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- 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.
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