105 lines
4.4 KiB
Plaintext
105 lines
4.4 KiB
Plaintext
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 it's 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_attempts - the number of times a search was attempted
|
|
w1_master_max_slave_count
|
|
- the maximum slaves that may be attached to a master
|
|
w1_master_name - the name of the device (w1_bus_masterX)
|
|
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 w1_master_search to a positive value to disable searches.
|
|
|
|
|
|
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.
|