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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.