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