1 ========================================= 2 Introduction to the 1-wire (w1) subsystem 3 ========================================= 4 5 The 1-wire bus is a simple master-slave bus that communicates via a single 6 signal wire (plus ground, so two wires). 7 8 Devices communicate on the bus by pulling the signal to ground via an open 9 drain output and by sampling the logic level of the signal line. 10 11 The w1 subsystem provides the framework for managing w1 masters and 12 communication with slaves. 13 14 All w1 slave devices must be connected to a w1 bus master device. 15 16 Example w1 master devices: 17 18 - DS9490 usb device 19 - W1-over-GPIO 20 - DS2482 (i2c to w1 bridge) 21 - Emulated devices, such as a RS232 converter, parallel port adapter, etc 22 23 24 What does the w1 subsystem do? 25 ------------------------------ 26 27 When a w1 master driver registers with the w1 subsystem, the following occurs: 28 29 - sysfs entries for that w1 master are created 30 - the w1 bus is periodically searched for new slave devices 31 32 When a device is found on the bus, w1 core tries to load the driver for its family 33 and check if it is loaded. If so, the family driver is attached to the slave. 34 If there is no driver for the family, default one is assigned, which allows to perform 35 almost any kind of operations. Each logical operation is a transaction 36 in nature, which can contain several (two or one) low-level operations. 37 Let's see how one can read EEPROM context: 38 1. one must write control buffer, i.e. buffer containing command byte 39 and two byte address. At this step bus is reset and appropriate device 40 is selected using either W1_SKIP_ROM or W1_MATCH_ROM command. 41 Then provided control buffer is being written to the wire. 42 2. reading. This will issue reading eeprom response. 43 44 It is possible that between 1. and 2. w1 master thread will reset bus for searching 45 and slave device will be even removed, but in this case 0xff will 46 be read, since no device was selected. 47 48 49 W1 device families 50 ------------------ 51 52 Slave devices are handled by a driver written for a family of w1 devices. 53 54 A family driver populates a struct w1_family_ops (see w1_family.h) and 55 registers with the w1 subsystem. 56 57 Current family drivers: 58 59 w1_therm 60 - (ds18?20 thermal sensor family driver) 61 provides temperature reading function which is bound to ->rbin() method 62 of the above w1_family_ops structure. 63 64 w1_smem 65 - driver for simple 64bit memory cell provides ID reading method. 66 67 You can call above methods by reading appropriate sysfs files. 68 69 70 What does a w1 master driver need to implement? 71 ----------------------------------------------- 72 73 The driver for w1 bus master must provide at minimum two functions. 74 75 Emulated devices must provide the ability to set the output signal level 76 (write_bit) and sample the signal level (read_bit). 77 78 Devices that support the 1-wire natively must provide the ability to write and 79 sample a bit (touch_bit) and reset the bus (reset_bus). 80 81 Most hardware provides higher-level functions that offload w1 handling. 82 See struct w1_bus_master definition in w1.h for details. 83 84 85 w1 master sysfs interface 86 ------------------------- 87 88 ========================= ===================================================== 89 <xx-xxxxxxxxxxxx> A directory for a found device. The format is 90 family-serial 91 bus (standard) symlink to the w1 bus 92 driver (standard) symlink to the w1 driver 93 w1_master_add (rw) manually register a slave device 94 w1_master_attempts (ro) the number of times a search was attempted 95 w1_master_max_slave_count (rw) maximum number of slaves to search for at a time 96 w1_master_name (ro) the name of the device (w1_bus_masterX) 97 w1_master_pullup (rw) 5V strong pullup 0 enabled, 1 disabled 98 w1_master_remove (rw) manually remove a slave device 99 w1_master_search (rw) the number of searches left to do, 100 -1=continual (default) 101 w1_master_slave_count (ro) the number of slaves found 102 w1_master_slaves (ro) the names of the slaves, one per line 103 w1_master_timeout (ro) the delay in seconds between searches 104 w1_master_timeout_us (ro) the delay in microseconds between searches 105 ========================= ===================================================== 106 107 If you have a w1 bus that never changes (you don't add or remove devices), 108 you can set the module parameter search_count to a small positive number 109 for an initially small number of bus searches. Alternatively it could be 110 set to zero, then manually add the slave device serial numbers by 111 w1_master_add device file. The w1_master_add and w1_master_remove files 112 generally only make sense when searching is disabled, as a search will 113 redetect manually removed devices that are present and timeout manually 114 added devices that aren't on the bus. 115 116 Bus searches occur at an interval, specified as a sum of timeout and 117 timeout_us module parameters (either of which may be 0) for as long as 118 w1_master_search remains greater than 0 or is -1. Each search attempt 119 decrements w1_master_search by 1 (down to 0) and increments 120 w1_master_attempts by 1. 121 122 w1 slave sysfs interface 123 ------------------------ 124 125 =================== ============================================================ 126 bus (standard) symlink to the w1 bus 127 driver (standard) symlink to the w1 driver 128 name the device name, usually the same as the directory name 129 w1_slave (optional) a binary file whose meaning depends on the 130 family driver 131 rw (optional) created for slave devices which do not have 132 appropriate family driver. Allows to read/write binary data. 133 =================== ============================================================
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