Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 The 1-wire (w1) subsystem 2 ------------------------------------------------------------------ 3 The 1-wire bus is a simple master-slave bus that communicates via a single 4 signal wire (plus ground, so two wires). 5 6 Devices communicate on the bus by pulling the signal to ground via an open 7 drain output and by sampling the logic level of the signal line. 8 9 The w1 subsystem provides the framework for managing w1 masters and 10 communication with slaves. 11 12 All w1 slave devices must be connected to a w1 bus master device. 13 14 Example 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 21 What does the w1 subsystem do? 22 ------------------------------------------------------------------ 23 When 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 28 When a device is found on the bus, w1 core tries to load the driver for its family 29 and check if it is loaded. If so, the family driver is attached to the slave. 30 If there is no driver for the family, default one is assigned, which allows to perform 31 almost any kind of operations. Each logical operation is a transaction 32 in nature, which can contain several (two or one) low-level operations. 33 Let's see how one can read EEPROM context: 34 1. one must write control buffer, i.e. buffer containing command byte 35 and two byte address. At this step bus is reset and appropriate device 36 is selected using either W1_SKIP_ROM or W1_MATCH_ROM command. 37 Then provided control buffer is being written to the wire. 38 2. reading. This will issue reading eeprom response. 39 40 It is possible that between 1. and 2. w1 master thread will reset bus for searching 41 and slave device will be even removed, but in this case 0xff will 42 be read, since no device was selected. 43 44 45 W1 device families 46 ------------------------------------------------------------------ 47 Slave devices are handled by a driver written for a family of w1 devices. 48 49 A family driver populates a struct w1_family_ops (see w1_family.h) and 50 registers with the w1 subsystem. 51 52 Current family drivers: 53 w1_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 57 w1_smem - driver for simple 64bit memory cell provides ID reading method. 58 59 You can call above methods by reading appropriate sysfs files. 60 61 62 What does a w1 master driver need to implement? 63 ------------------------------------------------------------------ 64 65 The driver for w1 bus master must provide at minimum two functions. 66 67 Emulated devices must provide the ability to set the output signal level 68 (write_bit) and sample the signal level (read_bit). 69 70 Devices that support the 1-wire natively must provide the ability to write and 71 sample a bit (touch_bit) and reset the bus (reset_bus). 72 73 Most hardware provides higher-level functions that offload w1 handling. 74 See struct w1_bus_master definition in w1.h for details. 75 76 77 w1 master sysfs interface 78 ------------------------------------------------------------------ 79 <xx-xxxxxxxxxxxx> - A directory for a found device. The format is family-serial 80 bus - (standard) symlink to the w1 bus 81 driver - (standard) symlink to the w1 driver 82 w1_master_add - (rw) manually register a slave device 83 w1_master_attempts - (ro) the number of times a search was attempted 84 w1_master_max_slave_count 85 - (rw) maximum number of slaves to search for at a time 86 w1_master_name - (ro) the name of the device (w1_bus_masterX) 87 w1_master_pullup - (rw) 5V strong pullup 0 enabled, 1 disabled 88 w1_master_remove - (rw) manually remove a slave device 89 w1_master_search - (rw) the number of searches left to do, 90 -1=continual (default) 91 w1_master_slave_count 92 - (ro) the number of slaves found 93 w1_master_slaves - (ro) the names of the slaves, one per line 94 w1_master_timeout - (ro) the delay in seconds between searches 95 w1_master_timeout_us 96 - (ro) the delay in microseconds beetwen searches 97 98 If you have a w1 bus that never changes (you don't add or remove devices), 99 you can set the module parameter search_count to a small positive number 100 for an initially small number of bus searches. Alternatively it could be 101 set to zero, then manually add the slave device serial numbers by 102 w1_master_add device file. The w1_master_add and w1_master_remove files 103 generally only make sense when searching is disabled, as a search will 104 redetect manually removed devices that are present and timeout manually 105 added devices that aren't on the bus. 106 107 Bus searches occur at an interval, specified as a summ of timeout and 108 timeout_us module parameters (either of which may be 0) for as long as 109 w1_master_search remains greater than 0 or is -1. Each search attempt 110 decrements w1_master_search by 1 (down to 0) and increments 111 w1_master_attempts by 1. 112 113 w1 slave sysfs interface 114 ------------------------------------------------------------------ 115 bus - (standard) symlink to the w1 bus 116 driver - (standard) symlink to the w1 driver 117 name - the device name, usually the same as the directory name 118 w1_slave - (optional) a binary file whose meaning depends on the 119 family driver 120 rw - (optional) created for slave devices which do not have 121 appropriate family driver. Allows to read/write binary data.