| Kernel driver w1_therm |
| ==================== |
| |
| Supported chips: |
| * Maxim ds18*20 based temperature sensors. |
| * Maxim ds1825 based temperature sensors. |
| |
| Author: Evgeniy Polyakov <johnpol@2ka.mipt.ru> |
| |
| |
| Description |
| ----------- |
| |
| w1_therm provides basic temperature conversion for ds18*20 devices, and the |
| ds28ea00 device. |
| supported family codes: |
| W1_THERM_DS18S20 0x10 |
| W1_THERM_DS1822 0x22 |
| W1_THERM_DS18B20 0x28 |
| W1_THERM_DS1825 0x3B |
| W1_THERM_DS28EA00 0x42 |
| |
| Support is provided through the sysfs w1_slave file. Each open and |
| read sequence will initiate a temperature conversion then provide two |
| lines of ASCII output. The first line contains the nine hex bytes |
| read along with a calculated crc value and YES or NO if it matched. |
| If the crc matched the returned values are retained. The second line |
| displays the retained values along with a temperature in millidegrees |
| Centigrade after t=. |
| |
| Parasite powered devices are limited to one slave performing a |
| temperature conversion at a time. If none of the devices are parasite |
| powered it would be possible to convert all the devices at the same |
| time and then go back to read individual sensors. That isn't |
| currently supported. The driver also doesn't support reduced |
| precision (which would also reduce the conversion time) when reading values. |
| |
| Writing a value between 9 and 12 to the sysfs w1_slave file will change the |
| precision of the sensor for the next readings. This value is in (volatile) |
| SRAM, so it is reset when the sensor gets power-cycled. |
| |
| To store the current precision configuration into EEPROM, the value 0 |
| has to be written to the sysfs w1_slave file. Since the EEPROM has a limited |
| amount of writes (>50k), this command should be used wisely. |
| |
| The module parameter strong_pullup can be set to 0 to disable the |
| strong pullup, 1 to enable autodetection or 2 to force strong pullup. |
| In case of autodetection, the driver will use the "READ POWER SUPPLY" |
| command to check if there are pariste powered devices on the bus. |
| If so, it will activate the master's strong pullup. |
| In case the detection of parasite devices using this command fails |
| (seems to be the case with some DS18S20) the strong pullup can |
| be force-enabled. |
| If the strong pullup is enabled, the master's strong pullup will be |
| driven when the conversion is taking place, provided the master driver |
| does support the strong pullup (or it falls back to a pullup |
| resistor). The DS18b20 temperature sensor specification lists a |
| maximum current draw of 1.5mA and that a 5k pullup resistor is not |
| sufficient. The strong pullup is designed to provide the additional |
| current required. |
| |
| The DS28EA00 provides an additional two pins for implementing a sequence |
| detection algorithm. This feature allows you to determine the physical |
| location of the chip in the 1-wire bus without needing pre-existing |
| knowledge of the bus ordering. Support is provided through the sysfs |
| w1_seq file. The file will contain a single line with an integer value |
| representing the device index in the bus starting at 0. |
