diff options
Diffstat (limited to 'Documentation/hwmon/ds1621')
| -rw-r--r-- | Documentation/hwmon/ds1621 | 108 |
1 files changed, 108 insertions, 0 deletions
diff --git a/Documentation/hwmon/ds1621 b/Documentation/hwmon/ds1621 new file mode 100644 index 00000000000..1fee6f1e6bc --- /dev/null +++ b/Documentation/hwmon/ds1621 @@ -0,0 +1,108 @@ +Kernel driver ds1621 +==================== + +Supported chips: + * Dallas Semiconductor DS1621 + Prefix: 'ds1621' + Addresses scanned: I2C 0x48 - 0x4f + Datasheet: Publicly available at the Dallas Semiconductor website + http://www.dalsemi.com/ + * Dallas Semiconductor DS1625 + Prefix: 'ds1621' + Addresses scanned: I2C 0x48 - 0x4f + Datasheet: Publicly available at the Dallas Semiconductor website + http://www.dalsemi.com/ + +Authors: + Christian W. Zuckschwerdt <zany@triq.net> + valuable contributions by Jan M. Sendler <sendler@sendler.de> + ported to 2.6 by Aurelien Jarno <aurelien@aurel32.net> + with the help of Jean Delvare <khali@linux-fr.org> + +Module Parameters +------------------ + +* polarity int + Output's polarity: 0 = active high, 1 = active low + +Description +----------- + +The DS1621 is a (one instance) digital thermometer and thermostat. It has +both high and low temperature limits which can be user defined (i.e. +programmed into non-volatile on-chip registers). Temperature range is -55 +degree Celsius to +125 in 0.5 increments. You may convert this into a +Fahrenheit range of -67 to +257 degrees with 0.9 steps. If polarity +parameter is not provided, original value is used. + +As for the thermostat, behavior can also be programmed using the polarity +toggle. On the one hand ("heater"), the thermostat output of the chip, +Tout, will trigger when the low limit temperature is met or underrun and +stays high until the high limit is met or exceeded. On the other hand +("cooler"), vice versa. That way "heater" equals "active low", whereas +"conditioner" equals "active high". Please note that the DS1621 data sheet +is somewhat misleading in this point since setting the polarity bit does +not simply invert Tout. + +A second thing is that, during extensive testing, Tout showed a tolerance +of up to +/- 0.5 degrees even when compared against precise temperature +readings. Be sure to have a high vs. low temperature limit gap of al least +1.0 degree Celsius to avoid Tout "bouncing", though! + +As for alarms, you can read the alarm status of the DS1621 via the 'alarms' +/sys file interface. The result consists mainly of bit 6 and 5 of the +configuration register of the chip; bit 6 (0x40 or 64) is the high alarm +bit and bit 5 (0x20 or 32) the low one. These bits are set when the high or +low limits are met or exceeded and are reset by the module as soon as the +respective temperature ranges are left. + +The alarm registers are in no way suitable to find out about the actual +status of Tout. They will only tell you about its history, whether or not +any of the limits have ever been met or exceeded since last power-up or +reset. Be aware: When testing, it showed that the status of Tout can change +with neither of the alarms set. + +Temperature conversion of the DS1621 takes up to 1000ms; internal access to +non-volatile registers may last for 10ms or below. + +High Accuracy Temperature Reading +--------------------------------- + +As said before, the temperature issued via the 9-bit i2c-bus data is +somewhat arbitrary. Internally, the temperature conversion is of a +different kind that is explained (not so...) well in the DS1621 data sheet. +To cut the long story short: Inside the DS1621 there are two oscillators, +both of them biassed by a temperature coefficient. + +Higher resolution of the temperature reading can be achieved using the +internal projection, which means taking account of REG_COUNT and REG_SLOPE +(the driver manages them): + +Taken from Dallas Semiconductors App Note 068: 'Increasing Temperature +Resolution on the DS1620' and App Note 105: 'High Resolution Temperature +Measurement with Dallas Direct-to-Digital Temperature Sensors' + +- Read the 9-bit temperature and strip the LSB (Truncate the .5 degs) +- The resulting value is TEMP_READ. +- Then, read REG_COUNT. +- And then, REG_SLOPE. + + TEMP = TEMP_READ - 0.25 + ((REG_SLOPE - REG_COUNT) / REG_SLOPE) + +Note that this is what the DONE bit in the DS1621 configuration register is +good for: Internally, one temperature conversion takes up to 1000ms. Before +that conversion is complete you will not be able to read valid things out +of REG_COUNT and REG_SLOPE. The DONE bit, as you may have guessed by now, +tells you whether the conversion is complete ("done", in plain English) and +thus, whether the values you read are good or not. + +The DS1621 has two modes of operation: "Continuous" conversion, which can +be understood as the default stand-alone mode where the chip gets the +temperature and controls external devices via its Tout pin or tells other +i2c's about it if they care. The other mode is called "1SHOT", that means +that it only figures out about the temperature when it is explicitly told +to do so; this can be seen as power saving mode. + +Now if you want to read REG_COUNT and REG_SLOPE, you have to either stop +the continuous conversions until the contents of these registers are valid, +or, in 1SHOT mode, you have to have one conversion made. |