projects / linux-2.6-block.git / blame
| Commit | Line | Data |
|---|---|---|
| 6cd15974 DF |
1 | Kernel driver w1_therm |
| 2 | ==================== | |
| 3 | ||
| 4 | Supported chips: | |
| 5 | * Maxim ds18*20 based temperature sensors. | |
| f3261dfb | 6 | * Maxim ds1825 based temperature sensors. |
| 6cd15974 DF |
7 | |
| 8 | Author: Evgeniy Polyakov <johnpol@2ka.mipt.ru> | |
| 9 | ||
| 10 | ||
| 11 | Description | |
| 12 | ----------- | |
| 13 | ||
| 14 | w1_therm provides basic temperature conversion for ds18*20 devices. | |
| 15 | supported family codes: | |
| 16 | W1_THERM_DS18S20 0x10 | |
| 17 | W1_THERM_DS1822 0x22 | |
| 18 | W1_THERM_DS18B20 0x28 | |
| f3261dfb | 19 | W1_THERM_DS1825 0x3B |
| 6cd15974 DF |
20 | |
| 21 | Support is provided through the sysfs w1_slave file. Each open and | |
| 22 | read sequence will initiate a temperature conversion then provide two | |
| 23 | lines of ASCII output. The first line contains the nine hex bytes | |
| 24 | read along with a calculated crc value and YES or NO if it matched. | |
| 25 | If the crc matched the returned values are retained. The second line | |
| 26 | displays the retained values along with a temperature in millidegrees | |
| 27 | Centigrade after t=. | |
| 28 | ||
| 29 | Parasite powered devices are limited to one slave performing a | |
| 30 | temperature conversion at a time. If none of the devices are parasite | |
| 31 | powered it would be possible to convert all the devices at the same | |
| 32 | time and then go back to read individual sensors. That isn't | |
| 33 | currently supported. The driver also doesn't support reduced | |
| 34 | precision (which would also reduce the conversion time). | |
| 35 | ||
| 36 | The module parameter strong_pullup can be set to 0 to disable the | |
| 29e5507a MA |
37 | strong pullup, 1 to enable autodetection or 2 to force strong pullup. |
| 38 | In case of autodetection, the driver will use the "READ POWER SUPPLY" | |
| 39 | command to check if there are pariste powered devices on the bus. | |
| 40 | If so, it will activate the master's strong pullup. | |
| 41 | In case the detection of parasite devices using this command fails | |
| 42 | (seems to be the case with some DS18S20) the strong pullup can | |
| 43 | be force-enabled. | |
| 44 | If the strong pullup is enabled, the master's strong pullup will be | |
| 45 | driven when the conversion is taking place, provided the master driver | |
| 46 | does support the strong pullup (or it falls back to a pullup | |
| 6cd15974 DF |
47 | resistor). The DS18b20 temperature sensor specification lists a |
| 48 | maximum current draw of 1.5mA and that a 5k pullup resistor is not | |
| 49 | sufficient. The strong pullup is designed to provide the additional | |
| 50 | current required. |