind of operations. Each logical operation is a transaction
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in nature, which can contain several (two or one) low-level operations.
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Let's see how one can read EEPROM context:
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1. one must write control buffer, i.e. buffer containing command byte
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and two byte address. At this step bus is reset and appropriate device
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is selected using either W1_SKIP_ROM or W1_MATCH_ROM command.
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Then provided control buffer is being written to the wire.
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2. reading. This will issue reading eeprom response.
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It is possible that between 1. and 2. w1 master thread will reset bus for searching
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and slave device will be even removed, but in this case 0xff will
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be read, since no device was selected.
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W1 device families
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------------------
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Slave devices are handled by a driver written for a family of w1 devices.
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A family driver populates a struct w1_family_ops (see w1_family.h) and
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registers with the w1 subsystem.
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Current family drivers:
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w1_therm
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- (ds18?20 thermal sensor family driver)
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provides temperature reading function which is bound to ->rbin() method
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of the above w1_family_ops structure.
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w1_smem
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- driver for simple 64bit memory cell provides ID reading method.
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You can call above methods by reading appropriate sysfs files.
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What does a w1 master driver need to implement?
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-----------------------------------------------
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The driver for w1 bus master must provide at minimum two functions.
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Emulated devices must provide the ability to set the output signal level
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(write_bit) and sample the signal level (read_bit).
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Devices that support the 1-wire natively must provide the ability to write and
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sample a bit (touch_bit) and reset the bus (reset_bus).
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Most hardware provides higher-level functions that offload w1 handling.
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See struct w1_bus_master definition in w1.h for details.
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w1 master sysfs interface
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-------------------------
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========================= =====================================================
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<xx-xxxxxxxxxxxx> A directory for a found device. The format is
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family-serial
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bus (standard) symlink to the w1 bus
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driver (standard) symlink to the w1 driver
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w1_master_add (rw) manually register a slave device
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w1_master_attempts (ro) the number of times a search was attempted
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w1_master_max_slave_count (rw) maximum number of slaves to search for at a time
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w1_master_name (ro) the name of the device (w1_bus_masterX)
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w1_master_pullup (rw) 5V strong pullup 0 enabled, 1 disabled
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w1_master_remove (rw) manually remove a slave device
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w1_master_search (rw) the number of searches left to do,
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-1=continual (default)
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w1_master_slave_count (ro) the number of slaves found
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w1_master_slaves (ro) the names of the slaves, one per line
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w1_master_timeout (ro) the delay in seconds between searches
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w1_master_timeout_us (ro) the delay in microseconds beetwen searches
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========================= =====================================================
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If you have a w1 bus that never changes (you don't add or remove devices),
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you can set the module parameter search_count to a small positive number
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for an initially small number of bus searches. Alternatively it could be
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set to zero, then manually add the slave device serial numbers by
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w1_master_add device file. The w1_master_add and w1_master_remove files
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generally only make sense when searching is disabled, as a search will
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redetect manually removed devices that are present and timeout manually
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added devices that aren't on the bus.
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Bus searches occur at an interval, specified as a sum of timeout and
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timeout_us module parameters (either of which may be 0) for as long as
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w1_master_search remains greater than 0 or is -1. Each search attempt
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decrements w1_master_search by 1 (down to 0) and increments
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w1_master_attempts by 1.
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w1 slave sysfs interface
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------------------------
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=================== ============================================================
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bus (standard) symlink to the w1 bus
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driver (standard) symlink to the w1 driver
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name the device name, usually the same as the directory name
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w1_slave (optional) a binary file whose meaning depends on the
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family driver
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rw (optional) created for slave devices which do not have
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appropriate family driver. Allows to read/write binary data.
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=================== ============================================================
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