| David Fries | 6cd1597 | 2008-10-15 22:04:43 -0700 | [diff] [blame] | 1 | Kernel driver w1_therm |
| 2 | ==================== |
| 3 | |
| 4 | Supported chips: |
| 5 | * Maxim ds18*20 based temperature sensors. |
| 6 | |
| 7 | Author: Evgeniy Polyakov <johnpol@2ka.mipt.ru> |
| 8 | |
| 9 | |
| 10 | Description |
| 11 | ----------- |
| 12 | |
| 13 | w1_therm provides basic temperature conversion for ds18*20 devices. |
| 14 | supported family codes: |
| 15 | W1_THERM_DS18S20 0x10 |
| 16 | W1_THERM_DS1822 0x22 |
| 17 | W1_THERM_DS18B20 0x28 |
| 18 | |
| 19 | Support is provided through the sysfs w1_slave file. Each open and |
| 20 | read sequence will initiate a temperature conversion then provide two |
| 21 | lines of ASCII output. The first line contains the nine hex bytes |
| 22 | read along with a calculated crc value and YES or NO if it matched. |
| 23 | If the crc matched the returned values are retained. The second line |
| 24 | displays the retained values along with a temperature in millidegrees |
| 25 | Centigrade after t=. |
| 26 | |
| 27 | Parasite powered devices are limited to one slave performing a |
| 28 | temperature conversion at a time. If none of the devices are parasite |
| 29 | powered it would be possible to convert all the devices at the same |
| 30 | time and then go back to read individual sensors. That isn't |
| 31 | currently supported. The driver also doesn't support reduced |
| 32 | precision (which would also reduce the conversion time). |
| 33 | |
| 34 | The module parameter strong_pullup can be set to 0 to disable the |
| 35 | strong pullup or 1 to enable. If enabled the 5V strong pullup will be |
| 36 | enabled when the conversion is taking place provided the master driver |
| 37 | must support the strong pullup (or it falls back to a pullup |
| 38 | resistor). The DS18b20 temperature sensor specification lists a |
| 39 | maximum current draw of 1.5mA and that a 5k pullup resistor is not |
| 40 | sufficient. The strong pullup is designed to provide the additional |
| 41 | current required. |